J.MoI.Evo1.

8,

175-195

Journalof MolecularEvolution

(1976)

(

by Springer-Verlag 1976

Synthesis Elemental Abundance as a Factor in the Origins of Mineral Nutrient Requirements* J. H, McCLENDON School of Life Sciences, Lincoln,

Nebr.

University of Nebraska-Li~coln,

68588, USA

Received January 26,

1976; March 30, 1976

Summary. No element is found to be commonly required if it has an abundance of less than about 2 nM in the ocean,

20 ~moles/kg

in the earth's crust, or

200 ~moles/iOO moles Si in the cosmos. More than 40 elements are above these limits,

but only 18 of them are commonly required

(6 of these being dis-

pensed with by some organism). It is postulated hypotheses:

that all of the required elements

fall under one of four

H-I--a unique requirement dating from the origin of life;

H-II--a unique requirement,

acquired later; H-III--a primordial

which was satisfied by a number of elements, made to the most abundant member; It is suggested that H, K

H-IV--same as III, but a later acquisition.

(vs. Na), Mg

under H-I. Special requirements

(vs. Ca), C, N, O, P, S and Fe fall

such as for B, Se and I fall under H-II.

H-III are K vs. Rb, Mg vs. Be(?), and Mn vs. various metals.

requirement

evolutionary adaptation being

In

S vs. Se, CI vs. Br, H vs. F(?), and Zn

In H-IV probably fall Ca vs. Sr, Na vs. Li(?),

Mo vs. V, and Si vs. Ge. The most abundant heavy metal in the ocean is Zn, which may account for its utilization;

other required heavy metals have

special utility as electron carriers.

Key words: Original of Life/Mineral Nutrition/Abundance

of Elements/

Macronutrients/Micronutrients

INTRODUCTION

Discussions constituents. spontaneous eral

of

the In

origin

environment

nevertheless,

origin

questing of

has

some

of

life

after

life, mostly

literature

the been on

deal

mostly

with

the

conditions

appropriate

fitness

the

of

taken

for

a

of

few

*A preliminary version of this paper was presented of Sciences, April 1970. Dedicated to my father, Jessie Francis McClendon, many years of research in nutrition.

for

inorganic

granted. the

organic

major

There

the

minis,

elements

to the Nebraska Academy aged 95, in honor of his

175

(Henderson, 1913; Wald, 1962), and a r e c e n t p a p e r on the m i c r o n u t r i e n t s (Egami, 1974). In addition, the q u e s t i o n of e x t r a t e r r e s t r i a l o r i g i n of life has b e e n r e o p e n e d , w i t h the a b u n d a n c e of m o l y b d e n u m as a factor in the a r g u m e n t (Crick & Orgel, 1973; C h a p p e l l et al., 1974; Jukes, 1974; Orgel, 1974; B a n i n & Navrot, 1975). It is o b v i o u s that the p r e s e n c e of an e l e m e n t is a n e c e s s a r y p r e r e q u i s i t e to the d e v e l o p m e n t of e s s e n t i a l m e t a b o l i s m b a s e d on that element. But it is not s u f f i c i e n t to a c c o u n t for the b i o c h e m i c a l roles of the u s e f u l e l e m e n t s . It is i m p o r t a n t to e x p l a i n w h y the m a j o r i t y of the e l e m e n t s in the p e r i o d i c t a b l e have n o t b e e n s e l e c t e d for a b i o c h e m i c a l role (or h a v e b e e n u t i l i z e d by o n l y s e l e c t types of o r g a n i s m s ) . One p o s s i b l e r e a s o n is that the p a r t i c u l a r e l e m e n t in q u e s t i o n m i g h t be s u i t a b l e for a b i o l o g i c a l role, b u t that its a b u n d a n c e in the a v a i l a b l e e n v i r o n m e n t is too low, e i t h e r on an a b s o l u t e basis, or in c o m p a r i s o n w i t h a n o t h e r e l e m e n t w h i c h can fill the same role. Since one c a n n o t c o n d u c t a d i r e c t e x p e r i m e n t to find the e f f e c t of v a r y i n g the a b u n d a n c e of e l e m e n t s on the e v o l u t i o n a r y d e v e l o p m e n t of life, I w i s h to a t t e m p t an e x a m i n a t i o n of the a v a i l a b l e d a t a to see h o w m u c h of the k n o w n n u t r i t i o n a l and b i o c h e m i c a l r e q u i r e m e n t s can be e x p l a i n e d on the b a s i s of a b u n dance. In o r d e r to f u l f i l l a role in m e t a b o l i s m , an e l e m e n t m u s t h a v e the a p p r o p r i a t e p r o p e r t i e s . Often, we do not k n o w w h a t t h e s e are in f u n d a m e n t a l detail, but do k n o w in some a p p r o x i m a t i o n , such as ionic c h a r g e or r e d o x p o t e n t i a l . If one and o n l y one e l e m e n t is e x p e r i m e n t a l l y found to c o m p l e t e l y f u l f i l l a role, t h e r e are two p o s s i b l e e x p l a n a t i o n s . E i t h e r o n l y one e l e m e n t in the p e r i o d i c t a b l e has the p r o p e r t i e s w h i c h p e r m i t it to p e r f o r m the f u n c t i o n or, d u r i n g the e v o l u t i o n a r y o r i g i n or d e v e l o p m e n t of life, a c h o i c e has b e e n m a d e b e t w e e n two or m o r e c a n d i d a t e s by m e a n s of an a d a p t i v e i n c r e a s e in the s p e c i f i c i t y (molecular affinity) to the m o r e a b u n d a n t element. If an e l e m e n t m a y f u n c t i o n in s e v e r a l u n r e l a t e d roles, a d a p t a t i o n in one r o l e m a y i n f l u e n c e its use in a n o t h e r role, e s p e c i a l l y if s p e c i f i c m e c h a n i s m s c h a n g e its i n t r a c e l l u l a r a b u n d a n c e . The b i o l o g i c a l e l e m e n t s h a v e a c t u a l l y b e e n o b s e r v e d to fall into two d i s t i n c t groups. In the f i r s t fall such as carbon, n i t r o g e n , and o x y g e n w h i c h are u n i q u e in the p e r i o d i c t a b l e so t h a t t h e r e can be no s u b s t i t u t i o n s e v e n a m o n g r e l a t e d e l e m e n t s (Wald, 1962). In the s e c o n d g r o u p are e l e m e n t s w i t h w h i c h t h e r e is some a m b i g u i t y , such t h a t p a i r s (or more) h a v e such c l o s e s i m i l a r i t y t h a t one is u s e f u l as a t r a c e r for the o t h e r in some role, for e x a m p l e r u b i d i u m and p o t a s s i u m . It is e v i d e n t t h a t a b u n d a n c e w i l l i n f l u e n c e u t i l i t y in d i f f e r e n t w a y s in the two sets. G r a n t i n g all the above, t h e r e are still a l t e r n a t i v e p o s s i b i l i t i e s as to the time c o u r s e of e v o l u t i o n . It m a y be t h a t ad176

d i t i o n s to or d e l e t i o n s from the list of r e q u i r e d e l e m e n t s have b e e n m a d e since the time of the o r i g i n of life. H u t c h i n s o n (1943), B e r t r a n d (1950), and P i r i e (1960) have t a k e n the posi t i o n that the p r i m i t i v e c o n d i t i o n was one in w h i c h a larger n u m b e r of e l e m e n t s was u t i l i z e d or r e q u i r e d , than at the p r e s e n t time - even as m u c h as the w h o l e p e r i o d i c table. In their view, n a t u r a l s e l e c t i o n has r e d u c e d the number, e i t h e r v e r y early, or r e l a t i v e l y r e c e n t l y . The r e v e r s e s e q u e n c e is also p o s s i b l e . I have t h e r e f o r e set up four h y p o t h e t i c a l c a t e g o r i e s of e s s e n t i a l elements: Hypothesis I. an e l e m e n t a l r e q u i r e m e n t is a b s o l u t e l y s p e c i f i c and a p r i m i t i v e c h a r a c t e r p r o b a b l y a r i s i n g in the H a d e a n p e r i o d (prior to 3.4 aeons ago; Cloud, 1974). Hypothesis II. an e l e m e n t a l r e q u i r e m e n t is a b s o l u t e l y s p e c i f i c and is a d e r i v e d a d d i t i o n d a t i n g from the A r c h a e a n p e r i o d or later. Hypothesis III. a r e q u i r e m e n t is e v o l u t i o n a r i l y d e r i v e d by s e l e c t i v e d e l e t i o n from a p r i m i t i v e set (Hadean) of r e q u i r e d r e l a t e d e l e m e n t s , by a d a p t a t i o n to the m o r e a b u n d a n t member. In some roles, s u b s t i t u t i o n of one e l e m e n t for a n o t h e r in the set is still p o s s i b l e . Hypothesis IV. a r e q u i r e m e n t is e v o l u t i o n a r i l y derived, as in III, but the set of r e l a t e d e l e m e n t s was not a p r i m i t i v e requirement. In the a r g u m e n t s to follow, the a s s u m p t i o n is m a d e that the r e q u i r e m e n t for those e l e m e n t s w h i c h are u n i v e r s a l l y r e q u i r e d arose in the H a d e a n (H-I or H-III), w h i l e those e l e m e n t s req u i r e d by a l i m i t e d set of o r g a n i s m s w e r e e v o l u t i o n a r i l y acq u i r e d (H-II or H-IV). O r d i n a r i l y , in H-II and H-IV, the function, as w e l l as the m i n e r a l r e q u i r e m e n t , m u s t be an a d d i t i o n to the b i o c h e m i s t r y of the organism. A g i v e n e l e m e n t m a y also fall u n d e r two h y p o t h e s e s . In p r i n c i p l e , all of these h y p o t h eses m a y also be stated in the negative. In the case of H-II, the m e a n i n g of the n e g a t i v e h y p o t h e s i s is that there is a dem o n s t r a b l e ( p r e s u m a b l y primitive) r e q u i r e m e n t for m a n y o r g a n isms, but that the e l e m e n t a l r e q u i r e m e n t has b e e n lost in some group. The n e g a t i v e of H-I and H-III(?) w o u l d c h a r a c t e r i z e e l e m e n t s w h i c h have such c h e m i c a l c h a r a c t e r i s t i c s or a b u n d a n c e s that they w e r e not u t i l i z e d by any e a r l y o r g a n i s m (a v e r y s p e c u l a t i v e c o n c l u s i o n ) . The n e g a t i v e of H - I V a p p e a r s to be m e a n i n g less, h o w e v e r (unless H - I V is the n e g a t i v e of H-III), so that there are o n l y 6 c a t e g o r i e s a l t o g e t h e r . It is clear that u n d e r any of the above h y p o t h e s e s , the a b s o l u t e a b u n d a n c e of an e l e m e n t w i l l d e t e r m i n e w h e t h e r it is (or could be) a m a c r o n u t r i e n t or a m i c r o n u t r i e n t or either.

177

MATERIALS

AND M E T H O D S

The b u l k of the d a t a is p r e s e n t e d as tables by p e r i o d i c g r o u p s of v a r i o u s m e a s u r e s of a b u n d a n c e plus n u t r i t i o n a l r e q u i r e m e n t s . T h e n o b l e g a s s e s have b e e n omitted. The e l e m e n t s are s h o w n in the p r i n c i p a l c h e m i c a l form of o c c u r r e n c e in the ocean, w h e r e k n o w n (Riley & Chester, 1971). E g a m i (1974) (see also C h a p p e l l et al., 1974; Jukes, 1974; Orgel, 1974; B a n i n & N a v r o t , 1975) has s u g g e s t e d that the c o n c e n t r a t i o n s in sea w a t e r w e r e those w h i c h w e r e c r u c i a l to the o r i g i n of e l e m e n t a l r e q u i r e m e n t s . This is not a f o r e g o n e c o n c l u s i o n , since the o c e a n is in a s t e a d y state r e l a t i o n s h i p w i t h the s e d i m e n t s and a t m o s p h e r e (MacIntyre, 1970; Hammond, 1975). Iron, p h o s p h o r u s , and carbon, in p a r t i c u l a r , are e l e m e n t s w h o s e bulk r e s e r v o i r s are insoluble. For this reason, the t a b l e s to f o l l o w show t h r e e sets of e n v i r o n m e n t a l a b u n d a n c e s : first, the c o s m i c a b u n d a n c e from w h i c h the e a r t h was f r a c t i o n a t e d in its c o n d e n s a t i o n f r o m the solar n e b u l a (data of Cameron, listed in R ~ s l e r & Lange, 1972); second, the a v e r a g e a b u n d a n c e in the c r u s t of the earth; third, the abunEnvironmental Abundances.

d a n c e in the ocean. For the crust, the d a t a u s e d are g i v e n and j u s t i f i e d by M a s o n (1966), p r i n c i p a l l y d e r i v e d from p r i o r est i m a t e s of G o l d s c h m i d t & T a y l o r (see R ~ s l e r & Lange, 1972). For the c o m p o s i t i o n of the o c e a n (the m i n o r c o n s t i t u e n t s are c o n t r o v e r s i a l or v a r i a b l e ) , I h a v e m a i n l y u s e d the d a t a of M a s o n (1966) (see also M a c I n t y r e , 1970; R i l e y & C h e s t e r , 1971; R ~ s l e r & Lange, 1972), but in o r d e r to a c k n o w l e d g e the r e s e r v o i r of n i t r o g e n in the a t m o s p h e r e , I have c a l c u l a t e d its a b u n d a n c e as if all the N 2 w e r e d i s s o l v e d in the ocean. Atm o s p h e r i c o x y g e n and c a r b o n a m o u n t to less than I% of o c e a n i c and w e r e ignored. The a b u n d a n c e in the c o s m o s is e x p r e s s e d , as is c u s t o m a r y , in terms of a t o m i c r a t i o s (per 100 m o l e s Si) ; the c r u s t and o c e a n have b e e n r e c a l c u l a t e d as g r a m - a t o m s per k i l o g r a m , w a t e r included, d e s i g n a t e d as m o l a r i t y in the tables. The H a d e a n e a r t h p r o b a b l y had d i f f e r e n t a b u n d a n c e s of some e l e m e n t s in the crust, o c e a n and a t m o s p h e r e (Cloud, 1974). The p r i n c i p a l forms of c a r b o n and n i t r o g e n , as w e l l as their surface a b u n d a n c e s , are u n k n o w n . In a d d i t i o n , the r e d u c i n g n a t u r e of the H a d e a n and A r c h a e a n a t m o s p h e r e s is w e l l e s t a b l i s h e d , p r i n c i p a l l y due to the g r e a t a b u n d a n c e of r e d u c e d iron and s u l f u r w h i c h w o u l d s c a v e n g e any free o x y g e n (Cloud, 1974). It seems c e r t a i n that m a n y e l e m e n t s w o u l d h a v e b e e n f o u n d in the H a d e a n o c e a n in a m o r e r e d u c e d state t h a n t h o s e l i s t e d in the tables, in p a r t i c u l a r , e l e m e n t s such as iron, m a n g a n e s e , copper, v a n a d i u m , c h r o m i u m , c o b a l t and m o l y b d e n u m . K r a u s k o p f (1956) found that of t h i r t e e n rate m e t a l s in the ocean, all w e r e p r e s e n t at far less than s a t u r a t i o n , due to local r e m o v a l as the s u l f i d e s or h y d r o x i d e s , a d s o r p t i o n or r e m o v a l by o r g a n i s m s . Since the s e q u e s t r a t i o n t a k e s p l a c e in o x y g e n p o o r s e d i m e n t s , 178

it m a y be that the p r e s e n t c o n c e n t r a t i o n s are not u n l i k e those of the p r i m i t i v e earth, but the s i t u a t i o n is c l e a r l y complex. Some o p p o s i n g e x a m p l e s m a y be cited. The p r e s e n t a b u n d a n c e of o c e a n i c iron is low due to p r e c i p i t a t i o n of the ferric h y d r o x ide, but was at least l o c a l l y a b u n d a n t as the f e r r o u s s u l f a t e or b i c a r b o n a t e d u r i n g the A r c h a e a n and P r o t e r o p h y t i c (Cloud, 1974). C h r o m i u m , w h e n r e d u c e d from Cr VI to Cr III by sulfide, p r e c i p i t a t e s as the e x t r e m e l y i n s o l u b l e h y d r o x i d e (Krauskopf, 1956), so that its e a r l y o c e a n i c c o n c e n t r a t i o n m a y have b e e n less than at present. K r a u s k o p f (1956) found that Mo, V, W, Ni, and Co are not r e m o v e d from sea w a t e r by s u l f i d e or h y d r o x i d e , so that he r e s o r t e d to b i o l o g i c a l a c c u m u l a t i o n , a p r o c e s s that p r o b a b l y c o u l d not be p o s t u l a t e d in the H a d e a n period. Since c o b a l t is n o w u n s a t u r a t e d by IO5-fold, its p r e s e n t c o n c e n t r a tion m a y be m i s l e a d i n g . O t h e r m i n e r a l s , such as s i l i c a and c a r b o n a t e , are also b i o l o g i c a l l y r e m o v e d from the p r e s e n t ocean.

Plant Abundance. R e c o g n i z i n g that o r g a n i s m i c c o m p o s i t i o n v a r i e s w i t h the species, health, n u t r i t i o n a l state, and m i n e r a l substrate, I have taken, by w a y of an example, the c o m p o s i t i o n of a l f a l f a (lucerne, Medicago sativa) as g i v e n by B e r t r a n d (1950), b e c a u s e of its e x t e n s i v e list of m i n o r and n o n - r e q u i r e d elements. The d a t a w e r e r e c a l c u l a t e d in g r a m - a t o m s per kg ("molarity"). Nutritional Requirements. The ease of p r e p a r a t i o n of h i g h l y pur i f i e d diets for p h o t o a u t o t r o p h i c o r g a n i s m s m a k e s the d a t a from p l a n t n u t r i t i o n at once m o r e p r e c i s e and taken from a m o r e div e r s e g r o u p of o r g a n i s m s than that for animals. I take as a w o r k i n g h y p o t h e s i s that there is no element, generally required by plants, that has not b e e n d i s c o v e r e d . If there are such r e q u i r e m e n t s u n d i s c o v e r e d , they m u s t be c o m m o n e l e m e n t s req u i r e d in small a m o u n t s or their n e c e s s a r y a m o u n t s m u s t be less than those of m o l y b d e n u m and cobalt, w h i c h seems u n l i k e l y . T h e r e are, of course, a n u m b e r of e l e m e n t s r e q u i r e d by part i c u l a r s p e c i e s or groups, w h i c h have not b e e n d e m o n s t r a t e d to be e s s e n t i a l for life in general. The t a b l e s show the minimum requirements (abundances) in a h i g h e r p l a n t in good n u t r i t i o n a l status (Epstein, 1965; rec a l c u l a t e d on a wet basis), plus an i n d i c a t i o n of r e q u i r e m e n t s (R) a m o n g o t h e r plants, animals, and m i c r o o r g a n i s m s . A m o n g h i g h e r plants, algae, fungi, and b a c t e r i a , only C, H, O, N, P, S, K, Mg, Fe, Mn, Zn, and Cu are r e q u i r e d by all t e s t e d s p e c i e s (Epstein, 1972). F u r t h e r d a t a and d i s c u s s i o n m a y be found in E p s t e i n (1965, 1972), F r i e d e n (1972), R u h l a n d (1958), S t e w a r d (1963), B o l l a r d & B u t l e r (1966) and O ' D e l l & C a m p b e l l (1970). In a d d i t i o n to r e q u i r e m e n t s , the final c o l u m n of the table i n d i c a t e s those e l e m e n t s w h i c h h a v e b e e n r e p o r t e d to s u b s t i t u t e for p a r t of the r e q u i r e m e n t for o t h e r e l e m e n t s (S), and some e l e m e n t s r e p o r t e d to a c c u m u l a t e (A) to a r e m a r k a b l e e x t e n t in 179

Table

i a

Concentrations

of frequently required elements

Cosmic

Crust

Plant

Sea & air

requirement A. Alkali,

alkaline-earth metals,

H

2.5 Mmole

O

O

2.5 kmole

H

29 1.4 1.23

non-metals

M

H

io1

M

H

iii

M

M

O

50

M

O

54

M mM

C

930

mole

Na

M

C

7

M

C1 535

N

240

mole

Ca 905

mM

N

200 mM

Na 456

mM

Mg

91

mole

Mg 860

mM

K

50 mM

N

mM

S

38

mole

K

662

mM

Ca

25 mM

Mg

56

mM

mole

P

34

mM

Mg

16 mM

S

28

mM

4.4

mole

C

17

mM

P

12 mM

Ca

iO

mM

1

mole

S

8

mM

(Na

14 mM)

K

mmole

Cl

3.7

mM

S

6 mM

C

mmole

N

1.4

mM

C1 600 ~M

B

zM

B

P

Ca

4.9

Na P K

316

C1 260

2.4 m_mole

B

B

195

930

400 ~M

9.7 mM 2.3 mM 430

~M

2.2 ~M

B. Heavy metals Fe 900

mM

Fe 400 pM

Fe iOO

nM

Mn 680

Fe

8.5

mmole

mole

Mn

mM

Mn 200 ~M

Mo i00

nM

Co 180

mmole

Zn

mM

Zn

60 ~M

Zn

75

nM

Cu

21

mmole

Cu 870

~M

Cu

20 DM

Cu

50

nM

Zn

20

mmole

Co 430

~M

(Co 300 nM)

Mn

36

nM

Mo 240

~mole

Mo

~M

Mo 200 nM

Co

2

nM

17 i.i

15

a

B, Ca, C1, Co, Mo, Na are widely but not universally required; in parenthesis

are content in alfalfa,

indicate the distinction between

not requirement.

"macro" and "micro"

the figures

Horizontal

lines

shown in Figure

I.

The order of heavy metals in the ocean is uncertain due to differences in published abundances.

one

or

but

non-essential

another

RESULTS The

AND

cosmic

explicable any

text

pattern

of

the

number even

and

B,

those 180

at

elements

commonly

required

plant.

of

the

to

iron. below

of The

logarithmic about

the

them

atomic

most

in

number and

the

light

46,

features

the

is

elements

greater rarity

which

table.

1966 of

with

abundance the

which

Mason,

abundance

periodic

periodic

a pattern

(see

obvious

discontinuity

only in

follow

synthesis

decrease

elements,

A major

them

elements

pathways

numbered

making just

known

geochemistry).

atomic as

denotes

some

abundances by

of

such

"N"

for

DISCUSSION

are

the

plant.

are

is

or

this higher

abundance peaks of

Li,

rarer

Be, than

"__" ~__ ~--~____~1 ®

Z

~

~

~

o

0

~

"

o

o

~" -

~

~

~

~

o

~

@

! ~

~m

0~

-~

~_ (y .,4

~

c~

0

4J

O

4J -4

@

#

.,4

~

0~ ~

O

~

0

~

•

~

00~

~

4~

~O

~0

~

-

-~

0

~

~

0

~

-p

--4 ,~

-

~

~

~]

~4 0 @

O

O

~-~

0

~

-~

O

q]

~m~

c~ ~

,~

.4

~

0

04

4J

9# m

H @

O

• -~

C~

~

o

--

~

O

CO

"q~'~

LZ')

qf)

I""-

"~

@

~]

P~ ~

N

0

@

~

•

0

O

m

o

4J

-H ~

~

~

~

~H

~

0

~

@

~

@ ~

~

~

~

~

b

T a b l e I d i s p l a y s the n u m e r i c a l a b u n d a n c e s of those 18 ele m e n t s w i t h the m o s t g e n e r a l (but not universal) b i o l o g i c a l req u i r e m e n t . T h e r e is a r o u g h c o r r e s p o n d e n c e b e t w e e n the four lists. A l t h o u g h it is o f t e n r e m a r k e d that the c o m p o s i t i o n of o r g a n i s m s r e s e m b l e s t h a t of the ocean, there does not s e e m to be m o r e r e s e m b l a n c e to the o c e a n than to the c r u s t or to the c o s m i c a b u n d a n c e s . In p a r t i c u l a r , the c o n t e n t of c a r b o n and n i t r o g e n in b o t h c r u s t and o c e a n is low c o m p a r e d to the p l a n t and c o s m i c a b u n d a n c e s , w h i l e s o d i u m c h l o r i d e is in g r e a t e x c e s s in the ocean. The s t r i k i n g f e a t u r e of the h e a v y m e t a l a b u n d a n c e s is the c l o s e o r d i n a l c o r r e s p o n d e n c e of cosmos, c r u s t and plant, 181

O~

Table 2 Group IA-alkali metals Cosmic

Crust

Sea & air

Alfalfa

Plant & animal requirements

+ Li

i0

rmnole

2.9

mM

25

~M

2.8 ~M a

A

M

456

mM

14

mM

A,S,R

+ Na + K

4.4

mole

1.23

316

mmole

662

mM

9.7 mM

44

mM

50 mM

650

~mole

1

mM

1.4 zM

54

~M

S,A

46

pmole

22

~M

4

+ Rh + Cs

nM

A

Fr a From Hutchinson

(1943).

w h i l e t h e o c e a n h a s an a p p a r e n t l y d i f f e r e n t o r d e r . In c o n t r a s t to e v e n so i n s o l u b l e an e l e m e n t as p h o s p h o r u s , the heavy metals a r e v e r y s t r o n g l y d e p l e t e d in t h e o c e a n as c o m p a r e d to t h e plant requirement ( e x c e p t for P a n d B, t h e r e a r e n o r e q u i r e d elements

in t h e

ocean

in t h e

four

orders

of m a g n i t u d e

between

C a n d Fe). A m o n g t h e c o s m i c a b u n d a n c e s , those of oxygen, pota s s i u m , c h l o r i d e a n d b o r o n s e e m l o w c o m p a r e d to t h e o r d e r of t h e e l e m e n t s as f o u n d o n e a r t h . M o l y b d e n u m and cobalt are the rarest elements utilized, their concentrations being above 100 ~ m o l e in t h e c o s m i c r a t i o , 10 m i c r o m o l a r in t h e c r u s t , a n d I n a n o m o l a r in t h e o c e a n . F i g u r e I s h o w s t h e r e l a t i v e a b u n d a n c e of t h e e l e m e n t s w i t h t w o d i f f e r e n t c u t - o f f p o i n t s , o n e d o u b l e the above limits and the other distinguishing "macro" and "micro" availability. In t h e d i s c u s s i o n b e l o w , e l e m e n t s w h o s e a b u n d a n c e s a r e b e l o w t h e l o w e r l i m i t s a r e c o n s i d e r e d to b e t o o r a r e to b e u t i l i z e d . N o s i g n i f i c a n c e is to b e a t t a c h e d to t h e h i g h e r l i m i t o t h e r t h a n to c a l l a t t e n t i o n to t h o s e e l e m e n t s w h i c h f a l l a t t h e l o w e r e n d of t h e a v a i l a b l e r a n g e . ~e

It is w i d e l y r e c o g n i z e d t h a t t h e c h e m i c a l e l e m e n t s of t h e s e c o n d p e r i o d d i f f e r m a r k e d l y

Second Period.

properties

of

f r o m t h o s e of t h e e l e m e n t s d i r e c t l y b e l o w t h e m in t h e p e r i o d i c t a b l e (Wald, 1962). T h i s m a k e s o x y g e n , n i t r o g e n , c a r b o n , a n d f l u o r i n e u n i q u e , w i t h no e l e m e n t s a b l e to s u b s t i t u t e for t h e m in b i o l o g i c a l systems. However, lithium, beryllium and boron, a l s o of t h e s e c o n d p e r i o d , a r e r a r e r t h a n a n y e l e m e n t ( e x c e p t Sc) of a t o m i c n u m b e r l e s s t h a n 31 (Ga). C o n s e q u e n t l y , it is i m p o s s i b l e to s a y w h e t h e r t h e y a r e a m o n g t h e g e n e r a l l y u n n e e d e d e l e m e n t s b e c a u s e t h e y a r e u n i q u e or b e c a u s e t h e y a r e r a r e , or b o t h . Group IA. The Alkali Metals. A l t h o u g h a l l b u t o n e of t h e s e e l e m e n t s a r e p r e s e n t in t h e o c e a n a n d c r u s t in a m o u n t s a b o v e t h e m i n i m u m , it is n o t e w o r t h y t h a t t h e t w o m o s t a b u n d a n t e l e m e n t s a r e t h e

182

ones c o m m o n l y required. N e v e r t h e l e s s , it is p o t a s s i u m that is m o s t s t r i n g e n t l y r e q u i r e d for n u t r i t i o n , as the m o n o v a l e n t c a t i o n of cell interiors. This p r e f e r e n c e could not have a r i s e n b e c a u s e of a g r e a t e r a b u n d a n c e b e c a u s e none of the three env i r o n m e n t a l a b u n d a n c e s shows m o r e p o t a s s i u m than sodium. The basis of this p r e s u m a b l y p r i m i t i v e r e q u i r e m e n t is not clear, but q u a l i f i e s for H-I (with r e s p e c t to Na vs. K). A n u m b e r of e n z y m e s are k n o w n in w h i c h a c o f a c t o r r e q u i r e m e n t for K + is also s a t i s f i e d by Rb + or NH4+, but it c a n n o t be s u p p o s e d that K + is a s u r r o g a t e for a p r i m i t i v e l y a b u n d a n t NH4 +, since there is no g r e a t e r a b u n d a n c e of N over K. No single e n z y m e or g r o u p of e n z y m e s can be p o s i t i v e l y i d e n t i f i e d as b e i n g the basis of the p o t a s s i u m r e q u i r e m e n t b e c a u s e of the n e c e s s a r y c o o r d i n a t e r e l a t i o n s h i p b e t w e e n the e n z y m e r e q u i r e m e n t s and the specif i c i t i e s of the a c t i v e t r a n s p o r t carriers. A simple c o u r s e of events m i g h t be that some e n z y m e (perhaps in the p r i m i t i v e g e n e t i c system) had the i n i t i a l H a d e a n r e q u i r e m e n t for K +, l e a d i n g to an a d a p t i v e d e v e l o p m e n t of a c t i v e t r a n s p o r t of K + and e x c l u s i o n of Na +. This s u b s e q u e n t l y led to (a tertiary) a d a p t a t i o n of other e n z y m e s to the d o m i n a n t m o n o v a l e n t cation, K +, (H-III, in the latter a d a p t a t i o n s . ) However, it is a b u n d a n t l y clear, at least for the u p t a k e m e c h a n i s m s and some i n d i v i d u a l enzymes, that r u b i d i u m is a good s u b s t i t u t e for p o t a s s i u m and is b e s t c o n s i d e r e d n o n - e s s e n tial only b e c a u s e of its rarity. That this i n t e r c h a n g a b i l i t y does not e x t e n d to 100% s u b s t i t u t i o n in n u t r i t i o n e x c e p t for s p e c i a l o r g a n i s m s (e.g. Streptococcus faecalis), can be a t t r i b u t e d to an a d a p t i v e s p e c i a l i z a t i o n (H-III or H-IV, for K vs. Rb). M a n y land p l a n t s a p p e a r to be a d e q u a t e l y n o u r i s h e d in the a b s e n c e of sodium, but it is u n l i k e l y that the H a d e a n o c e a n was low in this ion. Since at least part of the m o d e r n r e q u i r e m e n t for s o d i u m in m a r i n e p l a n t s can be s a t i s f i e d by an o s m o t i c s u b s t i t u t i o n of p o t a s s i u m or even m a g n e s i u m , and part of the m o n o v a l e n t c a t i o n r e q u i r e m e n t of land p l a n t s can be s a t i s f i e d by sodium, I s u g g e s t that there is a p r i m i t i v e , n o n - s p e c i f i c o s m o t i c a n d / o r ionic r e q u i r e m e n t that has b e c o m e v a r i o u s l y a d a p t a t i v e l y m o d i f i e d (H-III), such that h i g h e r a n i m a l s can no longer do w i t h o u t sodium, w h i l e m a n y h i g h e r p l a n t s can take it or leave it. The dual t r a n s p o r t m e c h a n i s m s d i s c o v e r e d by E p s t e i n and c o w o r k e r s (Epstein, 1965) i m p l y that s o d i u m is a c t i v e l y t a k e n up w h e n in s u f f i c i e n t s u p p l y but p o t a s s i u m is a l w a y s favored. R e p o r t s do not i n d i c a t e w h e t h e r l i t h i u m m a y s u b s t i t u t e for sodium, but it is as a b u n d a n t as r u b i d i u m and the p l a n t a n a l y sis shown i m p l i e s that it is not a c t i v e l y d i s c r i m i n a t e d against. It m a y fall u n d e r H-III vs. Na, but the u n i q u e p r o p e r t i e s of s e c o n d p e r i o d e l e m e n t s (above) r e q u i r e that m o r e i n f o r m a t i o n be available. Group IIA. Alkaline Earths.

The m o s t

abundant

divalent

cation

in 183

Table 3 Group IIA-alkaline earths Cosmic

Be (OH) n ++ Mg ++ Ca ++ Sr ++ Ba ++ Ra

Crust

Sea & air

Alfalfa

Plant & animal requirements

2

rmmole

310

~M

70 pM

91

mole

860

mM

56 mM

34 mM

mole

905

mM

iO mM

145 mM

25 mM(N)

4.3 mM

90 ~M

183 ~M a

S,A

3.3 mM

200 nM

188 DM a

S,A

4.9

6.1 m_mole 370

~mole tr

S 16 mM

tr

a

From Hutchinson

both the divalent however,

(1943).

o c e a n a n d in c e l l i n t e r i o r s is m a g n e s i u m (for o t h e r c a t i o n s see z i n c u n d e r t r a n s i t i o n m e t a l s ) . T h i s m a y , b e a c o i n c i d e n c e b e c a u s e , m o r e so t h a n in t h e c a s e of

t h e a l k a l i m e t a l s , t h e r e is a m a r k e d d i f f e r e n c e in c h e m i c a l properties between the third and fourth periods. I suggest that the magnesium requirement is f u n d a m e n t a l a n d p r i m i t i v e , b a s e d on the well known interaction with such condensed phosphates as A T P a n d e s p e c i a l l y t h e r i b o s o m e - m - R N A binding (H-I) . W h i l e land plants and vertebrates have a large requirement for c a l c i u m , t h e b u l k of t h i s is for e x t r a c e l l u l a r s t r u c t u r e a n d so can be considered a derived requirement. S i n c e in t h i s r e quirement strontium follows the same accumulation routes, this is n o t a n e x c l u s i v e r o l e (H-IV). T h i s c o n c l u s i o n is s u p p o r t e d b y t h e f i n d i n g s t h a t s o m e p l a n t s s e e m n o t to h a v e a n y m a c r o nutritient requirement for c a l c i u m at all. H o w e v e r , it is w e l l e s t a b l i s h e d that plasma membranes of a v a r i e t y of o r g a n i s m s a r e s t a b i l i z e d b y t h e p r e s e n c e of C a ++ in t h e m e d i u m . If t h i s is a p r i m i t i v e c h a r a c t e r , it s h o u l d b e a s s i g n e d to H - I (with r e s p e c t to M g + + ) . It m a y b e t h a t b e r y l l i u m c a n s u b s t i t u t e to s o m e e x t e n t for m a g n e s i u m , b u t its r a r i t y ( e s p e c i a l l y in t h e ocean) p r e c l u d e d a n y f a v o r a b l e a d a p t a t i o n , r e s u l t i n g in s e l e c t i o n for m a g n e s i u m and toxicity of b e r y l l i u m . L i k e w i s e , t h e p a u c i t y of b o t h s t r o n t i u m a n d b a r i u m r e l a t i v e to c a l c i u m w o u l d a p p e a r to p r e c l u d e t h e i r b e i n g a d o p t e d for e s s e n t i a l r o l e s (H-III) . T h e m o s t s p e c t a c u l a r r o l e of m a g n e s i u m in t h e w o r l d t o d a y is as an e s s e n t i a l p a r t of c h l o r o p h y l l . Its e a s e of r e p l a c e m e n t b y h y d r o g e n m a k e s it u n l i k e l y t h a t m a g n e s i u m t e t r a p y r r o l e s were s p o n t a n e o u s m e m b e r s of t h e p r i m o r d i a l s o u p , h o w e v e r . C h l o r o p h y l ] a i t s e l f c o u l d o n l y d a t e f r o m t h e A r c h a e a n o r i g i n s of b l u e - g r e e r algal photosynthesis; r e m n a n t s of p r i o r e v o l u t i o n a r y history a r e p r e s e n t as t h e b a c t e r i a l c h l o r o p h y l l s . Since the very first 184

Table 4 Group IlIA & B and the rare earths Cosmic

Crust

Plant &

Alfalfa

Sea & air

animal requirements B(OH)

2.4 mmole

3 AI(OH)

9.5

pM

430 ZM

650 ~M

400 ~M(N)

M

400 nM

930 ~M

A

490

~M

i00 pM

930

mole

3.01

3 Sc

2.8 nmlole

Ga

900

~mole

215

pM

7 nM

Y

890

~mole

370

pM

i nM

io

Dmole

1

~M

i pM

La

50

~mole

200

~M

20 pM

T1

31

zmole

iO

~M

50 pM

58

~mole

400

~M

3 nM

23

~mole

60

~M

4 pM

87

zmole

200

zM

16 pM

290

~mole

156

~M

22 pM

2.7 zmole

31

~M

220 pM

8

~M

5 nM

In +++

Ac Ce +++ +++ Pr +++ Nd Sum of At. No. 62-71 Th Pa 4UO2(C03) 3

800

nmole

cells

were

heterotrophs,

H-II,

even

though

we

do

in t h i s not

role magnesium

know why

it

must

is b e t t e r

fall

than

under

any

other

metal. G r o u p IIIA

~ B a n d the R a r e

Earths.

Of

these

25 e l e m e n t s ,

is v e r y a b u n d a n t , b u t t h e o n l y m e m b e r t h a t concentration is c o s m i c a l l y r a r e b o r o n . O n rare are

earths above

are

20

~M

MacIntyre, 1970) I riM. S i n c e o n l y ported appear at

no more in

the

rare crust,

and uranium boron among

than many and have this

has the

other

cerium

aluminum

a high oceanic other hand, the

elements;

(R~sler

many

& Lange,

1972;

oceanic concentrations above w h o l e s e t o f e l e m e n t s is r e -

to b e r e q u i r e d b y a n y o r g a n i s m , t h e c h e m i c a l p r o p e r t i e s to b e t h e l i m i t i n g f a c t o r . It is l i k e l y t h a t , in b u l k

least,

the

requirement

for boron

is d u e

to b o r a t e

binding

to c i s - d i o l s in c e l l w a l l p o l y s a c c h a r i d e s a n d is a d e r i v e d adaptation (H-II) ; s i n c e b o r a t e is n o t c o n s i d e r e d e s s e n t i a l f o r a n i m a l s , a n y m o r e f u n d a m e n t a l i n t r a c e l l u l a r f u n c t i o n of boron must likewise be derived rather than primordial. 185

Table 5 IVA & B-carbon, silicon, etc., plus hydrogen Cosmic

Crust

Sea & air

Alfalfa

Plant & animal requirements

H(I/2H 0 2

2.5 Mmole

HCO3

930

mole

Si(OH) 4

i00

mole

Ti(OH) 4

170

mmole

2.5 mmole

Ge(OH)

4

Zr

1.4

M

16.7

mM

9.86

130

Hf

M

87.2

2.3 mM

9.45

M

107

#M

92

mM

21

nM

21

zM

1

nM

mM

300

pM

i0

nM

+

iO

nM

+

1.4 mmole

Sn

iii

1.8

Hmole

17

pM

11.3 ~mole

17

~M

2.2 mmole

63

HM

3.3 iO

M

100.9 M

M

7.0 M

mM

A,R

HM

R

+ Pb(OH)

It

is c u r i o u s

ranking vantage mineral calcium

that

the

general

abundance

of

aluminum

- out-

all but two crustal elements - has not been taken ado f b y s o m e o r g a n i s m or o t h e r , if o n l y in t h e f o r m of d e p o s i t s , a f t e r t h e m a n n e r of d e p o s i t s of s i l i c a or of carbonate and phosphate. Some plants do accumulate

aluminum ( H u t c h i n s o n , 1943) b u t so far as I a m a w a r e , t h e y d o n o t r e q u i r e its p r e s e n c e . T h i s s e e m s to b e a c a s e of a n e g a t i v e Hypothesis

I.

Group IVA & B Carbon, Silicon, etc.; plus Hydrogen.

Although

all but

Ge, Zr, a n d Hf a r e p r e s e n t in a d e q u a t e a m o u n t s , o n l y H a n d C a r e g e n e r a l l y u s e f u l . H y d r o g e n is p l a c e d h e r e ( S a n d e r s o n , 1960) b e c a u s e its e l e c t r o n e g a t i v i t y a l l i e s it m o s t c l o s e l y w i t h t h e other elements with a half-filled outer electron shell; the close equivalence of e l e c t r o n e g a t i v i t y of carbon and hydrogen makes the hydrocarbons more exactly non-polar than any analogous compounds. Silicon, although much more abundant terrest r i a l l y t h a n c a r b o n , is l e s s e l e c t r o n e g a t i v e t h a n c a r b o n , so that silanes are decomposed in t h e p r e s e n c e of w a t e r . C a r b o n is a l s o d i s t i n g u i s h e d as a m e m b e r of t h e s e c o n d p e r i o d , in r e a d i l y f o r m i n g d o u b l e b o n d s , so t h a t e v e n t h e o x i d i z e d f o r m s [CO2, (SiO2)n] h a v e d r a s t i c a l l y different physical properties. G e r m a n i u m is m o r e e l e c t r o n e g a t i v e than hydrogen (Sanderson, 1 9 6 0 ) , so t h a t it m i g h t r e p l a c e c a r b o n , b u t b o t h its e x t r e m e insolubility a n d l a c k of d o u b l e b o n d f o r m a t i o n a r e u n f a v o r a b l e . It s e e m s c e r t a i n t h a t t h e b i o l o g i c a l r e q u i r e m e n t for c a r b o n c o u l d n o t b e f i l l e d b y a ~ y o t h e r e l e m e n t in i m a g i n a b l e c i r c u m s t a n c e s (H-I). 186

Table 6 Group VA-Nitrogen,

phosphorus,

Cosmic

N

2 HPO = 4 HAs04

Sb(OH)

6

etc.

Crust

1.4 mM

Sea & air

Alfalfa

Plant & animal requirements

195 mM a

590 mM

200 mM

23 mM

12 mM

240

mole

1

mole

34

mM

2 ~M

170 ~mole

24

~M

30 nM

23 ~mole

1.6 ~M

2 nM

30 ~mole

1

+

+

BiO

nM

i00 pM

a

Atmospheric N2; dissolved N is 36 ~M.

H y d r o g e n is d i s t i n c t l y d i f f e r e n t f r o m lithium, sodium, etc. (as m o n o v a l e n t e l e m e n t s ) , and is at the same time m u c h m o r e a b u n d a n t (H-I). For f l u o r i n e see below. S i l i c a is u t i l i z e d by a w i d e v a r i e t y of p l a n t s and a n i m a l s and is r e a s o n a b l y a b u n d a n t , even in the ocean. T i t a n i u m or g e r m a n i u m o x i d e s m i g h t s u b s t i t u t e , but are m u c h r a r e r (H-IV). G e r m a n i u m also has some r e s e m b l a n c e to b o r o n in G r o u p IIIA, in the f o r m a t i o n of c i s - d i o l s , but is so m u c h r a r e r that it w o u l d not h a v e b e e n u s e d (H-IV). Tin and lead are, c o n t r a r y to the u s u a l trend, h a r d l y r a r e r t h a n the l i g h t e r g e r m a n i u m , and more a b u n d a n t in sea water. This s o l u b i l i t y b r i n g s t h e m into the n a n o m o l a r r a n g e in the ocean, b u t no g e n e r a l b i o l o g i c a l u t i l i t y is found. The tin r e q u i r e m e n t in m a m m a l s m u s t fall into H-II or IV. L e a d ' s m o s t n o t e w o r t h y p r o p e r t y b i o l o g i c a l l y is as a poison, but t o l e r a n c e to its p r e s e n c e m u s t be w i d e s p r e a d . Group VA Nitrogen, Phosphorus, etc. L i k e c a r b o n vs. silicon, n i t r o gen and p h o s p h o r u s f o r m c o m p o u n d s of o n l y s u p e r f i c i a l similarity. The e l e c t r o n e g a t i v i t y of p h o s p h o r u s is such that p h o s p h i n e d o e s not o c c u r b i o l o g i c a l l y , w h i l e d o u b l e b o n d f o r m a t i o n by n i t r o g e n and acid a n h y d r i d e f o r m a t i o n by p h o s p h a t e m a k e e a c h of t h e m u n i q u e l y useful. E a c h falls u n d e r H-I w i t h r e s p e c t to e a c h other. H o w e v e r , a r s e n i c r e s e m b l e s p h o s p h o r u s g r e a t l y , and one m i g h t be t e m p t e d to s u g g e s t that H - I I I h o l d s w i t h r e s p e c t to p h o s p h a t e v e r s u s the less a b u n d a n t arsenate. T h e d a n g e r in s p e c u l a t i o n of this k i n d is b r o u g h t out, h o w e v e r , w h e n it is r e a l i z e d that the r e s e m b l a n c e does not e x t e n d to the a q u e o u s s t a b i l i t y of the a c i d a n h y d r i d e . It is p o s s i b l e to i m a g i n e a r s e n a t e a n a l o g u e s of c o m p l e x p h o s p h a t e esters, even n u c l e i c acids, but the m e c h a n i s m for t h e i r synt h e s i s via h i g h e n e r g y A T P a n a l o g u e s is a p p a r e n t l y u n a v a i l a b l e .

187

Table 7 Group VIA-oxygen,

etc.

Cosmic

H 0 2 SO = 4 SeO = 4 Te

Crust

2.5 kmole 37.5

mole

1.9 mmole 290

~mole

29

M

8mM 600 nM

Sea & air

Alfalfa

Plant & animal requirements

53.7

49

50

M

28

mM

30

nM

M

32 mM

M

6 mM A,S,R

78 nM

Po

P e r h a p s life w o u l d have e v o l v e d in the a b s e n c e of p h o s p h a t e , but it is d i f f i c u l t to p r o p o s e a model; the r e l a t i v e p a u c i t y of a r s e n i c m a y be looked on as a f o r t u n a t e c o n s e q u e n c e of the m e c h a n i s m of s t e l l a r s y n t h e s i s . On the o t h e r hand, the p r e s e n c e of p h o s p h a t e in the o c e a n in only m i c r o m o l a r q u a n t i t i e s is p r o b a b l y not r e l e v a n t to its c h o i c e as a m a c r o n u t r i e n t e l e m e n t . W h i l e less a b u n d a n t than m a n y e l e m e n t s , it is not r a r e in rocks, w h i c h f o r m the l o n g - t e r m b i o l o g i c a l r e s e r v o i r . The o r i g i n of life m a y h a v e t a k e n p l a c e in c l o s e a s s o c i a t i o n w i t h p h o s p h a t e bearing minerals. T h e r e l a t i v e r a r i t y of a n t i m o n y and b i s m u t h s e e m to h a v e p r e c l u d e d any b i o l o g i c a l role, even as c o m m o n p o i s o n s . Group VIA Oxygen, Sulfur, etc. O n c e again, o x y g e n is a u n i q u e , abund a n t element, f a l l i n g into H-I. U n l i k e p h o s p h o r u s , h o w e v e r , s u l f u r is b i o l o g i c a l l y r e q u i r e d in the r e d u c e d f o r m and sele n i u m a p p e a r s to be a c l o s e e n o u g h a n a l o g u e t h a t o n l y the r e l a t i v e a b u n d a n c e p r e v e n t e d the latter f r o m a s s u m i n g a m o r e p r o m i n e n t role. E v e n today, the m e c h a n i s m for r e d u c i n g s u l f a t e to s u l f i d e is t r a v e r s e d by s e l e n a t e as w e l l (Bandurski, 1965). C o n s e q u e n t l y , the s p e c i f i c i t y that m a k e s s e l e n i u m a p o i s o n to m o s t o r g a n i s m s is to be r e g a r d e d u n d e r H-III, w i t h rare tell u r i u m p e r h a p s t a g g i n g a l o n g as well. The b i o c h e m i c a l b a s i s of the a n i m a l m i c r o n u t r i e n t r e q u i r e m e n t for s e l e n i u m w o u l d have to i n v o l v e some s p e c i a l p r o p e r t y of s e l e n i u m not found in sulfur, f a l l i n g into H y p o t h e s i s II since it is a p p a r e n t l y not a g e n e r a l r e q u i r e m e n t . Some p l a n t s h a v e a d a p t e d to the r e l a t i v e l y h i g h c o n c e n t r a t i o n s of s e l e n i u m in c e r t a i n soils (H-IV?). E l e m e n t s of G r o u p V I B (chromate, m o l y b d a t e , tungstate) s t a n d in r e l a t i o n to s u l f a t e as a r s e n a t e is to p h o s p h a t e , f i t t i n g the s u l f a t e a c t i v a t i o n m e c h a n i s m , b u t b e i n g u n a b l e to f o r m s t a b l e a n a l o g u e s of a d e n o s i n e p h o s p h o s u l f a t e (Bandurski, 1965). Group VIIA. Halogens. W h i l e a t h o u s a n d - f o l d less a b u n d a n t (cosmically) than its n e i g h b o r s in the s e c o n d period, f l u o r i n e is still a c o m m o n e l e m e n t in the e a r t h ' s crust. It is less abun188

Table 8 Group VIIA-halogens Cosmic

Crust

F-

160 mmole

33

CI-

260 mmole

Br

395 zmole

31

60 ~mole

4

I

Sea & air

Alfalfa

Plant & animal requirements

mM

68 zM

80 pM

A,R

3.7 n~

535 mM

20 mM

600 ~M(N)

~M

810 ~M

6 ~M

S,A

zM

400 nM

200 nM

A,R

At

d a n t in the o c e a n than b r o m i n e , far less so than c h l o r i n e , p r e s u m a b l y due to the f o r m a t i o n of i n s o l u b l e m i n e r a l s c o m p a r a b l e to t h o s e w h i c h f o r m the b a s i s for its u t i l i t y as a t r a c e req u i r e m e n t in h u m a n n u t r i t i o n . T h e s e and o t h e r p r o p e r t i e s set it a p a r t f r o m the o t h e r h a l o g e n s , w h i c h are f o u n d as s o l u b l e m o n o v a l e n t ions for the m o s t part. The f l u o r i n e r e q u i r e m e n t in anim a l n u t r i t i o n is to be c o n s i d e r e d u n d e r H-II, since the f l u o r a p a t i t e s of t e e t h and b o n e are not p r i m i t i v e c h a r a c t e r i s t i c s of all o r g a n i s m s . On the o t h e r hand, the n o n - p r i m i t i v e n e s s of f l u o r i n e u t i l i z a t i o n is not d u e to some f u n d a m e n t a l i n s u i t a b i l i t y to b i o c h e m i c a l t r a n s f o r m a t i o n . The s t a b i l i t y of synt h e t i c f l u o r o c a r b o n s , and the n a t u r a l o c c u r r e n c e of m o n o f l u o r o a c e t a t e and r e l a t e d c o m p o u n d s in c e r t a i n p l a n t s t e s t i f y to this. S i n c e the e l e m e n t m o s t r e s e m b l i n g f l u o r i n e in such c o m p o u n d s is h y d r o g e n , the o v e r w h e l m i n g a b u n d a n c e of the l a t t e r m a y h a v e b e e n the c o n t r o l l i n g f a c t o r (H-III), but this a r g u m e n t c a n n o t be c a r r i e d too far, due to the d i f f e r e n c e in e l e c t r o n e g a t i v i t y of h y d r o g e n and fluorine. U t i l i t y as m o n o v a l e n t a n i o n s as b e t w e e n c h l o r i d e , b r o m i d e , and iodide, seems to d e p e n d u p o n t h e i r r e l a t i v e a b u n d a n c e (H-III), a l t h o u g h the s p e c i f i c i t y is p r o b a b l y not g r e a t even today. The u p t a k e m e c h a n i s m s w h i c h lead to e x t r e m e c o n c e n t r a t i o n r a t i o s b e t w e e n sea w a t e r and m a r i n e p l a n t s for i o d i d e m a y p o s s i b l y be c o i n c i d e n t a l , but some m a r i n e a n i m a l s and p l a n t s do s y n t h e s i z e c a r b o n c o m p o u n d s of b o t h b r o m i n e and iodine. The n e c e s s a r y s y n t h e s i s of t h y r o x i n e , in a n i m a l m e t a b olism, seems m o s t p r o b a b l y a d e r i v e d r e q u i r e m e n t , u n d e r H-II, s i n c e b r o m i n e and c h l o r i n e are m u c h m o r e a b u n d a n t . R a r e as i o d i n e is, it is a l m o s t in the m i c r o m o l a r r a n g e in the ocean, far a b o v e m a n y g e n e r a l l y r e q u i r e d n u t r i e n t s . It is i n t e r e s t i n g that in h i g h e r p l a n t s , w h e r e the n o n s p e c i f i c i n t r a c e l l u l a r a n i o n r e q u i r e m e n t can be m e t by b i o synthetic processes (as m a l a t e , c i t r a t e , etc.), c h l o r i d e is m e r e l y a m i c r o n u t r i e n t . The u b i q u i t o u s p r e s e n c e of c h l o r i d e

189

has e v i d e n t l y i n d u c e d some a d a p t i v e r e q u i r e m e n t in the p h o t o s y n t h e t i c m e c h a n i s m (H-IV). In fungi, c h l o r i d e is n o n - e s s e n t i a l . Groups VB, VIB, VIIB, VIIIB, IB, and IIB. Transition Metals. J u s t as the e l e m e n t s of the s e c o n d p e r i o d seem to be u n i q u e (above), the first set of t r a n s i t i o n e l e m e n t s (in the f o u r t h period) appear to be set a p a r t from those b e l o w t h e m in the table. Thus, iron, cobalt, nickel, and copper have p r o p e r t i e s d i s t i n c t from the p l a t i n u m metals, silver and gold. At the same time, o n l y the f o l l o w i n g e l e m e n t s have a d e q u a t e a b u n d a n c e s in b o t h the c r u s t and the ocean: V, Cr, Mo, Mn, Fe, Co, Ni, Cu, Zn; Cd m a y be a d e q u a t e in the o c e a n and crust, and Nb, Ta, and W in the c r u s t only. It is also of i n t e r e s t that if one r a i s e s the o c e a n i c c u t - o f f p o i n t to just above 10 nM, only Cr, Co and Ni d i s a p p e a r from the above list. The p r i m o r d i a l o c e a n was r e d u c i n g ; K r a u s kopf (1956) found that in the p r e s e n c e of 5 m M H2S, the l i m i t i n g c o n c e n t r a t i o n s of Ni, Co, Mo, and V w e r e all v e r y h i g h (above I ~M), w h i l e r e d u c e d Cr p r e c i p i t a t e s as the v e r y i n s o l u b l e hydroxide. N o t e m u s t be m a d e that m o l y b d e n u m is the only e l e m e n t in the f i f t h p e r i o d to be g e n e r a l l y r e q u i r e d and is the only t r a n s i t i o n e l e m e n t of the fifth p e r i o d w h i c h has an o c e a n i c a b u n d a n c e of m o r e than 10 n a n o m o l a r . (Iodine is the only o t h e r e s s e n t i a l ele m e n t b e y o n d the f o u r t h period.) It is s t r i k i n g that among the f o u r t h p e r i o d B g r o u p s (Sc-Zn), five e l e m e n t s are g e n e r a l l y r e q u i r e d (if we i n c l u d e cobalt), and five are not. S e a r c h i n g for some c o m m o n factor, c a t a l y t i c c a p a c i t y in some e n z y m e role is o b v i o u s for those w h i c h are u t i l i z e d . It is not obvious, however, w h y such e l e m e n t s as n i c k e l and c h r o m i u m are not m o r e w i d e l y u t i l i z e d in such roles. A f u r t h e r c o m m o n f e a t u r e for all b u t zinc of the u s e f u l e l e m e n t s is the p r e s e n c e of m u l t i p l e o x i d a t i o n states w i t h i n or near the s t a b i l i t y range in water; this i m p l i e s a role in c a t a l y t i c oxid a t i o n and r e d u c t i o n . A m o n g Mo, Mn, Fe, Co, Cu, and Zn, w h i c h element(s) can be a s s u m e d to have p l a y e d a role in the m e t a b o l i s m of the p r i m i t i v e H a d e a n h e t e r o t r o p h i c cells? F e r r e d o x i n o c c u r s in n o n - p h o t o s y n t h e t i c a n a e r o b e s and is g e n e r a l l y r e g a r d e d as a v e r y a n c i e n t protein. It also could be r e g a r d e d as an a d v a n c e d f o r m of ino r g a n i c iron sulfide, p e r h a p s in use from the v e r y b e g i n n i n g . The use of iron p o r p h y r i n s and n o n - h e m e iron p r o t e i n s in resp i r a t i o n m u s t p o s t d a t e the d e v e l o p m e n t of b l u e - g r e e n algal p h o t o s y n t h e s i s w i t h the p r o d u c t i o n of 02 (in m i d - A r c h a e a n , 3.3 aeons ago; Cloud, 1974). C o p p e r m a y have a s i m i l a r h i s t o r y since it now o c c u r s b o t h in the p h o t o s y n t h e t i c p a t h w a y as plastoc y a n i n and in c y t o c h r o m e oxidase. In p l a n t s the m o s t d e m a n d i n g role of m a n g a n e s e is in the o x y g e n l i b e r a t i n g p a t h w a y of p h o t o synthesis, so that one m u s t look e l s e w h e r e for an early role, if any. The other e n z y m a t i c roles of Mn a p p e a r to be for the d i v a l e n t cation; in this it is s i m i l a r to, even t h o u g h d i s t i n c t 190

Table

9

G r o u p VB, VIB, VIIB,

VIIIB,

Cosmic

IB, IIB Sea & air

Crust

Alfalfa

Plant & animal requirement

22

mmole

Nb

81

~mole

215

pM

i00 pM

pmole

ii

pM

I00 p M

Ta

1.5

2.7 m M

CrO = 4

780

mmole

2

mM

10 nM

MoO = 4

242

pmole

15

pM

100 nM

~mole

8

~M

500 pM

mmole

17

mM

36 nM

5.4

pmole

5

nM

50 p M

8.5

mole

900

mM

I00 n M 7 pM

WO = 4

10.5

Mn(OH)

685

pM

39 nM

VO-(?) 3

R,A

R iO

~M

200 nM (N)

66

~M

200 pM

480

pM

400 pM

3OO

nM

R,S,A

Tc

ReO 4 Fe (OH) Ru

87

pmole

100

nM

Os

64

pmole

26

nM

180

mmole

430

pM

Rh

15

pmole

50

nM

ir

50

pmole

5

nM

+ CoCl

2 nM

++ Ni

2.74

mole

1.3 m M

5 nM

Pd

68

pmole

94

nM

Pt

128

pmole

50

nM

21.2

mmole

870

pM

50 n M

26

pmole

650

nM

400 p M

20

nM

20 pM

8.5 pM

+ CuCl AgCl =

3 AuCl2 ++ Zn + CdCl

14.5

pmole

20.2

mmole

1.1 m M

75 nM

89

pmole

1.8 pM

i nM

HgCI3

41

pmole

400

nM

4O

pM

20 pM

5O

pM

6O pM

200 pM

191

from, the role of zinc. A r e a s o n a b l e p o s t u l a t e is that in the (Hadean) p r i m o r d i a l soup, all the a v a i l a b l e d i v a l e n t c a t i o n s formed c r o s s - l i n k i n g c o m p l e x e s of v a r y i n g stabilities, and that d u r i n g a few h u n d r e d m i l l i o n years an e v o l u t i o n a r y s e l e c t i o n o c c u r r e d for the v a r i o u s roles w h i c h they play today (see Zn below). C o b a l t p r o b a b l y was among this group, w i t h the o r i g i n of V i t a m i n B12 a later d e v e l o p m e n t , after the s y n t h e s i s of the m o r e r e g u l a r t e t r a p y r r o l e s (contrary to the v i e w of M a r g u l i s , 1970), due to the special r e q u i r e m e n t for a smaller c e n t r a l cavity to fit the cobalt ion. The r e q u i r e m e n t for cobalt, however early it m u s t have appeared, is not an o b l i g a t o r y one, however, since higher plants (and some algae) get along v e r y well w i t h o u t it (negative H-II). In the case of legumes, a req u i r e m e n t for cobalt is p r e s e n t w h e n the plants are fixing n i t r o g e n via their s y m b i o t i c bacteria, but not w h e n s u p p l i e d w i t h nitrate. Iron thus seems to s a t i s f y H-I; copper, H-I or H-II; m a n g a n e s e , zinc and cobalt, H-III or H-II. The role of m o l y b d e n u m is not clear on a h i s t o r i c a l basis. Its c r u s t a l r a r i t y and the c o m p l e x i t y of the systems (nitrate reductase, x a n t h i n e oxidase, n i t r o g e n a s e , etc.) in w h i c h m o l y b d e n u m is found may imply that it is a late a d d i t i o n to the b i o c h e m i c a l a r m a m e n t a r i u m . The v e r t e b r a t e r e q u i r e m e n t for x a n t h i n e o x i d a s e m u s t c e r t a i n l y q u a l i f y for H-II. M a r g u l i s (1970) regards n i t r o g e n a s e as a p r i m i t i v e r e q u i r e m e n t , but the r e d u c t i o n of n i t r a t e is easier to a c c o m p l i s h b i o c h e m i c a l l y and the Hadean o r i g i n r e q u i r e d the p r e s e n c e of NH 3. In some organisms, the m o l y b d e n u m r e q u i r e m e n t seems to v a n i s h w h e n n i t r o g e n is s u p p l i e d as ammonium. It seems r e a s o n a b l e to assign Mo to H-II (or H-IV, see below), but the d e v e l o p m e n t of the r e q u i r e m e n t could have taken place a l r e a d y by the end of the Hadean. C h r o m i u m has several o x i d a t i o n states, but c h r o m a t e and d i c h r o m a t e may p o s s i b l y be too strong o x i d a n t s for c o m m o n biochemical utility. The r e q u i r e m e n t of animals for c h r o m i u m app a r e n t l y falls into H-II. V a n a d i u m has no lack of c a p a b i l i t y in e l e c t r o n t r a n s p o r t roles and stable aqueous v a l e n c e states. It seems to be a closer n u t r i t i o n a l r e l a t i v e of m o l y b d e n u m than is c h r o m i u m and v a n a d i u m m i g h t have b e e n e x p e c t e d to fill some e l e c t r o n t r a n s p o r t function. The a b u n d a n c e of soluble m o l y b d e n u m is the greater, however, w h i c h may lend c r e d e n c e to a H-IV p r o p o s a l for d i s t i n g u i s h i n g b e t w e e n m o l y b d e n u m , vanadium, a n d also tungsten. In some bacteria, v a n a d i u m has a sparing effect on the m o l y b d e n u m r e q u i r e m e n t , but it only r a r e l y has been d e m o n s t r a t e d to be a r e q u i r e m e n t , in a d d i t i o n to Mo. The u t i l i z a t i o n of v a n a d i u m in t u n i c a t e s m u s t be considered a d e r i v e d function (H-II or IV). The c o m p l e x r e l a t i o n s b e t w e e n v a n a d i u m and m o l y b d e n u m and the e x i s t e n c e of v a n a d y l p o r p h y r i n s p e r h a p s m a k e it p o s s i b l e that H a d e a n o r g a n i s m s used V, Mo, Fe, and Cu somewhat i n t e r c h a n g a b l y in some roles, subsequent e v o l u t i o n very early e l i m i n a t i n g V from use in fundam e n t a l b i o c h e m i s t r y (H-III). 192

++ The o r i g i n of the zinc r e q u i r e m e n t m u s t r e l a t e to Zn , but at least some e n z y m e s are r e a c t i v a t e d by o t h e r d i v a l e n t m e t a l s w h e n the zinc is r e m o v e d in vitro. The d i v a l e n t ions w h i c h w i l l s u b s t i t u t e for zinc in one or a n o t h e r zinc e n z y m e or z i n c - a c t i v a t e d e n z y m e are (O'Dell & C a m p b e l l , 1971): Co, Ni, Fe, Cd, Hg, Pb, Mn, Sn, Pd, Mg, Ba, and Cu, the first t h r e e r e a c t i v a t i n g b o t h p e p t i d a s e and e s t e r a s e a c t i v i t i e s of c a r b o x y p e p t i d a s e A. The m e c h a n i s m of a c t i o n of zinc seems to be one of c o m p l e x f o r m a t i o n to b i n d s u b s t r a t e to enzyme, e n z y m e s u b u n i t s t o g e t h e r , or b i n d i n g c o e n z y m e (NAD) to enzyme. If h a v i n g c o m p l e t e d d s h e l l s is i m p o r t a n t to this function, zinc's c o n g e n e r s , c a d m i u m and m e r c u r y , w e r e e v i d e n t l y u n f a v o r e d due to t h e i r r e l a t i v e scarcity. M e r c u r y is c e r t a i n l y s u b j e c t to m e t a b o l i c attack, as r e c e n t o b s e r v a t i o n s of m e t h y l m e r c u r y in the e n v i r o n m e n t h a v e shown. I p r o p o s e H-II for zinc at least v i s - a - v i s Cd and Hg. Its c o n c e n t r a t i o n in sea w a t e r is above that of any d i v a l e n t t r a n s i t i o n m e t a l e x c e p t iron, and a b o v e those in the c r u s t e x c e p t Mn, Fe, and Ni. H-III also seems a p p r o p r i a t e for the zinc r e q u i r e m e n t as a whole, w i t h a f u n d a m e n t a l l y n o n - s p e c i f i c r e q u i r e m e n t for a d i v a l e n t , s t r o n g l y c o o r d i n a t e d b i n d i n g function in b i o c h e m i s t r y . A s i m i l a r a r g u m e n t m a y p o s s i b l e a p p l y to the b i o c h e m i c a l role of M n in such e n z y m e s as p y r u v a t e c a r b o x y l a s e , but the f u n d a m e n t a l p r o p e r t i e s w h i c h lead to a d i s t i n c t i o n b e t w e e n Zn and Mn are not clear. N i c k e l is also a v a i l a b l e as the d i v a l e n t ion, a b o u t as abund a n t as V, Cr, and Zn in the crust and above that of c o b a l t in the p l a n t e x a m p l e c i t e d in the table and in the ocean. A l t h o u g h an e x t e n s i v e b o t a n i c a l l i t e r a t u r e has a r i s e n (Mishra & Kar, 1974), m o s t d e a l s w i t h its t o x i c i t y , and it is not c o n s i d e r e d e s s e n t i a l for p l a n t life in general. It is p o s s i b l e that this is due to a f a i l u r e in the p u r i f i c a t i o n of diets, since it has b e e n r e c e n t l y r e p o r t e d that the p l a n t e n z y m e u r e a s e c o n t a i n s f i r m l y b o u n d n i c k e l (Dixon et al., 1976). D i x o n et al. (1976) s u g g e s t the e x i s t e n c e of o t h e r u n r e c o g n i z e d t r a n s i t i o n m e t a l enzymes, b u t their p r o p o s e d m o d e of a c t i o n does not p e r m i t one to d i s t i n g u i s h b e t w e e n d i f f e r e n t d i v a l e n t m e t a l s on a m e c h a n istic basis. S t u d y of u r e a s e from p l a n t s g r o w n on a low n i c k e l d i e t m i g h t show r e p l a c e m e n t by a n o t h e r metal, in w h i c h case H-III or H - I V w o u l d be e n t i r e l y justified. T h o s e p l a n t s and a n i m a l s for w h i c h Ni is d e m o n s t r a b l y e s s e n t i a l are p r o b a b l y to be c o n s i d e r e d u n d e r H-II.

CONCLUSION E x a m i n a t i o n of the w h o l e p e r i o d i c table r e v e a l s that b o t h l i m i t i n g cases fail: that the only f a c t o r in the o r i g i n of e l e m e n t a l n u t r i e n t r e q u i r e m e n t s has b e e n the a b u n d a n c e of the e l e m e n t s , or that the a b u n d a n c e of an e l e m e n t had n o effect on 193

the o r i g i n of t h o s e r e q u i r e m e n t s . For example, the fact t h a t i n g r o u p s III and IV, w i t h the e x c e p t i o n of carbon, t h e r e are no u n i v e r s a l l y r e q u i r e d elements, in spite of a p p a r e n t l y ade q u a t e a b u n d a n c e , can be t a k e n as e v i d e n c e that the c h e m i c a l p r o p e r t i e s of the e l e m e n t s are an at least e q u a l p a r t n e r in d e t e r m i n i n g the o r i g i n of r e q u i r e m e n t s . F u r t h e r m o r e , the conc e n t r a t i o n s of the r e q u i r e d h e a v y m e t a l s in a p l a n t in g o o d n u t r i t i o n a l status are not c o r r e l a t e d w i t h o c e a n i c c o n c e n trations, while being positively correlated with crustal abundances. On the o t h e r hand, t h e r e a p p e a r s to be an a b s o l u t e limit in a b u n d a n c e b e l o w w h i c h no e l e m e n t m a y fall and still be a v a i l a b l e in a d e q u a t e a m o u n t s e v e n as a m i c r o n u t r i e n t (10-20 z m o l e s / k g c r u s t or I-2 nM in the ocean). A l s o a p p a r e n t l y e s t a b l i s h e d is the fact t h a t some e l e m e n t s are a v a i l a b l e in a d e q u a t e a m o u n t s and h a v e s u i t a b l e c h e m i c a l p r o p e r t i e s , b u t w e r e r e d u n d a n t and e l i m i n a t e d from g e n e r a l use by the p r e s e n c e of a m o r e a b u n d a n t r e l a t i v e . A c l o s e r e x a m i n a t i o n w i l l be req u i r e d to e s t a b l i s h the m e c h a n i s t i c b a s i s of the o r i g i n s of the r e q u i r e m e n t s in a n u m b e r of cases such as K v e r s u s Na, M n v e r s u s Zn, or Zn v e r s u s Ni. On c l o s e r e x a m i n a t i o n of the r e q u i r e m e n t s and a b u n d a n c e s , a m o d i f i c a t i o n of the a b u n d a n c e h y p o t h e s e s m a y p r o v e n e c e s s a r y . F o r t h o s e e l e m e n t s w h i c h are on the b o r d e r l i n e of b e i n g una v a i l a b l e , the c o m p e t i t i v e e x c l u s i o n p r i n c i p l e m a y apply. For example, if zinc, n i c k e l and m a n g a n e s e are, in p r i n c i p l e , i n t e r c h a n g e a b l e b u t rare, e v o l u t i o n a r y a d a p t a t i o n m a y p r o v i d e a d i v i s i o n of labor b e t w e e n them, p r o v i d i n g the e n z y m i c s p e c i f i c i t y r e q u i r e d but u t i l i z i n g t h e m all.

REFERENCE S Bandurski, R.S. (1965). Biological reduction of sulfate and nitrate. In: Plant biochemistry, J. Bonnet, J.E. Varner, eds., p. 467. New York: Academic Press Banin, A., Navrot, J. (1975). Science 189, 550 Bertrand, D. (1950). Bull.Am.Museum Nat.Hist. 94, 403 Bollard, E.G., Butler, G.W. (1966). Ann.Rev.Plant Physiol. 17, 77 Chappell, W.R., Meglen, R.R., Runnells, D.D. (1974). Icarus. 21, 513 Cloud, P. (1974). Am.Scientist. 62, 54 Crick, F.H.C., Orgel, L.E. (1973). Icarus. 19, 341 Dixon, N.E., Gazzola, C., Blakeley, R.L., Zerner, B. (1976). Science 191, 1144 Egami, F. (1974). J.Mol.Evol. 4, 113 Epstein, E. (1965). Mineral metabolism. In: Plant biochemistry, J. Bonnet, J.E. Varner, eds., p. 438. New York: Academic Press Epstein, E. (1972). Mineral nutrition of plants. New York: Wiley Frieden, E. (1972). Sci.Am. 229(7), 52 Hammond, A.L. (1975). Science 189, 868 194

Henderson, L.J. Hutchinson, G.E. Jukes, T.H.

(1943). Quart.Rev.Biol.

18, i

(1974). Icarus. 21, 516

Krauskopf, K.B. MacIntyre; F. Mason, B.

(1913). The fitness of the environment. New York: Macmillan

(1956). Geochim. Cosmochim. 9, 1

(1970). Sci.Am. 227(11), 104

(1966). Principles of geochemistry. New York: John Wiley

Mishra, D., Kar, M.

(1974). Bot.Rev. 40, 395

O'Dell, B.L., Campbell, B.J.

(1971). Trace elements. In: Comprehensive

biochemistry, M. Florkin, E.H. Stotz, eds., pp. 21, 179. Amsterdam: Elsevier Orgel, L.E.

(1974). Icarus. 21, 518

Pirie, N.W.

(1960). Chemical diversity and the origin of life. In: Aspects

of the origin of life, M. Florkin, ed. New York: Pergamon Press Riley, J.P., Chester, R.

(1971). Introduction to marine chemistry. New

York: Academic Press R~sler, H.J., Lange, H.

(1972). Geochemical tables. Amsterdam: Elsevier

Ruhland, W., ed. (1958). Encycl. plant physiology, Vol. IV. Berlin, G6ttingen, Heidelberg: Springer Sanderson, R.T.

(1960). Chemical periodicity. New York: Reinhold

Steward, F.C., ed. Press Wald, G.

(1963). Plant physiology, Vol. III. New York: Academic

(1962). Life in the second and third periods. In: Horizons in

biochemistry, M. Kasha, B. Pullman, eds., p. 127. New York: Academic Press

Note A d d e d in Proof. Broda

(1975) presented a strong argument for the nonexistence of nitrate prior to the origin of free atmospheric oxygen. The

use of molybdenum in nitrate reductase cannot, therefore, have arisen before the origin of blue-green algal photosynthesis and was most likely preceded by its use in nitrogenase, in anaerobes and photosynthetic bacteria. REFERENCE Broda, E.

(1975). J.Mol.Evol. 7, 87

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