EVOLUTION
OF O X Y G E N
BY P L A N T S
BIOSPHERE
IN R E L A T I O N
TO
EVOLUTION
V. M. K U T Y U R I N
V. I. Vernadsky Geochemistry Institute, Academy of Science, Moscow, U.S,S.R. Abstract. The process of water decomposition by plants is discussed in connection with biosphere evolution. This process consists of two parts: water oxidation and oxygen evolution. The origin of the water oxidation process took place after the synthesis of chlorine-type pigments, the structure of which corresponds to a more oxidized state than the bacteriochlorophyll type. The ability of plants to evolve oxygen is the result of a long evolution process. The capability of decomposing water in the long-wave length spectral region by algae and higher plants, which can be only seen under anaerobic conditions was discovered. This mechanism was suggested to be a reflection of a relict form of plant apparatus having operated under ancient, strictly anaerobic, conditions.
The appearance of blue-green algae on the Earth was one of the most important events of its history. At this time (approximately three billion years ago) the evolution of photosynthetic plants with the capability to decompose water and evolve oxygen took place, The oxygen production by plants caused the development of the ozone screen and of aerobic metabolism. The accumulation of oxygen in the atmosphere greatly changed the character of geochemical processes on the earth's surface. The data of Table I show the amount of oxygen being evolved by plants per year in comparison with the amount in several natural sources. TABLE I The content of oxygen in several natural sources and oxygen production by plants (per year) Source Atmosphere Hydrosphere
Sedimentary rocks Photosynthesis
Oxygen content (tons) 1 x 1015 Oxygen dissolved in: fresh water sea water
2 x 1 x 1.5 x ! x
l014 10 is 1015 101~
If we assume the same order of oxygen production by plants during the last few million years, we come to the conclusion that all the hydrosphere water undergoes decomposition through photosynthesis during a period of about ten million years. To connect the origin of water decomposition by plants and its evolution with certain steps of biosphere evolution we must know the mechanism of water decomposition and oxygen evolution. Unfortunately this mechanism belongs to the least developed problem of photosynthesis. Only in 194I was it shown that photosynthetic oxygen originates from water and not from carbon dioxide (Vinogradov and Teiss, 1941; Ruben et al., 1941). Systematic investigation of the mechanism of water decomposition and oxygen evolution began only ten years ago. A scheme describing this process is shown in Figure 1. It consists of two parts: the first part represents Origins of Life 6 (1975) 257-263. All Rights Reserved Copyright 9 1975 by D. Reidel Publishing Company, Dordrecht-Holland
258
V.M. KUTYURIN
T
r_t
I
I
: m
31
.-_
_J
I'- - - I
I
',0.
k_3
Fig. 1. Scheme for the water decomposition process by plants. I P.S. - the first photochemical system ot chloroplasts. I - water oxidation mechanism; II - oxygen formation mechanism; 1 - pool of chlorophyll molecules transferring light energy to photochemical centre, whose m e c h a n i s m is shown in Figure 2a; 2 - structure-bonding water as shown in Figure 2b; 3 - oxidation-reduction system containing metal ion Me, perhaps manganese, and conjugated to the oxidized form of the chlorophyll as shown in Figure 2c; 4 - u n k n o w n enzyme system E evolving oxygen : system 3 m a y be enclosed in a system 4,
water oxidation, the second one shows oxygen formation. As was shown by Kutyurin et al. (1969), water oxidation occurs under special conditions without oxygen evolution. The latter process is more sensitive to the action of detergents than the former, showing its more intimate dependence on chloroplast structure. Water oxidation may be supposed to be the first stage of the water decomposition process. It only began after the synthesis of chlorine-like pigment such as chlorophylls a and b. Bacterial pigments are not able to oxidize the water molecule, as shown in Table II. Its oxidized forms do not have a sufficiently high potential (E0 ~> + 0.8 v). (Kutyurin et al., 1968a). It is interesting to note that this high potential form of oxidized chlorophyll T A B L E II Oxidation-reduction potentials of chlorophylls and related substances Substance
E01 a
Eoll
Solvent
Chlorophyll a Chlorophyll a Chlorophyll b Mn-protoporphyrin Chlorobium Chlorophyll Chlorobium Chlorophyll Bacteriochlorophyll
0.77 0.79 0.80 0.85 0.62 0.62 0.62
0.90 0.93 0.93 0.74 0.72 b
Acetone Acetonitrile Acetone Phosphate buffer p H 7.5 Acetone Acetonitrile Acetone
a E o ! corresponds
to the normal oxidation-reduction potential of the system chl+/chl and Eo ~ to the system chl + +/chl § at p H 7.0. b The transfer of the second electron was not completely reversible under experimental conditions.
259
EVOLUTION OF OXYGEN BY PLANTS
forms occurs in lipophilic media or through complex formation with basic aminoacids. This indicates that the formation of a water oxidation mechanism by plants was the result of special binding of chlorophyll with lipoprotein in photosynthetic lamellae. Since the chlorine-type pigments have higher oxidation potentials than the bacterial one, it is clear either that its formation might be a later step of the biosphere evolution than the appearance of photosynthetic bacteria or that there were some regions on Earth with sharply different oxidation-reduction conditions. It should be emphasized that there are only two possible mechanisms of oxygen formation from the phenomenological point of view" --4e
@
--2e
4H20
,4"OH+4H +
4 "OH
,2 H z O + O /
@
)2 "OH+2 H +
2H20 --2e
2 "OH
,2 " O ' + 2 H +
2 "O" )O2 So the oxygen molecule formation is either the result of dismutation of 4 hydroxyl radicals or the twofold oxidation of an oxygen atom firstly in the form of a water molecule and secondly in the form of the hydroxyl radical (or hydroxyl group connected with a metal ion). The hydroxyl radical is a substance of extremely high reactivity; it exists in a free state for only 10-1~ -9 s. Therefore plants must have a mechanism for collecting and transforming of hydroxyl radicals. We have supposed that this mechanism is connected with the mode of water oxidation which,
*
Chl h -LChl
I
H
~L§
-Chl + +
I
H
I
H
Fig. 2a. A scheme proposed to show the possibility of water oxidation by the oxidized form of chlorophyll. The upper line shows the completion of the conjugated chlorophyll system on the carbocyclic ring when the pigment is oxidized. Chl - chlorophyll, Q - acceptor of electron.
in its turn, is able to separate the reduced form (basic state of chlorophyll) from an oxidized substance (hydroxyl radical) as is shown in Figure 2a. This first step of organization is based on the property of the chlorophyll molecule to attach the water molecule to its carbocyclic ring (Kutyurin, 1972), a hypothesis that was experimentally confirmed in our laboratory. The second step - a conservation and collection of hydroxyl radicals would be realized by a connection with a metal ion in the manner shown in Figure 2b. This
260
v.M. KUTYURIN
\N - . t
'~N --
9
o...,-'-'9 .,
-;,,' o.(H,
z
u,~,
~z
i
Fig. 2b. Conjugate oxidation of a metal ion linked with the water chain as a result of the primary oxidation of a water molecule by chlorophyll. mechanism explains the simultaneous spatial partition and cooperation of oxidative action and the result of it. According to experimental results obtained by Kessler (1960) the manganese ion must play an important role in the overall processes leading to oxygen evolution. T h a t is, the manganese ion may be a participant of the mechanism shown above. We did not succeed in finding the binding form of manganese - it occurred neither in porphyrins nor in structural proteins. Manganese may be supposed to be an essential structural factor itself in addition to its participation in the chemical reaction as suggested in Figure 2c (Kutyurin, 1970). This hypothesis is based on experimental results obtained in Gaffron's laboratory ( H o m a n n et al., 1968) which have shown the necessity of two lamellae layers touching, or of a spatial arrangement between them in order to evolve photosynthetic oxygen. If we assume oxygen evolution to be the result of twofold oxidation of each oxygen atom originating from water we shall expect the participation of two different chlorophyll forms in the mechanism of water decomposition. To investigate this +
2%o
0,]
Me--o H-,-2 ,+
2 M@~OHc-~2 Me"*=--OH--'2H++2Me"+Oa i
DI
M e n*= It
O*-H
H--*O ,,
02+2H+
li
M e ..2 ..... d . b
........
Fig. 2c. Reaction scheme of two-fold oxidation of oxygen atom originating from water and random cooperation of two lipoprotein lamellae with metal ions.
261
E V O L U T I O N OF O X Y G E N BY P L A N T S
problem we have paid special attention to the oxygen evolution by plants in the long-wave spectral region (2>~700 nm). As was shown by Kutyurin et al. (1968b) under anaerobic conditions there is a special mechanism of water decomposition in the long-wavelength spectral region in algae which is masked by oxygen uptake under the usual aerobic conditions, as shown in Figures 3a, b. Recently we have confirmed this ability in higher plants (Kutyurin et al., 1973). The sensitivity of this mechanism to a special poison monuron, is greater than that in the short-wavelength spectral region, as is shown in Figure 3c. In searching for the second photochemical
[0,] He g ....
0
.~,.
5
I0
.
JO
gO
50
60 "t, rrli.R'w
Figs. 3a-c. Some characteristic features of oxygen evolution in the long-wave spectral region (,~,i> 700 nm). (a) Decrease of apparent rate of oxygen evolution as the oxygen concentration increases in the nutrient solution 1"switch on, ~ switch off the light, $ He - removal of oxygen by inlet of helium.
"1,0
0.8 0,6
0,~ 0,2 L
850 Fig. 3b.
880
700 7'20 7k.O 760
a ,ma
Relative quantum efficiency of oxygen evolution under aerobic (1) and (2) anaerobic conditions.
262
v.M. KUTYURIN
[0,] ~,
Url,
MU
6 4 g mmmmmD
0
Fig. 3c.
'
t
Inhibition of oxygen evolution by monuron: - O - O - O - / ~ - / ~ - A - light of 730 nm.
CMU •
rain
monochromatic light of 680 nm;
centre in the proposed mechanism (Figure 1) we have found another mechanism of water decomposition. Its specific peculiarity is the sensitivity to oxygen concentration in the medium. It is to some extent similar in its property to blue-green algae and this makes it possible to suggest that this mechanism represents a relict form of a photosynthetic apparatus which operated under ancient, strictly anaerobic, conditions. Space does not permit us to go into detail about the possible correlation between the chloroplast structure of plants belonging to different systematic groups and variability of its oxygen metabolism, as there are few unambiguous facts to be discussed. It may only be said that the mechanism of water decomposition and oxygen evolution is the result of a long evolutionary process. The capacity to oxidize water seems to be the result of the suitable arrangement of chlorophyll on the lipoprotein molecule. As to the oxygen formation it should be emphasized that this process has undergone many alterations which are reflected in the difference between long- and short-wavelength water decomposition by plants as shown above. The first outlines of this mechanism have become apparent. The next step in the elucidation of this process gives useful information for the problem of biosphere evolution and the origin of life, the investigation of which was greatly advanced thanks to the efforts of the prominent heroes of our Assembly.
EVOLUTIONOF OXYGENBY PLANTS
References Homann, P. H. et al. : 1968, Comp. Biochem. and Biophys. of Photosynthesis, Tokyo, p. 50. Kessler, E. : 1960, Handb. Pflanzenphysiol. 5, 923. Kutyurin, V. M. : 1970, Izvest. Akad. Nauk S S S R , ser. biol., N 4, 569. Kutyurin, V. M. : 1972, Proceed. lind Internat. Congr. on Photosynth. Research, I, The Hague, p. 93. Kutyurin, V. M. et al.: 1968a, Dokl. Aead. Nauk S S S R 171, 215. Kutyurin, V. M. et al.: 1968b, Dokt. Aead. Nauk S S S R 181, 1270. Kutyurin, V. M. et al.: 1969, Fiziol. Rast. 16, 181. Kutyurin, V. M. et al. : 1973, Fiziol. Rast. in press. Ruben, S. et al. : 1941, J. Amer. Chem. Soe. 63, 877. Vinogradov, A. P. and Teiss, R. V.: 1941, Dokl. Akad. Nauk S S S R 33, 497.
263
OF O X Y G E N
BY P L A N T S
BIOSPHERE
IN R E L A T I O N
TO
EVOLUTION
V. M. K U T Y U R I N
V. I. Vernadsky Geochemistry Institute, Academy of Science, Moscow, U.S,S.R. Abstract. The process of water decomposition by plants is discussed in connection with biosphere evolution. This process consists of two parts: water oxidation and oxygen evolution. The origin of the water oxidation process took place after the synthesis of chlorine-type pigments, the structure of which corresponds to a more oxidized state than the bacteriochlorophyll type. The ability of plants to evolve oxygen is the result of a long evolution process. The capability of decomposing water in the long-wave length spectral region by algae and higher plants, which can be only seen under anaerobic conditions was discovered. This mechanism was suggested to be a reflection of a relict form of plant apparatus having operated under ancient, strictly anaerobic, conditions.
The appearance of blue-green algae on the Earth was one of the most important events of its history. At this time (approximately three billion years ago) the evolution of photosynthetic plants with the capability to decompose water and evolve oxygen took place, The oxygen production by plants caused the development of the ozone screen and of aerobic metabolism. The accumulation of oxygen in the atmosphere greatly changed the character of geochemical processes on the earth's surface. The data of Table I show the amount of oxygen being evolved by plants per year in comparison with the amount in several natural sources. TABLE I The content of oxygen in several natural sources and oxygen production by plants (per year) Source Atmosphere Hydrosphere
Sedimentary rocks Photosynthesis
Oxygen content (tons) 1 x 1015 Oxygen dissolved in: fresh water sea water
2 x 1 x 1.5 x ! x
l014 10 is 1015 101~
If we assume the same order of oxygen production by plants during the last few million years, we come to the conclusion that all the hydrosphere water undergoes decomposition through photosynthesis during a period of about ten million years. To connect the origin of water decomposition by plants and its evolution with certain steps of biosphere evolution we must know the mechanism of water decomposition and oxygen evolution. Unfortunately this mechanism belongs to the least developed problem of photosynthesis. Only in 194I was it shown that photosynthetic oxygen originates from water and not from carbon dioxide (Vinogradov and Teiss, 1941; Ruben et al., 1941). Systematic investigation of the mechanism of water decomposition and oxygen evolution began only ten years ago. A scheme describing this process is shown in Figure 1. It consists of two parts: the first part represents Origins of Life 6 (1975) 257-263. All Rights Reserved Copyright 9 1975 by D. Reidel Publishing Company, Dordrecht-Holland
258
V.M. KUTYURIN
T
r_t
I
I
: m
31
.-_
_J
I'- - - I
I
',0.
k_3
Fig. 1. Scheme for the water decomposition process by plants. I P.S. - the first photochemical system ot chloroplasts. I - water oxidation mechanism; II - oxygen formation mechanism; 1 - pool of chlorophyll molecules transferring light energy to photochemical centre, whose m e c h a n i s m is shown in Figure 2a; 2 - structure-bonding water as shown in Figure 2b; 3 - oxidation-reduction system containing metal ion Me, perhaps manganese, and conjugated to the oxidized form of the chlorophyll as shown in Figure 2c; 4 - u n k n o w n enzyme system E evolving oxygen : system 3 m a y be enclosed in a system 4,
water oxidation, the second one shows oxygen formation. As was shown by Kutyurin et al. (1969), water oxidation occurs under special conditions without oxygen evolution. The latter process is more sensitive to the action of detergents than the former, showing its more intimate dependence on chloroplast structure. Water oxidation may be supposed to be the first stage of the water decomposition process. It only began after the synthesis of chlorine-like pigment such as chlorophylls a and b. Bacterial pigments are not able to oxidize the water molecule, as shown in Table II. Its oxidized forms do not have a sufficiently high potential (E0 ~> + 0.8 v). (Kutyurin et al., 1968a). It is interesting to note that this high potential form of oxidized chlorophyll T A B L E II Oxidation-reduction potentials of chlorophylls and related substances Substance
E01 a
Eoll
Solvent
Chlorophyll a Chlorophyll a Chlorophyll b Mn-protoporphyrin Chlorobium Chlorophyll Chlorobium Chlorophyll Bacteriochlorophyll
0.77 0.79 0.80 0.85 0.62 0.62 0.62
0.90 0.93 0.93 0.74 0.72 b
Acetone Acetonitrile Acetone Phosphate buffer p H 7.5 Acetone Acetonitrile Acetone
a E o ! corresponds
to the normal oxidation-reduction potential of the system chl+/chl and Eo ~ to the system chl + +/chl § at p H 7.0. b The transfer of the second electron was not completely reversible under experimental conditions.
259
EVOLUTION OF OXYGEN BY PLANTS
forms occurs in lipophilic media or through complex formation with basic aminoacids. This indicates that the formation of a water oxidation mechanism by plants was the result of special binding of chlorophyll with lipoprotein in photosynthetic lamellae. Since the chlorine-type pigments have higher oxidation potentials than the bacterial one, it is clear either that its formation might be a later step of the biosphere evolution than the appearance of photosynthetic bacteria or that there were some regions on Earth with sharply different oxidation-reduction conditions. It should be emphasized that there are only two possible mechanisms of oxygen formation from the phenomenological point of view" --4e
@
--2e
4H20
,4"OH+4H +
4 "OH
,2 H z O + O /
@
)2 "OH+2 H +
2H20 --2e
2 "OH
,2 " O ' + 2 H +
2 "O" )O2 So the oxygen molecule formation is either the result of dismutation of 4 hydroxyl radicals or the twofold oxidation of an oxygen atom firstly in the form of a water molecule and secondly in the form of the hydroxyl radical (or hydroxyl group connected with a metal ion). The hydroxyl radical is a substance of extremely high reactivity; it exists in a free state for only 10-1~ -9 s. Therefore plants must have a mechanism for collecting and transforming of hydroxyl radicals. We have supposed that this mechanism is connected with the mode of water oxidation which,
*
Chl h -LChl
I
H
~L§
-Chl + +
I
H
I
H
Fig. 2a. A scheme proposed to show the possibility of water oxidation by the oxidized form of chlorophyll. The upper line shows the completion of the conjugated chlorophyll system on the carbocyclic ring when the pigment is oxidized. Chl - chlorophyll, Q - acceptor of electron.
in its turn, is able to separate the reduced form (basic state of chlorophyll) from an oxidized substance (hydroxyl radical) as is shown in Figure 2a. This first step of organization is based on the property of the chlorophyll molecule to attach the water molecule to its carbocyclic ring (Kutyurin, 1972), a hypothesis that was experimentally confirmed in our laboratory. The second step - a conservation and collection of hydroxyl radicals would be realized by a connection with a metal ion in the manner shown in Figure 2b. This
260
v.M. KUTYURIN
\N - . t
'~N --
9
o...,-'-'9 .,
-;,,' o.(H,
z
u,~,
~z
i
Fig. 2b. Conjugate oxidation of a metal ion linked with the water chain as a result of the primary oxidation of a water molecule by chlorophyll. mechanism explains the simultaneous spatial partition and cooperation of oxidative action and the result of it. According to experimental results obtained by Kessler (1960) the manganese ion must play an important role in the overall processes leading to oxygen evolution. T h a t is, the manganese ion may be a participant of the mechanism shown above. We did not succeed in finding the binding form of manganese - it occurred neither in porphyrins nor in structural proteins. Manganese may be supposed to be an essential structural factor itself in addition to its participation in the chemical reaction as suggested in Figure 2c (Kutyurin, 1970). This hypothesis is based on experimental results obtained in Gaffron's laboratory ( H o m a n n et al., 1968) which have shown the necessity of two lamellae layers touching, or of a spatial arrangement between them in order to evolve photosynthetic oxygen. If we assume oxygen evolution to be the result of twofold oxidation of each oxygen atom originating from water we shall expect the participation of two different chlorophyll forms in the mechanism of water decomposition. To investigate this +
2%o
0,]
Me--o H-,-2 ,+
2 M@~OHc-~2 Me"*=--OH--'2H++2Me"+Oa i
DI
M e n*= It
O*-H
H--*O ,,
02+2H+
li
M e ..2 ..... d . b
........
Fig. 2c. Reaction scheme of two-fold oxidation of oxygen atom originating from water and random cooperation of two lipoprotein lamellae with metal ions.
261
E V O L U T I O N OF O X Y G E N BY P L A N T S
problem we have paid special attention to the oxygen evolution by plants in the long-wave spectral region (2>~700 nm). As was shown by Kutyurin et al. (1968b) under anaerobic conditions there is a special mechanism of water decomposition in the long-wavelength spectral region in algae which is masked by oxygen uptake under the usual aerobic conditions, as shown in Figures 3a, b. Recently we have confirmed this ability in higher plants (Kutyurin et al., 1973). The sensitivity of this mechanism to a special poison monuron, is greater than that in the short-wavelength spectral region, as is shown in Figure 3c. In searching for the second photochemical
[0,] He g ....
0
.~,.
5
I0
.
JO
gO
50
60 "t, rrli.R'w
Figs. 3a-c. Some characteristic features of oxygen evolution in the long-wave spectral region (,~,i> 700 nm). (a) Decrease of apparent rate of oxygen evolution as the oxygen concentration increases in the nutrient solution 1"switch on, ~ switch off the light, $ He - removal of oxygen by inlet of helium.
"1,0
0.8 0,6
0,~ 0,2 L
850 Fig. 3b.
880
700 7'20 7k.O 760
a ,ma
Relative quantum efficiency of oxygen evolution under aerobic (1) and (2) anaerobic conditions.
262
v.M. KUTYURIN
[0,] ~,
Url,
MU
6 4 g mmmmmD
0
Fig. 3c.
'
t
Inhibition of oxygen evolution by monuron: - O - O - O - / ~ - / ~ - A - light of 730 nm.
CMU •
rain
monochromatic light of 680 nm;
centre in the proposed mechanism (Figure 1) we have found another mechanism of water decomposition. Its specific peculiarity is the sensitivity to oxygen concentration in the medium. It is to some extent similar in its property to blue-green algae and this makes it possible to suggest that this mechanism represents a relict form of a photosynthetic apparatus which operated under ancient, strictly anaerobic, conditions. Space does not permit us to go into detail about the possible correlation between the chloroplast structure of plants belonging to different systematic groups and variability of its oxygen metabolism, as there are few unambiguous facts to be discussed. It may only be said that the mechanism of water decomposition and oxygen evolution is the result of a long evolutionary process. The capacity to oxidize water seems to be the result of the suitable arrangement of chlorophyll on the lipoprotein molecule. As to the oxygen formation it should be emphasized that this process has undergone many alterations which are reflected in the difference between long- and short-wavelength water decomposition by plants as shown above. The first outlines of this mechanism have become apparent. The next step in the elucidation of this process gives useful information for the problem of biosphere evolution and the origin of life, the investigation of which was greatly advanced thanks to the efforts of the prominent heroes of our Assembly.
EVOLUTIONOF OXYGENBY PLANTS
References Homann, P. H. et al. : 1968, Comp. Biochem. and Biophys. of Photosynthesis, Tokyo, p. 50. Kessler, E. : 1960, Handb. Pflanzenphysiol. 5, 923. Kutyurin, V. M. : 1970, Izvest. Akad. Nauk S S S R , ser. biol., N 4, 569. Kutyurin, V. M. : 1972, Proceed. lind Internat. Congr. on Photosynth. Research, I, The Hague, p. 93. Kutyurin, V. M. et al.: 1968a, Dokl. Aead. Nauk S S S R 171, 215. Kutyurin, V. M. et al.: 1968b, Dokt. Aead. Nauk S S S R 181, 1270. Kutyurin, V. M. et al.: 1969, Fiziol. Rast. 16, 181. Kutyurin, V. M. et al. : 1973, Fiziol. Rast. in press. Ruben, S. et al. : 1941, J. Amer. Chem. Soe. 63, 877. Vinogradov, A. P. and Teiss, R. V.: 1941, Dokl. Akad. Nauk S S S R 33, 497.
263
