Protective Action of Selenium Against Mercury in Northern Creek Chubs Jong Hwan Kim, Eric Birks and J. F. Heisinger Biology Department University of South Dakota Vermillion, S.D. 57069
L a r g e q u a n t i t i e s of s e l e n i u m as well as m e r c u r y were found in fishes, shell fishes, cats and humans in the M i n a m a t a Bay area of J a p a n in 1960 (IWATA et al. 1973). G A N T H E R et al. (1972) found that l o w - H g tuna c o n t a i n e d less s e l e n i u m t h a n h i g h - H g tuna. D e s p i t e h i g h - H g c o n c e n t r a t i o n s , tuna added to m e t h y l m e r c u r y t r e a t e d foods of J a p a n e s e quail and rats p r o t e c t e d these animals. The i n v e s t i g a t o r s h y p o t h e s i z e d that s e l e n i u m in the tuna was a n t a g o n i s t i c t o w a r d the d a m a g i n g a c t i o n s of m e t h y l m e r c u r y . K O E M A N et al. (1973) found that m a r i n e m a m m a l s w i t h h i g h m e r c u r y b u r d e n s also c o n t a i n e d high q u a n tities of selenium. This s u g g e s t e d that the s e l e n i u m p r o t e c t e d the m a m m a l s a g a i n s t the toxic a c t i o n of m e r c u r y c o m p o u n d s and that the two e l e m e n t s c o a c c u mulate. As early as 1967 P A R I Z E K and O S T A D A L O V A had demonstrated selenium's protective action against m e r c u r y in rats. Since that time several s t u d i e s have been p u b l i s h e d c o n c e r n i n g this i n t e r a c t i o n . IWA~A et al. (1973) s u g g e s t e d that s e l e n i u m might p r o m o t e the e l i m i n a t i o n of m e t h y l m e r c u r y from tissues. This is c o n t r a r y to most other studies w h i c h i n d i c a t e that s e l e n i u m a c t u a l l y p r o m o t e s the a c c u m u l a t i o n of m e r c u r y in the o r g a n i s m s . POTTER and M A T R O N E (1974) felt that a d e q u a t e e v i d e n c e was a v a i l a b l e to show that the p r o t e c t i o n a f f o r d e d by s e l e n i u m is not due to i n c r e a s e d m e r c u r y e x c r e t i o n . The m e c h a n i s m of this p r o t e c t i o n r e m a i n s an enigma. While many l a b o r a t o r y s t u d i e s have b e e n p u b l i s h e d c o n c e r n i n g this s i t u a t i o n in birds and m a m m a l s , t h e r e is a p a u c i t y of i n f o r m a t i o n c o n c e r n i n g m e r c u r y and s e l e n i u m i n t e r a c t i o n s in fish. H U C K A B E E (1974) r e p o r t e d that s e l e n i u m and m e r c u r y m i x t u r e s were m o r e toxic to carp eggs than m e r c u r y alone. Is s e l e n i u m p r o t e c t i v e a g a i n s t m e r c u r y at this p h y l o g e n e t i c level? Does it p r o m o t e m e r c u r y r e t e n t i o n or lead to a d e c r e a s e in t i s s u e mercury? It was the p u r p o s e of this study to b e g i n to i n v e s t i g a t e these q u e s t i o n s .
132 Bulletin of Environmental Contamination & Toxicology, Vol. 17, No. 2 9 1977 by Springer-Verlag New York Inc.
METHODS
AND M A T E R I A L S
N o r t h e r n c r e e k chubs ( S e m o t i l u s a t r o m a c u l a t u s ) were c o l l e c t e d from small s t r e a m s a r o u n d V e r m i l l i o n , South Dakota. F i s h b e t w e e n 3.5-5 cm were used. Each group of 20 was p l a c e d in a p p r o x i m a t e l y 14 liters of d e m i n e r a l i z e d water (about i 0 , 0 0 0 ohms) to w h i c h 50 p p m c a l c i u m c a r b o n a t e was added. The t e m p e r a t u r e was m a i n t a i n e d at 24-25~ The a q u a r i a w e r e o x y g e n a t e d and lined w i t h p o l y e t h y l e n e sheets. The test s u b s t a n c e s were Hg g i v e n as m e r c u r i c c h l o r i d e and s e l e n i u m as s e l e n i u m d i o x i d e . Exposures to test s u b s t a n c e s were made for 48 hours; the fish were c h e c k e d for m o r t a l i t y at eight hour i n t e r v a l s . F i s h were not fed d u r i n g p r e - t r e a t m e n t or test periods. At the end of the test p e r i o d s s u r v i v o r s were c o l l e c t e d , rinsed, f r o z e n and s t o r e d in p l a s t i c bags. Later, they were t a k e n to c o n s t a n t weight at 100~ They w e r e d i g e s t e d in m i x t u r e s of n i t r i c and s u l f u r i c acid w i t h v a n a d i u m p e n t o x i d e as a c a t a l y s t (DEITZ et al. 1973). A P e r k i n - E l m e r m o d e l 360 A t o m i c A b s o r p t i o n S p e c t r o p h o t o m e t e r was used for the flameless analysis. This m e t h o d y i e l d e d e x c e l l e n t r e c o v e r y f r o m spiked samples. T y p i c a l r e c o v e r y d a t a from a c o m p o site fish sample spiked w i t h one m i c r o g r a m of Hg was 1 0 0 . 0 9 % + 2 . 6 5 % (x + S.D., N = 9).
RESULTS
AND D I S C U S S I O N
C r e e k chubs p r e t r e a t e d (48 hrs.) w i t h Se (3 ppm) have lower m o r t a l i t y rates t h a n t h o s e s u b j e c t e d to i d e n t i c a l m e r c u r y c o n c e n t r a t i o n s for 48 hours w i t h o u t p r e t r e a t m e n t (Fig. i). All of the fish that died w e r e dead by 32 hours of e x p o s u r e . C o n s e q u e n t l y f i g u r e one r e p r e s e n t s the d e a t h rate for t h r e e time p e r i o d s (32, 40 and 48 hours) of m e r c u r y e x p o s u r e . S e l e n i u m p r e t r e a t m e n t at 3 p p m had no o b v i o u s e f f e c t s u p o n the fish. In p r e l i m i n a r y e x p e r i m e n t s we found that h i g h e r c o n c e n t r a t i o n s ( ~ 1 2 ppm) cause h e a v y m u c o u s a c c u m u l a t i o n s on the gills and death. This s e l e n i u m p r e t r e a t m e n t (3 ppm) w h i l e close to fatal levels, a p p a r e n t l y p r o t e c t e d the o r g a n i s m s f r o m subsequent mercury exposure. Was this p r o t e c t i o n p r o moted because selenium prevents mercury accumulation?
tions
W h e n we c o m p a r e the w h o l e body m e r c u r y a c c u m u l a (dry weight) of the Se p r e t r e a t e d fish w i t h
133
those not p r e t r e a t e d , p a t t e r n (Fig. 2).
we o b t a i n
a rather
complicated
At r e l a t i v e l y low m e r c u r y c o n c e n t r a t i o n s (0.01, 0.04 and 0.07 ppm) Se p r e t r e a t m e n t a p p e a r s to favor the a c c u m u l a t i o n of Hg. At 0.07 m i c r o g r a m s / m l the m e a n m e r c u r y b u r d e n in the p r e t r e a t e d a n i m a l s was 24.47 ppm, in the u n t r e a t e d only 18.60 ppm. However, none of these p r e t r e a t e d a n i m a l s died d e s p i t e their h i g h m e r c u r y b u r d e n s w h i l e 30Z (Fig. i) of the unt r e a t e d a n i m a l s were dead. The p r o t e c t i v e a c t i o n of s e l e n i u m p r o b a b l y does not r e s i d e in its a b i l i t y to p r e v e n t a c c u m u l a t i o n of total body m e r c u r y . We n o t e d that s l i g h t l y h i g h e r levels of Se p r e t r e a t m e n t c a u s e d c o p i o u s m u c u s secretion. M e r c u r y is r e a d i l y a b s o r b e d by mucus (USHA et al. 1975). P e r h a p s the h i g h e r levels of w h o l e body m e r c u r y in the p r e t r e a t e d a n i m a l s r e f l e c t the a c c u m u l a t i o n of that e l e m e n t in the skin, a c o m p a r a t i v e l y h a r m l e s s location. Of course, the coa c c u m u l a t i o n of m e r c u r y and s e l e n i u m is well k n o w n (GANTHER et al. 1972, K O E M A N et al. 1973, and K O S T A et al. 1975) in o t h e r o r g a n i s m s and our data is consistent w i t h that of most other studies. However, at the m o r e lethal levels Of m e r c u r y e x p o s u r e (0.i, 0.13, and 0.16 ppm) a new trend is o b s e r v e d (Fig. 2). S e l e n i u m p r e t r e a t e d a n i m a l s have lower body b u r d e n s than those not p r e t r e a t e d . IWATA et al. (1973) found that s o d i u m s e l e n i t e s i g n i f i c a n t l y d e c r e a s e d r e t e n t i o n of m e t h y l m e r c u r y in v a r i o u s organs. We are u n a b l e to find p u b l i s h e d s t u d i e s on i n o r g a n i c m e r c u r y a c c u m u l a t i o n s in fish w i t h w h i c h to c o m p a r e our results. We chose i n o r g a n i c m e r c u r y b e c a u s e m e r c u r i a l s o t h e r t h a n m e t h y l m e r c u r y p r o b a b l y p r e d o m i n a t e in s t r e a m and lake w a t e r and sediments. It was known as early as 1967 (HANNERZ) that fish can c o n c e n t r a t e i n o r g a n i c m e r c u r y d i r e c t l y f r o m w a t e r w i t h o u t the i n v o l v e m e n t of a food chain. In this study we p l o t t e d m i c r o g r a m s of H g / g r a m of s u r v i v i n g fish (wet weight) a g a i n s t m i c r o g r a m s of Hg/ml of w a t e r and found a good c o r r e l a t i o n (Y : 0.63 + 32 X, r = 0.87. P ~ 0 . 0 0 2 ) , i n d i c a t i n g a c o n c e n t r a t i o n f a c t o r of about 32 times in the 48 hour test period. U n d e r e n v i r o n m e n t a l c o n d i t i o n s Hg and Se and the c o m p l e x p r o d u c t s of their i n t e r a c t i o n s w o u l d simult a n e o u s l y a s s a u l t a fish. The r e s u l t s might be very d i f f e r e n t from those o b s e r v e d in this study. In a study of that type, H U C K A B E E and G R I F F I T H (1974) found a s y n e r g i s t i c effect of these two e l e m e n t s on
134
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Fig 2. Comparison of accumulation of Hg (dry weight basis) in creek chub survivors subjected to various concentrations of Hgwith and wlthoutSe pretJreatment. Points are means_ + SE. An S indicatesa significant difference between groups by Student's test(P
L a r g e q u a n t i t i e s of s e l e n i u m as well as m e r c u r y were found in fishes, shell fishes, cats and humans in the M i n a m a t a Bay area of J a p a n in 1960 (IWATA et al. 1973). G A N T H E R et al. (1972) found that l o w - H g tuna c o n t a i n e d less s e l e n i u m t h a n h i g h - H g tuna. D e s p i t e h i g h - H g c o n c e n t r a t i o n s , tuna added to m e t h y l m e r c u r y t r e a t e d foods of J a p a n e s e quail and rats p r o t e c t e d these animals. The i n v e s t i g a t o r s h y p o t h e s i z e d that s e l e n i u m in the tuna was a n t a g o n i s t i c t o w a r d the d a m a g i n g a c t i o n s of m e t h y l m e r c u r y . K O E M A N et al. (1973) found that m a r i n e m a m m a l s w i t h h i g h m e r c u r y b u r d e n s also c o n t a i n e d high q u a n tities of selenium. This s u g g e s t e d that the s e l e n i u m p r o t e c t e d the m a m m a l s a g a i n s t the toxic a c t i o n of m e r c u r y c o m p o u n d s and that the two e l e m e n t s c o a c c u mulate. As early as 1967 P A R I Z E K and O S T A D A L O V A had demonstrated selenium's protective action against m e r c u r y in rats. Since that time several s t u d i e s have been p u b l i s h e d c o n c e r n i n g this i n t e r a c t i o n . IWA~A et al. (1973) s u g g e s t e d that s e l e n i u m might p r o m o t e the e l i m i n a t i o n of m e t h y l m e r c u r y from tissues. This is c o n t r a r y to most other studies w h i c h i n d i c a t e that s e l e n i u m a c t u a l l y p r o m o t e s the a c c u m u l a t i o n of m e r c u r y in the o r g a n i s m s . POTTER and M A T R O N E (1974) felt that a d e q u a t e e v i d e n c e was a v a i l a b l e to show that the p r o t e c t i o n a f f o r d e d by s e l e n i u m is not due to i n c r e a s e d m e r c u r y e x c r e t i o n . The m e c h a n i s m of this p r o t e c t i o n r e m a i n s an enigma. While many l a b o r a t o r y s t u d i e s have b e e n p u b l i s h e d c o n c e r n i n g this s i t u a t i o n in birds and m a m m a l s , t h e r e is a p a u c i t y of i n f o r m a t i o n c o n c e r n i n g m e r c u r y and s e l e n i u m i n t e r a c t i o n s in fish. H U C K A B E E (1974) r e p o r t e d that s e l e n i u m and m e r c u r y m i x t u r e s were m o r e toxic to carp eggs than m e r c u r y alone. Is s e l e n i u m p r o t e c t i v e a g a i n s t m e r c u r y at this p h y l o g e n e t i c level? Does it p r o m o t e m e r c u r y r e t e n t i o n or lead to a d e c r e a s e in t i s s u e mercury? It was the p u r p o s e of this study to b e g i n to i n v e s t i g a t e these q u e s t i o n s .
132 Bulletin of Environmental Contamination & Toxicology, Vol. 17, No. 2 9 1977 by Springer-Verlag New York Inc.
METHODS
AND M A T E R I A L S
N o r t h e r n c r e e k chubs ( S e m o t i l u s a t r o m a c u l a t u s ) were c o l l e c t e d from small s t r e a m s a r o u n d V e r m i l l i o n , South Dakota. F i s h b e t w e e n 3.5-5 cm were used. Each group of 20 was p l a c e d in a p p r o x i m a t e l y 14 liters of d e m i n e r a l i z e d water (about i 0 , 0 0 0 ohms) to w h i c h 50 p p m c a l c i u m c a r b o n a t e was added. The t e m p e r a t u r e was m a i n t a i n e d at 24-25~ The a q u a r i a w e r e o x y g e n a t e d and lined w i t h p o l y e t h y l e n e sheets. The test s u b s t a n c e s were Hg g i v e n as m e r c u r i c c h l o r i d e and s e l e n i u m as s e l e n i u m d i o x i d e . Exposures to test s u b s t a n c e s were made for 48 hours; the fish were c h e c k e d for m o r t a l i t y at eight hour i n t e r v a l s . F i s h were not fed d u r i n g p r e - t r e a t m e n t or test periods. At the end of the test p e r i o d s s u r v i v o r s were c o l l e c t e d , rinsed, f r o z e n and s t o r e d in p l a s t i c bags. Later, they were t a k e n to c o n s t a n t weight at 100~ They w e r e d i g e s t e d in m i x t u r e s of n i t r i c and s u l f u r i c acid w i t h v a n a d i u m p e n t o x i d e as a c a t a l y s t (DEITZ et al. 1973). A P e r k i n - E l m e r m o d e l 360 A t o m i c A b s o r p t i o n S p e c t r o p h o t o m e t e r was used for the flameless analysis. This m e t h o d y i e l d e d e x c e l l e n t r e c o v e r y f r o m spiked samples. T y p i c a l r e c o v e r y d a t a from a c o m p o site fish sample spiked w i t h one m i c r o g r a m of Hg was 1 0 0 . 0 9 % + 2 . 6 5 % (x + S.D., N = 9).
RESULTS
AND D I S C U S S I O N
C r e e k chubs p r e t r e a t e d (48 hrs.) w i t h Se (3 ppm) have lower m o r t a l i t y rates t h a n t h o s e s u b j e c t e d to i d e n t i c a l m e r c u r y c o n c e n t r a t i o n s for 48 hours w i t h o u t p r e t r e a t m e n t (Fig. i). All of the fish that died w e r e dead by 32 hours of e x p o s u r e . C o n s e q u e n t l y f i g u r e one r e p r e s e n t s the d e a t h rate for t h r e e time p e r i o d s (32, 40 and 48 hours) of m e r c u r y e x p o s u r e . S e l e n i u m p r e t r e a t m e n t at 3 p p m had no o b v i o u s e f f e c t s u p o n the fish. In p r e l i m i n a r y e x p e r i m e n t s we found that h i g h e r c o n c e n t r a t i o n s ( ~ 1 2 ppm) cause h e a v y m u c o u s a c c u m u l a t i o n s on the gills and death. This s e l e n i u m p r e t r e a t m e n t (3 ppm) w h i l e close to fatal levels, a p p a r e n t l y p r o t e c t e d the o r g a n i s m s f r o m subsequent mercury exposure. Was this p r o t e c t i o n p r o moted because selenium prevents mercury accumulation?
tions
W h e n we c o m p a r e the w h o l e body m e r c u r y a c c u m u l a (dry weight) of the Se p r e t r e a t e d fish w i t h
133
those not p r e t r e a t e d , p a t t e r n (Fig. 2).
we o b t a i n
a rather
complicated
At r e l a t i v e l y low m e r c u r y c o n c e n t r a t i o n s (0.01, 0.04 and 0.07 ppm) Se p r e t r e a t m e n t a p p e a r s to favor the a c c u m u l a t i o n of Hg. At 0.07 m i c r o g r a m s / m l the m e a n m e r c u r y b u r d e n in the p r e t r e a t e d a n i m a l s was 24.47 ppm, in the u n t r e a t e d only 18.60 ppm. However, none of these p r e t r e a t e d a n i m a l s died d e s p i t e their h i g h m e r c u r y b u r d e n s w h i l e 30Z (Fig. i) of the unt r e a t e d a n i m a l s were dead. The p r o t e c t i v e a c t i o n of s e l e n i u m p r o b a b l y does not r e s i d e in its a b i l i t y to p r e v e n t a c c u m u l a t i o n of total body m e r c u r y . We n o t e d that s l i g h t l y h i g h e r levels of Se p r e t r e a t m e n t c a u s e d c o p i o u s m u c u s secretion. M e r c u r y is r e a d i l y a b s o r b e d by mucus (USHA et al. 1975). P e r h a p s the h i g h e r levels of w h o l e body m e r c u r y in the p r e t r e a t e d a n i m a l s r e f l e c t the a c c u m u l a t i o n of that e l e m e n t in the skin, a c o m p a r a t i v e l y h a r m l e s s location. Of course, the coa c c u m u l a t i o n of m e r c u r y and s e l e n i u m is well k n o w n (GANTHER et al. 1972, K O E M A N et al. 1973, and K O S T A et al. 1975) in o t h e r o r g a n i s m s and our data is consistent w i t h that of most other studies. However, at the m o r e lethal levels Of m e r c u r y e x p o s u r e (0.i, 0.13, and 0.16 ppm) a new trend is o b s e r v e d (Fig. 2). S e l e n i u m p r e t r e a t e d a n i m a l s have lower body b u r d e n s than those not p r e t r e a t e d . IWATA et al. (1973) found that s o d i u m s e l e n i t e s i g n i f i c a n t l y d e c r e a s e d r e t e n t i o n of m e t h y l m e r c u r y in v a r i o u s organs. We are u n a b l e to find p u b l i s h e d s t u d i e s on i n o r g a n i c m e r c u r y a c c u m u l a t i o n s in fish w i t h w h i c h to c o m p a r e our results. We chose i n o r g a n i c m e r c u r y b e c a u s e m e r c u r i a l s o t h e r t h a n m e t h y l m e r c u r y p r o b a b l y p r e d o m i n a t e in s t r e a m and lake w a t e r and sediments. It was known as early as 1967 (HANNERZ) that fish can c o n c e n t r a t e i n o r g a n i c m e r c u r y d i r e c t l y f r o m w a t e r w i t h o u t the i n v o l v e m e n t of a food chain. In this study we p l o t t e d m i c r o g r a m s of H g / g r a m of s u r v i v i n g fish (wet weight) a g a i n s t m i c r o g r a m s of Hg/ml of w a t e r and found a good c o r r e l a t i o n (Y : 0.63 + 32 X, r = 0.87. P ~ 0 . 0 0 2 ) , i n d i c a t i n g a c o n c e n t r a t i o n f a c t o r of about 32 times in the 48 hour test period. U n d e r e n v i r o n m e n t a l c o n d i t i o n s Hg and Se and the c o m p l e x p r o d u c t s of their i n t e r a c t i o n s w o u l d simult a n e o u s l y a s s a u l t a fish. The r e s u l t s might be very d i f f e r e n t from those o b s e r v e d in this study. In a study of that type, H U C K A B E E and G R I F F I T H (1974) found a s y n e r g i s t i c effect of these two e l e m e n t s on
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Flg. I. The percent mortalltyot creek chubs subjected to various concentrations of HgCI2 with and without Se pretreotment, Twenty animals were utilized in each of the two groups at each concentration.
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Fig 2. Comparison of accumulation of Hg (dry weight basis) in creek chub survivors subjected to various concentrations of Hgwith and wlthoutSe pretJreatment. Points are means_ + SE. An S indicatesa significant difference between groups by Student's test(P
