18 Fate of Hydrophobic Organic Contaminants Downloaded by UNIV OF ROCHESTER on January 19, 2018 | http://pubs.acs.org Publication Date: May 5, 1994 | doi: 10.1021/ba-1994-0237.ch018
Processes Affecting Uptake by Phytoplankton Robert S. Skoglund and Deborah L. Swackhamer Environmental and Occupational Health, School of Public Health, Box 807 UMHC, University of Minnesota, Minneapolis, MN 55455
The accumulation of hydrophobic contaminants in phytoplankton plays a significant role in the transport and fate of these potentially toxic compounds. However, the limited amount of available field data indicate that partitioning models fail to adequately predict the distribution of these compounds in the water column. Several hypotheses have been proposed to explain these differences. In this chapter we propose additional explanations for these differences. We hypothesize that assumptions in the partitioning model about the rate of uptake, mechanism of uptake, and effect of phytoplankton growth also contribute to these deviations.
G R O W I N G C O N C E R N OVER T H E POTENTIAL T O X I C O L O G I C A L a n d e c o l o g i c a l
effects o f h y d r o p h o b i c organic c o m p o u n d s ( H O C s ) has m a d e t h e d e t e r m i nation o f the e n v i r o n m e n t a l a n d h u m a n h e a l t h i m p a c t o f these c o m p o u n d s one o f t h e i m p o r t a n t objectives of e n v i r o n m e n t a l science. A t o o l often u s e d i n s o l v i n g ecotoxicological p r o b l e m s is the p r e d i c t i o n o f fluxes, d i s t r i b u t i o n s , a n d toxicology o f c o m p o u n d s f r o m a c o m b i n a t i o n o f p h y s i c a l - c h e m i c a l g e n eralizations a n d a l i m i t e d a m o u n t o f c o m p o u n d - s p e c i f i c data. T h e m o v e m e n t of d i s s o l v e d H O C s into the aquatic food w e b is an i m p o r t a n t flux because this d i s t r i b u t i o n can greatly increase t h e exposure o f h i g h e r organisms, i n c l u d i n g h u m a n s , to these c o m p o u n d s . P h y t o p l a n k t o n play an i m p o r t a n t role i n t h e i n c o r p o r a t i o n o f H O C s i n t o the aquatic food w e b . T h e l i p o p h i l i c i t y o f H O C s results i n a n e n h a n c e d 0065-2393/94/0237-0559$06.00/0 © 1994 American Chemical Society
Baker; Environmental Chemistry of Lakes and Reservoirs Advances in Chemistry; American Chemical Society: Washington, DC, 1994.
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association w i t h l i p i d - r i c h p h y t o p l a n k t o n i n the water c o l u m n . O n c e associated w i t h p h y t o p l a n k t o n , the d i s t r i b u t i o n a n d fate of H O C s are c o n t r o l l e d b y that o f the p h y t o p l a n k t o n . T h e p r i n c i p a l source o f organic c a r b o n to the aquatic food w e b is p h y t o p l a n k t o n , a n d a significant p o r t i o n of the H O C s f o u n d i n the food w e b p r o b a b l y e n t e r e d w i t h this organic c a r b o n (J). T h u s w a t e r - p h y t o p l a n k t o n p a r t i t i o n i n g is an i m p o r t a n t p a r a m e t e r i n the flux o f H O C s i n t o food w e b s . T h e most c o m m o n l y h e l d t h e o r y is that w a t e r - p h y t o p l a n k t o n p a r t i t i o n i n g o f H O C s is a passive t h e r m o d y n a m i c process (2, 3). M a s s transfer o f these c o m p o u n d s appears to be d r i v e n b y a net decrease i n the free e n e r g y of the system associated w i t h m o v e m e n t of H O C s from a n aqueous to a n organic phase. T h u s , at e q u i l i b r i u m , p a r t i t i o n i n g o f H O C s s h o u l d b e d i r e c t l y p r o p o r t i o n a l to t h e i r o c t a n o l - w a t e r p a r t i t i o n coefficients ( K ) a n d i n v e r s e l y p r o p o r t i o n a l to t h e i r aqueous solubilities. P a r t i t i o n i n g is p a r a m e t e r i z e d b y a p a r t i t i o n coefficient, r e f e r r e d to as the b i o a c c u m u l a t i o n factor ( B A F ) , a n d d e f i n e d as the concentration of a c o m p o u n d i n p h y t o p l a n k t o n d i v i d e d b y the d i s s o l v e d concentration i n a m b i e n t w a t e r i n e q u i v a l e n t u n i t s . S e v e r a l researchers (4-6) r e p o r t e d a c o r r e l a t i o n b e t w e e n l o g - t r a n s f o r m e d values o f B A F a n d K . A l t h o u g h the slopes o f these relationships are consistently less than 1, w h i c h is the value suggested b y the fugacity c o n c e p t (2), these data s u p p o r t the t h e o r y that b i o a c c u m u l a t i o n o f H O C s i n p h y t o p l a n k t o n results f r o m t h e r m o d y n a m i c p a r t i t i o n i n g . ow
o w
H o w e v e r , B a k e r et a l . (2) r e p o r t e d that field data suggest that several assumptions of H O C p a r t i t i o n i n g m o d e l s are v i o l a t e d i n surface waters a n d that these m o d e l s fail to adequately p r e d i c t H O C d i s t r i b u t i o n i n the w a t e r c o l u m n . F u r t h e r m o r e , a consistent slope of < 1 for the l o g B A F - l o g K regression i n laboratory studies a n d reports o f species-specific differences i n a c c u m u l a t i o n (I, 8, 9) indicate that a c c u m u l a t i o n appears to b e i n f l u e n c e d b y o t h e r factors. o w
P u b l i s h e d values o f B A F s for H O C s , s u m m a r i z e d b y S w a c k h a m e r a n d S k o g l u n d (JO), range f r o m 18 to 1,000,000. T h e v a r i e t y of m e t h o d s u s e d i n these studies prevents d i r e c t c o m p a r i s o n , so it is u n c l e a r w h a t factors are responsible for the large variation i n B A F s . S e v e r a l hypotheses have b e e n p r o p o s e d to e x p l a i n these variations i n a c c u m u l a t i o n a n d t h e i r deviations from Ko -based p r e d i c t i o n s . O n e hypothesis ( I I , 12) p r o p o s e d that a lack o f c o m p l e t e r e v e r s i b i l i t y i n the p a r t i t i o n i n g process is r e s p o n s i b l e for the d e viations. A second (13-16) t h e o r y a t t r i b u t e d the deviations to the p r e s e n c e of a t h i r d phase (colloids or d i s s o l v e d organic matter) a n d the i n a b i l i t y to accurately separate the d i s s o l v e d a n d s o r b e d states. A t h i r d (17) p r o p o s a l is that p a r t i t i o n i n g is d e p e n d e n t o n sorbent concentration. A n d finally, a f o u r t h (IS, 19) hypothesis holds that this d e v i a t i o n is a f u n c t i o n o f the effects o f m o l e c u l a r size a n d shape o n c e l l u l a r transport. W
T h e data indicate that a l l of these processes m a y affect a c c u m u l a t i o n a n d thus c o n t r i b u t e to deviations from the p r e d i c t e d a c c u m u l a t i o n values.
Baker; Environmental Chemistry of Lakes and Reservoirs Advances in Chemistry; American Chemical Society: Washington, DC, 1994.
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Hydrophobic
Organic Contaminants
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In this chapter w e present t h r e e a d d i t i o n a l factors that have b e e n o b s e r v e d to affect a c c u m u l a t i o n . W e h y p o t h e s i z e that the assumptions i n the p a r t i t i o n i n g m o d e l about the rate of a c c u m u l a t i o n , the m e c h a n i s m of a c c u m u lation, a n d the effect of g r o w t h are i n a p p r o p r i a t e a n d thus c o n t r i b u t e to the deviations from the p r e d i c t e d values. W e c o n d u c t e d laboratory experiments d e s i g n e d to measure the i n f l u e n c e of various sorbent a n d sorbate properties o n the a c c u m u l a t i o n of H O C s b y p h y t o p l a n k t o n . T h e specifics of these e x p e r i m e n t s w e r e p u b l i s h e d e l s e w h e r e (JO, 20). I n s u m m a r y , 40 p o l y c h l o r i n a t e d b i p h e n y l s ( P C B s ) (Table I) w e r e a l l o w e d to p a r t i t i o n to a u n i a l g a l c u l t u r e of Scenedesmus quadricauda under batch conditions. A t various t i m e points b e t w e e n 0.02 a n d 30 days, the phases w e r e operationally separated b y centrifugation. P a r t i t i o n i n g was d e t e r m i n e d b y the congener-specific m e a s u r e m e n t of P C B s i n each phase. A t the b e g i n n i n g of each e x p e r i m e n t , P C B congener concentrations r a n g e d from 30 to 1100 n g / L , a n d p h y t o p l a n k t o n mass was a p p r o x i m a t e l y 10 mg/L. E x p e r i m e n t s w e r e c o n d u c t e d u n d e r g r o w t h - l i m i t i n g c o n d i t i o n s (average g r o w t h rate = 0.03 d o u b l i n g p e r day) a n d g r o w t h - u n l i m i t e d c o n d i t i o n s (average g r o w t h rate = 0.13 d o u b l i n g p e r day). Parameters t h o u g h t to i n fluence p a r t i t i o n i n g w e r e m e a s u r e d c o n c u r r e n t l y . D a t a from these studies w e r e u s e d to evaluate the rate a n d m a g n i t u d e of w a t e r - p h y t o p l a n k t o n part i t i o n i n g a n d the factors that i n f l u e n c e the process.
Rate of Accumulation E q u i l i b r i u m i n an a c c u m u l a t i o n process is e m p i r i c a l l y d e f i n e d as the p o i n t at w h i c h a p a r t i t i o n i n g coefficient becomes invariant (reaches steady state), and theoretically as the p o i n t at w h i c h the fugacity ratio equals 1 (21). W i t h p h y t o p l a n k t o n , most p u b l i s h e d reports i n d i c a t e d that e q u i l i b r i u m was r e a c h e d i n a matter of hours (6, 8, 22-26). A s a result, p r e d i c t i o n s of H O C a c c u m u l a t i o n i n p h y t o p l a n k t o n have b e e n expressed as e q u i l i b r i u m - b a s e d equations exclusively. O u r data demonstrate that a c c u m u l a t i o n of P C B s b y p h y t o p l a n k t o n is not as r a p i d as was i n i t i a l l y thought; thus e q u i l i b r i u m - b a s e d equations are i n a p p r o p r i a t e i n some instances. W o r k b y M a c k a y (2) a n d C o n n o l l y a n d P e d e r s e n (27) d e m o n s t r a t e d that w h e n the fugacity ratio is 1, the l i p i d n o r m a l i z e d B A F ( B A F ) is e q u a l to K . T h e r e f o r e at e q u i l i b r i u m , the BAF -K relationship s h o u l d have a slope of 1 a n d an i n t e r c e p t of 0. F i g u r e 1 is a p l o t o f log-transformed values o f B A F and K for five o f the seven t i m e points of o u r g r o w t h - l i m i t e d e x p e r i m e n t . Biomass increase was m i n i m i z e d b y m a i n t a i n i n g the i n c u b a t i o n t e m p e r a t u r e at 1 0 - 1 2 °C to r e d u c e the c o n f o u n d i n g effects of g r o w t h . A r e d u c t i o n i n t e m p e r a t u r e f r o m 22 to 12 °C was r e p o r t e d (J) to have l i t t l e or no effect o n a c c u m u l a t i o n of H O C s by phytoplankton. ( l i p )
( l i p )
o w
0 W
( l i p )
ow
Baker; Environmental Chemistry of Lakes and Reservoirs Advances in Chemistry; American Chemical Society: Washington, DC, 1994.
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E N V I R O N M E N T A L CHEMISTRY O F L A K E S A N D RESERVOIRS
Table I. Results from Applying the Modified Richards M o d e l to Data from the Limited-Growth Experiment with 40 P C B s
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IUPAC Number 001 003 004 008 018 022 031 052 053 054 072 077 080 081 097 100 101 104 105 118 126 138 141 151 154 155 156 169 170 180 183 185 188 189 194 195 199 206 207 209
Substitution Pattern 2 4 2,2' 2,4' 2,2',5 2,3,4' 2,4',5 2,2',5,5' 2,2',5,6' 2,2',6,6' 2,3',5,5' 3,3,4,4' 3,3',5,5' 3,4,4',5 2,2',3',4,5 2,2',4,4',6 2,2',4,5,5' 2,2',4,6,6' 2,3',4,4',5 2,3',4,4',5 3,3',4,4',5 2,2',3,4,4',5 2,2',3,4,5,5' 2,2',3,5,5',6 2,2',4,4',5,6 2,2',4,4',6,6' 2,3,3',4,4',5 3,3',4,4',5,5' 2,2',3,3',4,4',5 2,2',3,4,4',5,5' 2,2',3,4,4',5',6 2,2',3,4',5,6,6' 2,2',3,4',5,6,6' 2,3,3',4,4',5,5' 2,2',3,3',4,4',5,5' 2,2',3,3',4,4',5,6 2,2',3,3',4,5,6,6' 2,2',3,3',4,4',5,5',6 2,2',3,3',4,4',5,6,6' 2,2',3,3',4,4',5,5',6,6'
Log 4.46 4.69 4.65 5.07 5.24 5.58 5.67 5.84 5.62 5.21 6.26 6.36 6.48 6.36 6.29 6.23 6.38 5.81 6.65 6.74 6.89 6.83 6.82 6.64 6.76 6.41 7.18 7.42 7.27 7.36 7.20 7.11 6.82 7.71 7.80 7.56 7.20 8.09 7.74 8.18
m
F Value
ρ Value
-0.64 -0.26 -0.50 -0.72 -0.60 -0.45 -0.67 -0.03 -0.33 -0.09 -0.29 -0.36 -0.36 -0.60 -0.08 0.02 0.30 0.12 -0.01 -0.05 0.08 -0.07 0.39 0.68 0.49 0.46 0.35 0.37 0.27 0.69 0.52 0.44 0.41 0.36 0.35 0.02
4.31 0.99 1.93 4.90 4.99 3.20 1.11 0.02 1.54 0.21 1.24 1.46 1.66 4.58 0.12 0.01 5.86 0.49 0.00 0.02 0.10 0.12 9.62 31.41 20.08 19.83 7.56 9.02 4.37 21.56 14.25 7.08 3.09 2.51 2.40 0.00
0.08 >0.25 0.25 0.06 0.06 0.13 >0.25 >0.25 >0.25 >0.25 >0.25 >0.25 >0.25 0.07 >0.25 >0.25 0.04 >0.25 >0.25 >0.25 >0.25 >0.25 0.02
