Plasma cholinesterase activity as a biomarker for quantifying exposure of green sturgeon (Acipenser medirostris) to carbaryl following applications to control burrowing shrimp in Washington State

Abstract

Willapa Bay, located in Southwestern Washington State (USA), is one of the rare intertidal locations where large-scale pesticide applications occur. Since the 1960s, carbaryl has been applied to mudflats in Willapa Bay to control populations of burrowing shrimp (<italic>Neotrypaea californiensis</italic> and <italic>Upogebia pugettensis</italic>) that decrease oyster (<italic>Crassostrea gigas</italic>) productivity. Green sturgeon (<italic>Acipenser medirostris</italic>) are present in Willapa Bay, and population declines (Southern Distinct Population Segment) have resulted in their listing under the Endangered Species Act (ESA). White sturgeon (<italic>Acipenser transmontanus</italic>) are also found in Willapa Bay, are closely related to green sturgeon, but are not ESA listed. To determine potential impacts of carbaryl on cholinesterases in sturgeon brain and plasma, seawater-acclimated and laboratory maintained white sturgeon (average wt = 1.1 kg) were exposed to each of six concentrations of carbaryl (0, 30, 100, 300, 1,000 and 3,000 µg L<super>-1</super>, n = 5 per treatment) for 6 h and brain acetylcholinesterase (AChE) and plasma butyrylcholinesterase (BChE) activities were measured. Enzyme recovery was measured in additional cohorts (n = 4 sturgeon per time point, average wt = 1.8 kg) exposed to 1,000 µg L<super>-1</super> carbaryl for 6 h with brain AChE and plasma BChE activity measured at 0, 24, 48 and 72 h post-exposure. Brain AChE activity was statistically (p ≤ 0.05) reduced at all concentrations except 30 µg L<super>-1</super>. Compared to controls, brain AChE was inhibited 15% in sturgeon exposed to 30 µg L<super>-1</super>, and 27%, 32%, 35% and 42% respectively at the higher concentrations. In the enzyme recovery trial, brain AChE was inhibited (p ≤ 0.05) after the 6-h exposure to carbaryl, followed by recovery over 72 h. Compared to controls, brain AChE was inhibited 36%, 23%, 28% and 13% at 0, 24, 48 and 72 h recovery post-exposure. Plasma BChE activities in the dose-response and recovery fish were not statistically different from controls, with activities indicative of elevation of the enzyme as opposed to inhibition. Another cohort of sturgeon (n = 16, average wt = 0.5 kg) was exposed to 0 or 3,000 µg L<super>-1</super> carbaryl for 6 h to clarify the results. Compared to controls, brain AChE was inhibited 29% in treatment sturgeon while plasma BChE was elevated by 30%, with both differences in enzyme activity statistically significant (p ≤ 0.05). Plasma samples were also collected from white sturgeon in the Columbia River and green sturgeon in Willapa Bay prior to application of carbaryl in 2012, and from green sturgeon in Willapa Bay 4-5 d after the last application. On average, white sturgeon plasma BChE activity (0.326 units/ml plasma) was statistically greater than that of green sturgeon (0.151 units/ml plasma; p ≤ 0.05). Plasma BChE activity from green sturgeon collected post-carbaryl application was statistically lower (37%) than that pre-application, indicative of carbaryl exposure. IC50s were calculated for captive white sturgeon brain AChE (8.7 µM) and plasma BChE (15.8 µM) and suggest brain AChE is slightly more sensitive than plasma BChE. Comparable values for plasma BChE from wild white (20.3 µM) and green (18.5 µM) sturgeon were similar and suggest white sturgeon are a good surrogate for green sturgeon. At the lowest carbaryl concentration to which white sturgeon were exposed in the laboratory (30 µg L<super>-1</super>), both brain AChE and plasma BChE activities were inhibited. At the higher doses (≥ 100 µg L<super>-1</super>), brain AChE activity was further inhibited, while plasma BChE activity was elevated. This suggests a concentration threshold (e.g., < 100 µg L<super>-1</super>) above which white sturgeon livers appear to release stored BChE into the plasma and/or increase BChE synthesis (induction) and secretion. The laboratory results suggest that no significant brain AChE inhibition would be expected at lower concentrations, indicative of little risk to sturgeon. If, however, the release of BChE from white sturgeon liver is masking plasma BChE inhibition, it is possible that the magnitude of brain AChE and plasma BChE inhibition mirror each other, and plasma BChE inhibition in Willapa Bay green sturgeon is indicative of similar brain AChE inhibition, which would indicate greater risk to the fish. Plasma BChE activity appears to be a conservative biomarker of exposure of sturgeon to carbaryl when inhibition is detected, as in the case of green sturgeon in Willapa Bay following carbaryl applications to control burrowing shrimp. Although it is unlikely that the level of BChE inhibition detected in wild green sturgeon is life-threatening, the relationship between plasma BChE inhibition and brain AChE activity is unclear. Further studies are needed to better understand the magnitude and effects of carbamate exposure on threatened and non-threatened sturgeon in Willapa Bay, and other Western US surface waters, including green sturgeon spawning grounds.