The role of various dissolved organic matter forms on chlorpyrifos bioavailability to the estuarine bivalve Mercenaria mercenaria

Dissolved organic matter (DOM) is comprised of a myriad of macromolecules with specific physical and chemical properties that may influence the bioavailability of hydrophobic pesticides to animals. This study was conducted to assess the role of various forms of DOM on the uptake and bioconcentration of the organophosphate insecticide chlorpyrifos (CHPY) to the bivalve Mercenaria mercenaria. Bivalves were exposed to DOM-free seawater (30‰) or to seawater containing a single form of DOM. DOM forms included two filtrate fractions of natural salt-marsh sediment DOM (DOM-(<0.45 μm) and DOM-(<3 kDa)); natural purified humic (HA) and fulvic (FA) acids; and water soluble cyclic oligosaccharides alpha and beta cyclodextrins (CD-α and CD-β). In 14C-CHPY uptake and elimination experiments, juvenile bivalves were exposed to uniformly-labeled 14C-CHPY and collected at time intervals during 48 h. The remaining bivalves were transferred to 14C-CHPY-free elimination chambers with bivalve collection at time intervals over 144 h. Total uptake of 14C-CHPY by bivalves in DOM-free seawater was >40% greater than in bivalves exposed to 14C-CHPY in the presence of most DOM forms. These results are consistent with much faster 14C-CHPY uptake rates estimated using a simple two parameter model. After the elimination period, bivalves exposed to DOM-free seawater had 14C-CHPY body residue concentrations between 25% and 86% greater than bivalves in the presence of DOM forms. Experiments with larger bivalves showed that pulse-chase exposures with a 1.5 h exposure period to 14C-CHPY was not long enough to detect differences in 14C-CHPY tissue accumulation efficiencies across treatments. Our findings suggest that natural forms of DOM, at environmentally realistic organic carbon concentrations, reduced pesticide uptake and bioconcentration, consistent with much lower uptake rates relative to bivalves exposed to 14C-CHPY in the absence of DOM. Interestingly, at the tested organic carbon concentrations CD-α and CD-β did not reduce 14C-CHPY bioconcentration in M. mercenaria.