Sedimentary metal bioavailability determined by the digestive constraints of marine deposit feeders: gut retention time and dissolved amino acids

Contaminant metals bound to sediments are subject to considerable solubiliz ation during passage of the sediments through the digestive systems of depo sit feeders. We examined the kinetics of this process, using digestive flui ds extracted from deposit feeders Arenicola marina and Parastichopus califo rnicus and then incubated with contaminated sediments. Kinetics are complex , with solubilization followed occasionally by readsorption onto the sedime nt. In general, solubilization kinetics are biphasic, with an initial rapid step followed by a slower reaction. For many sediment-organism combination s, the reaction will not reach a steady state or equilibrium within the gut retention time (GRT) of the organisms, suggesting that metal bioavailabili ty in sediments is a time-dependent parameter. Experiments with commercial protein solutions mimic the kinetic patterns observed with digestive fluids , which corroborates our previous study that complexation by dissolved amin o acids (AA) in digestive fluids leads to metal solubilization (Chen & Maye r 1998b; Environ Sci Technol 32:770-778). The relative importance of the fa st and slow reactions appears to depend on the ratio of ligands in gut flui ds to the amount of bound metal in sediments. High ligand to solid metal ra tios result in more metals released in fast reactions and thus higher labil ity of sedimentary metals. Multiple extractions of a sediment with digestiv e fluid of A. marina confirm the potential importance of incomplete reactio ns within a single deposit-feeding event, and make clear that bioavailabili ty to a single animal is Likely different from that to a community of organ isms. The complex kinetic patterns lead to the counterintuitive prediction that toxification of digestive enzymes by solubilized metals will occur mor e readily in species that dissolve less metals.