Digestive solubilization of particle-associated arsenate by deposit-feeders: The roles of proteinaceous and surfactant materials

Solubilization of arsenate in guts of deposit-feeders is a key process for their dietary uptake of arsenate from contaminated sediments. The present study explored this digestive solubilization with in vitro extraction experiments that quantified arsenic (As) release from substrates (natural sediment and As-enriched iron oxides) in the presence of various digestive agents (proteins, amino acids and surfactants collected from gut fluid of a sipunculan worm). To investigate potential mechanisms for the influence of digestive agents, analyses determined correlations between As and Fe concentrations, the size distribution of the As bound to the digestive agents, and the adsorption of the digestive agents on the substrates. Both the digestive surfactants and proteinaceous materials increased arsenate mobilization, with the surfactants enhancing the effects of the proteinaceous materials. Arsenate reduction and reductive dissolution of iron oxides were not observed and correlations between the concentrations of released As and Fe were weak. These findings indicate that dissolution release of Fe did not appear to be the main route by which the digestive agents mobilized particle-associated As. Most of the released As (>70%) was distributed in the <10 kDa fraction of the digestive agents, showing that the As mobilization was also not caused by complexation with proteins in the digestive agents. In contrast, adsorption of the digestive agents occurred along with the release of arsenate from the arsenate-rich substrates, suggesting that competitive adsorption was the mechanism by which the digestive agents mobilized sedimentary arsenate. Our work demonstrated that the presence of digestive surfactants significantly enhances arsenate availability during deposit feeding.