Methylmercury extraction from artificial sediments by the gut juice of the sipunculan, Sipunculus nudus

Artificially prepared sediments were used to investigate their binding with methylmercury (MeHg). The bioavailability of sediment-bound MeHg then was quantified by measuring the extraction by gut juice of the sipunculan Sipunculus nudus, as well as by different free amino acids and bovine serum albumin (BSA). Methylmercury distribution in different molecular weight- size fractions of gut juice also was determined using an ultrafiltration methodology. Organic and clay content were the two most important sediment components in MeHg partitioning, but most of the sediment-bound MeHg was complexed by organic matter (fulvic acid > humic acid) in sediments. Treatment with humic or fulvic acid generally increased the amount of bioavailable MeHg. Cysteine was more important than other amino acids in MeHg extraction. Proteins (especially >100 kDa fraction) in gut juice rather than free amino acids were the main agents in gut juice that extracted MeHg from sediments. Most extracted MeHg from artificial sediments was associated with the >100 kDa fraction (probably proteins) of gut juice but not with organic matter from sediments (humic acid and fulvic acid). Our results suggested that competition among different agents in gut juice (especially the large molecular-weight proteins) and the organic content of the sediments controlled the bioavailability of sediment-bound MeHg.