Metal bioavailability in marine sediments measured by chemical extraction and enzymatic mobilization

In this work we compared metal bioavailable fractions in marine sediments operationally measured by sequential selective extractions (in particular the labile fraction released by acetic acid) and by a biomimetic approach using a commercially available proteolytic enzyme, proteinase K, chosen because of its non-specific behaviour and because it is representative of the enzymes encountered in many deposit-feeding organisms. Different experimental conditions for enzymatic extraction were investigated and the enzyme was employed both untreated and thermally denatured to study metal release mechanisms (i.e. enzymatic hydrolysis and/or complexation). Extractable protein content was also measured to verify possible correlations with some metals.Considering the two sediments used in this work, the available fraction evaluated with a biomimetic approach was often significantly lower than the fraction released by the first step of selective extractions, as expected. Moreover a good correlation was found between the fraction of metal available to the enzyme and enrichment factors in the gut fluids of deposit-feeding organisms, suggesting that enzymatic approach is qualitatively more reliable than chemical extraction in replicating the mechanism of metal mobilization that occurs in the guts of deposit-feeders.