Interactions of thiols with sedimentary particulate phase: studies of 3-mercaptopropionate in salt marsh sediments from Shelter Island, New York

Abstract

3-Mercaptopropionate (3-MPA) is a major hydrophilic thiol in salt marsh sediments from Shelter Island, New York. It is associated mainly with the sedimentary particulate phase, although the mechanism of its binding has not been clear. In this study, we show that humic substances coated on clay minerals are involved in binding 3-MPA through di- and poly-sulfide linkages. With increasing depth, the abundance of 3-MPA in the fulvic acid fraction decreases with a concomitant increase in the humic acid fraction, suggesting that, with depth, fulvic-bound 3-MPA is converted into humic-bound 3-MPA. We modeled the depth variation with a multi-G model which suggests that there are two types of reactive components in both fulvic acid-bound and humic acid-bound 3-MPA, one changing about 10 times more rapidly than the other. These two 3-MPA pools probably reflect different types of sulfur bridging, with the polysulfide-bridged pool changing more rapidly than the disulfide-bridged pool. As free thiol, 3-MPA is attacked relatively rapidly by anaerobic bacteria, its degradation following firstorder kinetics. However, in the sediment core we studied, humic-bound 3-MPA was present throughout the sediment column, down to 22 cm, corresponding to an age of about 90 years. These results suggest that thiols can escape bacterial degradation in sediments by binding to humic substances. The sedimentary distribution of 3-MPA is an important illustration of the interplay of biological and chemical processes in the fate of thiols in anoxic marine sediments.