Distribution of the thiols glutathione and 3-mercaptopropionic acid in Connecticut lakes

The spatiotemporal chemical characteristics of the water column in Linsley Pond, a freshwater lake in Connecticut, were studied from just before its stratification in April through the fall turnover in December. Two low-molecular-weight thiols, glutathione and 3-mercaptopropionic acid (MPA), were detected at nanomolar concentrations in the water column. GSH was detected (average concentrations were ca. 5 nmol L21 in the particulate phase and ca. 3 nmol L21 in the dissolved phase) in surface and near-surface waters only and covaried with chlorophyll a. Both particulate and dissolved MPA were measured in the oxic and anoxic regions. In the metalimnion, MPA concentrations were greater than in the oxic water layers above. At times, the MPA concentrations in the metalimnion were as high as those found in the anoxic hypolimnion. The MPA present at different depths in the Linsley Pond water column is most likely produced through dissimilar mechanisms. Throughout the water column, MPA may be a product of the metabolic degradation of sulfur-containing organic compounds; however, in the hypolimnetic waters, dissolved sulfide may play an important role in MPA formation through abiotic nucleophillic addition to unsaturated functionalities in dissolved organic matter. Parallel laboratory experiments were performed to assess the importance of MPA in copper speciation. The results confirmed that despite nanomolar levels in Linsley Pond, MPA does not play an important role in copper speciation. A survey of six additional lakes in Connecticut was made for detectable thiols. Only MPA was detected in four of these lakes, and its distribution was similar to that in Linsley Pond. Thiols play many important biochemical roles. The pKa of the sulfhydryl group generally ranges between 9–11, and is therefore of relevance to intercellular pH. Additionally the sulfhydryl group can be easily oxidized to form disulfide bonds that can be converted back to corresponding thiols through enzymatic activities (Meister and Anderson 1983). Such properties make thiols very useful as structural elements in the disulfide bridges of proteins, as part of oxidization defense systems, and as regulatory messengers via addition of small molecular weight thiols to proteins (Gilbert 1984). Additionally, the sulfhydryl group in thiol molecules has an extremely high affinity for some transition metal cations (Boulegue et al. 1982). Through the sequestration of these transition metals, thiols may also serve as detoxifying agents under conditions of high metal loading (Meister and Anderson 1983; Grill et al. 1985). The important role that thiols play in the biogeochemical cycling of sulfur in a natural aquatic system is perhaps the biggest reason they have been widely studied (Mopper and Taylor 1986; Tang et al. 2000; Al-Farawati and Van Den Berg 2001). Thiols also serve as intermediates in the formation of many sulfur compounds. For example, cysteine (Cys) and glutathione (c-glutamylcysteinylglycine; GSH) can serve as precursors for environmentally important sulfur gases, such as carbonyl sulfide (Flock et al. 1997). In sediments, thiols readily react to form disulfide and polysulfide bridges (Boulegue et al. 1982). These processes may enhance the cross-linking between organic molecules, serving as a key step in the formation of highmolecular-weight organic matter fractions (Aizenshtat et al. 1995). As strong ligands for transition metals, thiols have been shown to promote the mobilization of metals in aquatic environments through complexation reactions (Cullen et al. 1984). However, because of their extremely reactive nature, the steady state concentrations of thiols in natural waters tend to be low, and therefore their detection has been difficult. Based on a review of the available literature, Cys and GSH, two sulfur-containing amino acids, and 3-mercaptopropionic acid (MPA) have been among the most frequently detected thiols in natural aquatic environments (Mopper and Taylor 1986; Tang et al. 2000; Al-Farawati and Van Den Berg 2001). Previous work on naturally occurring thiols has concentrated on their roles in marine environments. Studies have shown the presence of significant amounts of thiols in anoxic marine waters and sediments (Boulegue et al. 1982; Dyrssen et al. 1985; Mopper and Taylor 1986). Thiols at submicromolar levels have been measured in the Black Sea 1 Corresponding author. Acknowledgments We thank Benjamin Twining for his assistance with field work and his valuable comments on this manuscript. We also thank two anonymous reviewers whose thoughtful comments have helped make this manuscript a better one. This study was funded by NSF Hydrologic Sciences: EAR-0337201. Limnol. Oceanogr., 51(6), 2006, 2763–2774 E 2006, by the American Society of Limnology and Oceanography, Inc.