Microbial sulfur cycling across a 13 500-year-old lake sediment record

Abstract. The sulfur cycle is very important in lake sediments, despite the much lower sulfate concentrations in freshwater than seawater. To date, little is known about the formation and preservation of organic and inorganic sulfur compounds in such sediments, especially in the sulfate-depleted subsurface. Here, we investigated the fate of buried S-compounds down to 10 m sediment depth, which represents the entire ∼ 13.5 kyr sedimentary history of the sulfate-rich alpine Lake Cadagno. Chemical profiles of sulfate and reduced sulfur reveal that sulfate from lake water is depleted at the sediment surface with the concomitant formation of iron sulfide minerals. An underlying aquifer provides a second source of sulfate and other oxidants to the deepest and oldest sediment layers, generating an inverse redox gradient with ongoing sulfate consumption. The isotopic offsets between pools of humic acid sulfur, acid-volatile sulfur (AVS), and chromium-reducible sulfur (CRS) in both surface and deep sediments suggest differential timing of formation, with sulfide oxidation to sulfur/polysulfides playing an integral role in organic matter sulfurization. Although sulfate is depleted in the central part of the sediment column, dsrB gene libraries reveal a potential for microbial sulfur reduction throughout the sediment column, with sequences in sulfate-depleted layers being dominated by Chloroflexota. Collectively, our data indicate an active sulfur cycle that is driven by uncultivated microorganisms in deep sulfate-depleted sediments of Lake Cadagno.