Abstract
Studies on microbial sulfur cycling in marine sediment have primarily centered on the cycling of inorganic sulfur. The microbial diversity underlying the cycling of organosulfur compounds is largely unexplored. In this study, we present the first quantification of dissolved organic sulfur (DOS) microbial assimilation in marine surface sediments using (13)C-DOS quantitative DNA stable isotope probing (qSIP). We sampled marine sediment from 493 m water depth on the Puerto Rico continental slope, measured (13)C-assimilation from two DOS substrates ((13)C-taurine and (13)C-methionine), and compared the (13)C-DOS assimilation to (13)C-glucose uptake. Taurine utilization was confined to bacteria, whereas methionine was degraded by bacteria and archaea, including methanogenic Methanococcoides. Globally widespread uncultivated clades of Gammaproteobacteria and Deltaproteobacteria were the main drivers of DOS cycling and exhibited increased assimilation of carbon from taurine and methionine, compared to glucose. Only one operational taxonomic unit (OTU) affiliated with Neptuniibacter was found to assimilate taurine and methionine, but not glucose, implying that microbes exclusively utilizing both DOS substrates as a carbon source in marine sediments are rare. Still, a substantial number of bacterial taxa exhibited a higher assimilation of (13)C from taurine or methionine, compared to glucose, indicating their preference for both DOS substrates over glucose as a carbon source in the sediment. These results represent the first quantitative assessment of organosulfur cycling from taurine and methionine by uncultivated microbes in a marine benthic environment.