Sulfur Metabolites in the Pelagic Ocean
Mary Ann Moran and Bryndan P. Durham
Marine microbes play crucial roles in Earth’s element cycles through the production and consumption of organic matter. One of the elements whose fate is governed by microbial activities is sulfur, an essential constituent of biomass and a critical player in climate processes. Already well-studied in the ocean in its inorganic forms, organic sulfur compounds are now emerging as important chemical links between marine phytoplankton and bacteria. The high concentration of inorganic sulfur in seawater, which can be readily reduced by phytoplankton that are not limited for energy, provides an easy source of sulfur for biomolecule synthesis. Mechanisms such as exudation and cell lysis release these phytoplankton-derived sulfur metabolites into seawater, from which they are rapidly incorporated by marine bacteria and archaea. Energy-limited bacteria use scavenged sulfur metabolites as substrates or for the synthesis of vitamins, co-factors, signaling compounds, and antibiotics. Here we review current knowledge of the sulfur metabolites released into and taken up from the marine dissolved organic matter pool by microbial producers and consumers, and the ecological links facilitated by their diversity in structures, oxidation states, and chemistry.