Dynamic cycling and redox transformations of dissolved organic sulfur in the surface ocean

Abstract The marine dissolved organic sulfur (DOS) pool contains both labile metabolites that are rapidly cycled in the surface ocean and compounds that may persist for thousands of years in the marine environment. Here, we describe the sulfur speciation of marine solid‐phase extracted DOS (DOS SPE ) from the North Atlantic surface ocean, revealing a diversity of oxidation states that include alkyl sulfides, disulfides, aromatics, sulfonates, and sulfate esters. As dissolved organic carbon concentrations decrease with depth in the water column, relative abundances of reduced sulfur species decrease while those of oxidized sulfur species increase, indicating preferential remineralization of reduced organic sulfur (OS) compounds. Although microorganisms are considered to be the origin of a vast majority of marine DOS SPE , the DOS SPE produced by cultures of phototrophs and heterotrophic bacteria is compositionally distinct and 34 S‐enriched relative to environmental DOS SPE . Ultimately, mechanisms spanning different timescales, including microbial reworking, uptake, and photochemical oxidation transform labile DOS in the surface ocean, and may generate recalcitrant compounds that contribute to the long‐lived dissolved organic matter pool. The oxidation states of OS function as powerful tools for tracking the sources, sinks, and transformations of organic matter in complex biogeochemical systems.