The sulfur cycle is an important, although understudied facet of today's modern oxygen minimum zones (OMZs). Sulfur cycling is most active in highly productive coastal OMZs where sulfide-rich sediments interact with the overlying water column, forming a tightly coupled benthic-pelagic sulfur cycle. In such productive coastal systems, highly eutrophic and anoxic conditions can result in the benthic release of sulfide leading to an intensification of OMZ-shelf biogeochemistry. Active blooms involving a succession of sulfide-oxidizing bacteria detoxify sulfide and reduce nitrate to N2, while generating nitrite and ammonium that augment anammox and nitrification. Furthermore, the abiotic interactions of sulfide with trace metals may have the potential to moderate nitrous oxide emissions. While sulfide/sulfur accumulation events were previously considered to be rare, new evidence indicates that events can develop in OMZ shelf waters over prolonged periods of anoxia. The prevalence of these events has ramifications for nitrogen loss and greenhouse gas emissions, including other linked cycles involving carbon and phosphorous. Sulfur-based metabolisms and activity also extend into the offshore OMZ as a result of particle microniches and lateral transport processes. Moreover, OMZ waters ubiquitously host a community of organosulfur-based heterotrophs that ostensibly moderate the turnover of organic sulfur, offering an exciting avenue for future research. Our synthesis highlights the widespread distribution and multifaceted nature of the sulfur cycle in oceanic OMZs.
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The authors would like to thank Gesa Eirund for plotting of the World Ocean Atlas data 2013. The authors are thankful to Frank Stewart and three anonymous reviewers for their insightful comments and discussion. This work was supported by the MPG and the Sonderforschungsbereich (SFB754) GEOMAR, Kiel as well as the Villum Foundation (DEC) and the European Research Council (NOVAMOX to B.T., Grant no. 695599). The stipend of C.M.C was also supported by a Natural Sciences and Engineering Research Council of Canada (NSERC) scholarship.
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