Cabrol Léa auteur aut IRD Capo E. auteur aut IRD Van Vliet D.M. auteur aut IRD Bastiaan von Meijenfeldt F.A. auteur aut IRD Bertilsson S. auteur aut IRD Villanueva L. auteur aut IRD Sánchez-Andrea I. auteur aut IRD Björn E. auteur aut IRD Bravo A.G. auteur aut IRD Heimburger Boavida L.E. auteur aut IRD text journalArticle eng text/pdf reformatted digital access In the global context of seawater deoxygenation triggered by climate change and anthropogenic activities, changes in redox gradients impacting biogeochemical transformations of pollutants, such as mercury, become more likely. Being the largest anoxic basin worldwide, with high concentrations of the potent neurotoxic methylmercury (MeHg), the Black Sea is an ideal natural laboratory to provide new insights about the link between dissolved oxygen concentration and hgcAB gene-carrying (hgc+) microorganisms involved in the formation of MeHg. We combined geochemical and microbial approaches to assess the effect of vertical redox gradients on abundance, diversity, and metabolic potential of hgc+ microorganisms in the Black Sea water column. The abundance of hgcA genes [congruently estimated by quantitative PCR (qPCR) and metagenomics] correlated with MeHg concentration, both maximal in the upper part of the anoxic water. Besides the predominant Desulfobacterales, hgc+ microorganisms belonged to a unique assemblage of diverse previously underappreciated anaerobic fermenters from Anaerolineales, Phycisphaerae (characteristic of the anoxic and sulfidic zone), Kiritimatiellales, and Bacteroidales (characteristic of the suboxic zone). The metabolic versatility of Desulfobacterota differed from strict sulfate reduction in the anoxic water to reduction of various electron acceptors in the suboxic water. Linking microbial activity and contaminant concentration in environmental studies is rare due to the complexity of biological pathways. In this study, we disentangle the role of oxygen in shaping the distribution of Hg-methylating microorganisms consistently with MeHg concentration, and we highlight their taxonomic and metabolic niche partitioning across redox gradients, improving the prediction of the response of marine communities to the expansion of oxygen-deficient zones. specialized 032MILPHY 064GEOCHI 032CLIMET 8 4 en ligne [18 ] 2023 2379-5077 https://www.documentation.ird.fr/hor/fdi:010093633 10.1128/msystems.00537-23 2379-5077 [F B010093633] https://www.documentation.ird.fr/hor/fdi:010093633 https://horizon.documentation.ird.fr/exl-doc/pleins_textes/2025-05/010093633.pdf IRD - Base Horizon / Pleins textes 2025-04-10 2025-05-16 fdi:010093633 fre