The ultimate fate of hydrothermal sulphides on the seafloor depends on the nature and rate of abiotic and microbially catalysed reactions where sulphide minerals are exposed to oxic seawater. This study combines organic and inorganic geochemical with microbiological measurements across a suboxic transition zone of highly altered sulphidic sediments from the Trans-Atlantic Geotransverse hydrothermal field to characterize the reaction products and microbial communities present. There is distinct biogeochemical zonation apparent within the sediment sequence from oxic surface layers through a suboxic transition zone into the sulphide material. The microbial communities in the sediment differ significantly between the biogeochemical horizons sampled, with the identified microbes inferred to be associated with Fe and S redox cycling. In particular, Marinobacter species, organisms associated with circumneutral Fe oxidation, are dominant in a sulphide lens present in the lower core. The dominance of Marinobacter-related sequences within the relict sulphide lens implies that these organisms play an important role in the alteration of sulphides at the seafloor once active venting has ceased.