Denitrification by sulphur oxidizing Beggiatoa spp. mats on freshwater sediments

AT the surface of sulphide-rich marine and freshwater sediments, the gliding filamentous sulphur bacteria Beggiatoa spp. often grow abundantly to form conspicuous white mats^1-6. Beggiatoa may be responsible for the whole benthic consumption of oxygen, converting sulphide to either intracellular elemental sulphur, or to extracellular sulphate^7-9. Various strains of Beggiatoa have shown a capacity for nitrogen fixation and assimilation of ammonium and nitrate¹⁰. We now report that denitrification activity may also be attributed to Beggiatoa spp., which indicates that Beggiatoa mats could have a key role in the benthic cycling of oxygen, sulphur and nitrogen. Microprofiles of nitrate through a Beggiatoa mat on a lake sediment showed that all nitrate taken up from the water phase was consumed within the upper 150 μ m of the mat. Complete denitrification was demonstrated by the conversion of ¹⁵NO^-₃ to ¹⁵N₂ in semi-purified tufts of Beggiatoa, and the coupling of denitrification and sulphide oxidation was indicated by micro-profiles of oxygen and sulphide in pure cultures.