Anaerobic sulfide oxidation with nitrate by a freshwater Beggiatoa enrichment culture

A lithotrophic freshwater Beggiatoa strain was enriched in O-2-H2S gradient tubes to investigate its ability to oxidize sulfide with NO3- as an alternative electron acceptor. The gradient tubes contained different NO3- concentrations, and the chemotactic response of the Beggiatoa mats was observed. The effects of the Beggiatoa sp. on vertical gradients of O-2, H2S, pH, and NO3- were determined with microsensors. The more NO3- that was added to the agar, the deeper the Beggiatoa filaments glided into anoxic agar layers, suggesting that the Beggiatoa sp. used NO3- to oxidize sulfide at depths below the depth that O-2 penetrated. In the presence of NO3- Beggiatoa formed thick mats (> 8 mm), compared to the thin mats (ca. 0.4 mm) that were formed when no NO3- was added. These thick mats spatially separated O-2 and sulfide but not NO3- and sulfide, and therefore NO3- must have served as the electron acceptor for sulfide oxidation. This interpretation is consistent with a fourfold-lower O-2 flux and a twofold-higher sulfide flux into the NO3--exposed mats compared to the fluxes for controls without NO3-. Additionally, a pronounced pH maximum was observed within the Beggiatoa mat; such a pH maximum is known to occur when sulfide is oxidized to S-0 with NO3- as the electron acceptor.