Anaerobic Sulfide Oxidation with Nitrate by a Freshwater Beggiatoa Enrichment Culture

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Abstract
A lithotrophic freshwater Beggiatoa strain was enriched in O 2 -H 2 S gradient tubes to investigate its ability to oxidize sulfide with NO 3 − as an alternative electron acceptor. The gradient tubes contained different NO 3 − concentrations, and the chemotactic response of the Beggiatoa mats was observed. The effects of the Beggiatoa sp. on vertical gradients of O 2 , H 2 S, pH, and NO 3 − were determined with microsensors. The more NO 3 − that was added to the agar, the deeper the Beggiatoa filaments glided into anoxic agar layers, suggesting that the Beggiatoa sp. used NO 3 − to oxidize sulfide at depths below the depth that O 2 penetrated. In the presence of NO 3 − Beggiatoa formed thick mats (>8 mm), compared to the thin mats (ca. 0.4 mm) that were formed when no NO 3 − was added. These thick mats spatially separated O 2 and sulfide but not NO 3 − and sulfide, and therefore NO 3 − 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 NO 3 − -exposed mats compared to the fluxes for controls without NO 3 − . 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 NO 3 − as the electron acceptor.