Nitrification in Beggiatoa mats in the Guaymas Basin

Hydrothermal sediments in the Guaymas Basin are covered by microbial mats that are dominated by nitrate-respiring and sulfide-oxidizing Beggiatoa. The presence of these mats strongly correlates with sulfide- and ammonium-rich fluids venting from the subsurface. Since ammonium and oxygen form opposed gradients at the sediment surface, we hypothesized that nitrification is a significant process in these Beggiatoa mats. Using biogeochemical and molecular methods we measured nitrification and determined the diversity and abundance of nitrifiers. Nitrification rates ranged from 74 to 605 µmol N l 1 mat d-1, which exceeded those previously measured in hydrothermal plumes and other deep-sea habitats. Diversity and abundance analyses of archaeal and bacterial amoA genes, archaeal 16S rRNA pyrotags and fluorescence in situ hybridization (FISH) confirmed that ammonia- and nitrite-oxidizing microorganisms were associated with Beggiatoa mats. Intriguingly, we observed cells of bacterial and potential thaumarchaeotal ammonia-oxidizers attached to narrow, Beggiatoa-like filaments. Such a close spatial coupling of nitrification and nitrate respiration in mats of large sulfur bacteria is novel and may facilitate mat-internal cycling of nitrogen, thereby reducing loss of bioavailable nitrogen in deep-sea sediments.