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Sarah C. McHattont, Azeem Ahmad1, James P. Barry2 and Douglas C. Nelson'
'Section of Microbiology, University of California, Davis, CA 95616, USA
2Monterey Bay Aquarium Research Institute, Moss Landing, CA 95039, USA
Mats of conspicuous sulfur bacteria belonging to the genus Beggiatoa have been found in Monterey Canyon cold seeps. Individual disk-shaped cells, roughly 75 micrometers in diameter, form filaments one to several centimeters long. These bacteria have been recently described as sulfur oxidizing, nitrate respiring autotrophs that contain a central vacuole accounting for roughly 80% of the cellular biovolume. Nitrate is concentrated 400~fold over ambient concentration and is presumably stored in the vacuole. In these traits, as well as the extraordinary biomass achieved in surf~ial marine sediments, these Beggiatoa filaments show striking parallels with those of the extensive Thioploca mats found off the western coast of South America. Ribosomal RNA sequence analyses of Beggiatoa spp and Thioploca spp. support the view that the nitrate-accumulating, vacuolate phenotype is monophyletic. In cores from Monterey Canyon seeps, the vertical distribution of the Beggiatoa filaments is bimodal with dense populations in the top 3 cm of sediment and between S to 7 cm. Studies of sediment chemistry over the depths occupied by Beggiatoa filaments have revealed pronounced sulfide depletion and an ammonia maximum. In vitro enzyme assays for a nitrite reductase (NO2 ~ NH4+ ) yielded specific activities of 0.5 to 16 micromoles NH4+ formed/min/mg protein in washed Beggiatoa samples. This, coupled with a previous demonstration of high activity of a membrane-assodated nitrate reductase (NONE NO2-), strongly suggests that these bacteria are using nitrate stored in the vacuole to oxidize sulfide and that they are forming NH4+ as the product. At sediment depths well below any possible penetration of oxygen, this postulated use of internally stored nitrate provides a likely explanation for the observed sulfide depletion. Our proposal that nitrate respiration by vacuolate, filamentous, sulfur bacteria results in a'Tunonia as a waste product is in contrast to the literature view that Thioploca species are denitrifiers
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