9513962 Nelson The surfaces of a variety of marine sediments are dominated by colorless sulfur bacteria of the genera Thioploca and Beggiatoa. Although few of these bacteria have been grown in pure culture, the best characterized representatives can multiply at the oxic-anoxic interface where there are steep gradients of hydrogen sulfide and oxygen, which can serve as their oxidizable substrate and oxidant, respectively. There is emerging evidence that these bacteria are also autotrophs, i.e. they can synthesize their cell material starting with carbon dioxide as their sole carbon source. In some environments with little or no available oxygen, the dominant representatives of these bacteria are extremely large and they contain a membrane-bound central vacuole, comprising roughly 80% of cell volume. These are the only bacteria for which this structure has been reported. In dense samples from three different locations, vacuolate Beggiatoa and Thioploca were very recently demonstrated to contain nitrate in concentrations 10,000-fold higher than in their surrounding habitat. This proposal postulates that these bacteria use this internal nitrate as a terminal electron acceptor facilitating their respiration of soluble sulfide and/or organic compounds. Proposed studies include microelectrode and chemical gradient analysis of in situ consumption rates of potential electron donors (sulfide and organics) and electron acceptors (nitrate and oxygen). Assays of diagnostic enzymes and studies using radiolabelled organic and inorganic carbon compounds will help elucidate the relative roles of these compounds in their metabolism. Studies of the time-course of utilization of internal nitrate and the resultant products, coupled with various enzyme assays, will measure the capacity these bacteria have for denitrification. Additionally, using pure cultures in spatially structured cultu re media the relative rates by which these bacteria use organic and inorganic energy and carbon sources sill be assessed. Vacuolate Beggiatoa are proposed as a model of an important link between the vigorous sulfur cycle of eutrophic marine sediments and the nitrogen cycle. If they and Thioploca prove to be denitrifiers, they will be the first example of an ecologically dominant bacterium capable of removing combined nitrogen from a marine ecosystem.
