Ecophysiology and Phylogeny of Vacuolate, Nitrate-Accumulating Sulfur Bacteria

University Of California-Davis, Davis CA

Investigators

Abstract

Vacuoles, fluid-filled membrane-bound spaces within cells, may serve a variety of functions, including storage. Until recently, it has been categorically stated that vacuoles do not occur in bacteria. In recent studies, however, including NSF-supported studies undertaken in the PI's laboratory, vacuoles have been discovered in large bacteria belonging to the genera Beggiatoa, Thioploca and Thiomargarita. These structures, which comprise roughly 80% of the biovolume of these cells, are believed to be involved in the accumulation and storage of nitrate, which averages 0.1 to 0.5 molar (10,000-fold above ambient levels) computed over the entire cell volume. The stored nitrate can serve as an oxidant, thereby being converted to ammonia while allowing the generation of biologically useful energy driven by the anaerobic oxidation of hydrogen sulfide. These nitrate-accumulating bacteria form a single evolutionary cluster, and, judging from field studies of others, they may play a quantitatively important role in the marine nitrogen cycle. The current research will examine the ecophysiology and metabolism of these bacteria, focusing on Beggiatoa, and will extend the study of vacuolate bacteria to attached, filamentous, Thiothrix-like strains that dominate the biomass on a variety of surfaces at deep-sea hydrothermal vents off the coast of Washington and elsewhere. Studies of the attached forms will be undertaken to determine the evolutionary relationships between them and the previously identified vacuolate bacteria, and to determine whether the vacuole serves the same physiological role in the diverse species. To delve more deeply into the ecophysiology of vacuolate, nitrate reducing, sulfide-oxidizing bacteria, Beggiatoa populations readily available from sulfide-rich seeps (900 m depth) in Monterey Canyon will be studied in detail. Their ability to use both oxygen and nitrate as oxidants as well as their ability to use organic vs. inorganic sources of cell carbon will be determined. Because these bacteria dominate the upper 15 centimeters of the Monterey Canyon seep sediments, these studies will yield information on the bacteria's in situ metabolism through analyses of their abundance vs. depth in conjunction with a study of sediment porewater profiles of sulfide, sulfate and ammonia and the influence of certain metabolic inhibitors on sediment processes. No vacuolate bacterium has ever been grown in pure culture, but the populations under study can be harvested in sufficient purity to allow chemical, enzymological and molecular characterizations. The investigators will attempt to clone the genes encoding the central metabolic processes in Beggiatoa and will employ molecular procedures to determine whether the cloned genes were actually derive from Beggiatoa. Analyses of this type may eventually make it possible to understand the genetic basis for vacuole formation or gigantism in bacteria. This offers the long-term possibility of engineering these seemingly desirable traits into other cultivated bacteria