8. Large Sulfur-Oxidizing Bacteria at Gulf of Mexico Hydrocarbon Seeps

Large sulfide-oxidizing bacteria occur as visually conspicuous microbial mats in a wide range of sedimentary habitats, including estuarine and coastal marine sediments, and deep-sea vents and seeps. The microbial mats spread on the surface of sulfide-rich sediments, thus intercepting and oxidizing sulfide that diffuses upwards from the underlying sulfate reduction zone, or, that reaches the surface by advection of reduced fluids. The first intermediate of bacterial sulfide oxidation, elemental sulfur, is stored within the cytoplasm as globules, and serves as energy reserve. Large sulfide oxidizers have a wide metabolic repertoire, including autotrophic carbon fixation, sulfide and sulfur oxidation to sulfuric acid, nitrate reduction to ammonia or nitrogen gas, as well as polyphosphate storage and release causing local phosphate supersaturation and precipitation. Large sulfur-oxidizing bacteria are widespread at hydrocarbon seeps in the Gulf of Mexico, where numerous types with different morphology, phylogenetic affiliation, and physiology have been documented. In this chapter, we provide an overview of large sulfur-oxidizing bacteria in the Gulf of Mexico. We also incorporate previously unpublished sequencing data for selected filaments, and include recent observations of new morphological variants, including one that resembles sheathed marine Thioploca spp., but which shows a distinguished, unique branching morphology.