The Genera Beggiatoa and Thioploca
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The Genera Beggiatoa and Thioploca ANDREAS TESKE AND DOUGLAS C. NELSON
Introduction Filamentous sulfur-oxidizing bacteria of the genera Beggiatoa and Thioploca are some of the largest and most conspicuous bacteria in nature (Schulz and Jørgensen, 2001). Their white or yellow color, their filamentous morphology, the large width and length of their filaments, and their growth pattern in flocs and mats on sediment surfaces makes them highly conspicuous even to the unaided eye. The two genera are distinguished by a single morphological character: Thioploca filaments occur in bundles surrounded by a common sheath, whereas Beggiatoa filaments do not form this structure and occur as individual filaments. All Beggiatoa and Thioploca strains have the ability to oxidize sulfide to elemental sulfur that is stored as intracellular sulfur globules, which make the cells highly refractory and conspicuous under the microscope. This characteristic, together with the absence of photosynthetic pigments, has distinguished the genera Beggiatoa and Thioploca as filamentous members of the “colorless sulfur bacteria” from other filamentous bacteria, for example the cyanobacteria or nonsulfuroxidizing heterotrophs such as Cytophaga and Flexibacter. Contrary to earlier assumptions (Reichenbach and Dworkin, 1981), there is no close evolutionary relationship between other gliding filamentous bacteria, and Beggiatoa and Thioploca. The metabolic spectrum in Beggiatoa ranges from obligate autotrophy with sulfide and other reduced sulfur compounds as energy source, so far found in marine strains only (Nelson and Jannasch, 1983), to seemingly strict heterotrophy based on organic carbon and energy sources (Nelson and Castenholz, 1981a). The Thioploca spp. that have been characterized so far, have autotrophic potential (Otte et al., 1999). Biogenic or geothermal sulfide is oxidized to elemental sulfur, and subsequently in autotrophic strains to sulfate, to provide energy and reducing power for CO2 fixation and biomass synthesis. The electron acceptor for of sulfide oxidation is oxygen (Nelson et al., 1986b) or in some cases
nitrate (McHatton et al., 1996; Otte et al., 1999). The requirement for oxygen and sulfide, two mutually highly reactive chemical species, forces Beggiatoa and Thioploca into a specialized niche at naturally occurring oxic/anoxic interfaces where these bacteria compete efficiently with chemical sulfide oxidation (Jørgensen, 1982). Beggiatoa grows at oxic/anoxic interfaces, where sulfide as electron donor and oxygen as electron acceptor are present simultaneously (Nelson et al., 1986b). Large marine Beggiatoa also adapt to fluctuating gradients by increased intracellular storage capacity for oxidants (McHatton et al., 1996). Thioploca filaments can bridge spatially separated pools of oxidant and reductant by moving up and down and adjusting their position in a sediment-embedded sheath (Hüttel et al., 1996). Beggiatoa and Thioploca are effective mat-forming microorganisms; their mats may be
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