Allochromatium vinosum DSM 180

Light-field micrograph containing intracellular sulfur globules. Photo provided by Hans G. Trüper Bonn.

Anoxygenic purple sulfur bacteria flourish wherever light reaches sulfidic water layers or sediments and often occur as dense accumulations in conspicuous blooms in freshwater as well as in marine aquatic ecosystems. Here they are not only major players in the reoxidation of sulfide produced by sulfate-reducing bacteria in deeper anoxic layers but also important primary producers of fixed carbon (up to 83% of primary production in lakes can be anoxygenic). In spite of the high biogeochemical and environmental importance of these organisms, the only available genome sequence for a purple sulfur bacterium is that for the haloalkaliphilic Halorhodospira halophila SL1, an organism occurring in a rather restricted number of hypersaline alkaline habitats. Sequencing of a purple sulfur bacterium thriving in globally occurring habitats is therefore urgently required. Allochromatium vinosum is an ideal candidate for genome sequencing as it is environmentally abundant and occurs not only in pelagic communities but also in littoral sediments like sandy beaches, salt marches or intertidal mud flats. Furthermore it is arguably the best studied representative of this metabolic group and a huge body of literature exists on enzymology and biochemistry of carbon, sulfur, nitrogen and hydrogen metabolism in this organism. A genome sequence would greatly facilitate interpretation of the existing data and allow to gain a complete understanding of the metabolic network in this and other purple sulfur bacteria. In contrast to Halorhodospira halophila and green sulfur bacteria (for which genome sequences are available), members of the family Chromatiaceae like A. vinosum store sulfur globules inside of the cells when oxidizing sulfide or thiosulfate. They have this trait in common with a large number of environmentally important chemotrophic sulfur oxidizers like Beggiatoa and with sulfur-oxdizing bacterial symbionts of marine animals like Riftia pachyptila. On a biochemical level, oxidative sulfur metabolism of A. vinosum is among the best studied of all bacteria and the organism therefore already serves as a model for sulfur-storing bacteria. Nonetheless, important steps could so far not be clarified and a genome sequence would enable identification of further candidate genes, the function of which will be elucidated by mutational studies. A genome sequence of an environmentally abundant purple sulfur bacterium like Allochromatium vinosum will greatly contribute to our understanding of fundamental carbon cycling mechanisms of the biosphere. Purple sulfur bacteria fix carbon along the Calvin cycle. Anoxygenic photosynthesis depends on reduced inorganic sulfur compounds originating from anaerobic degradation of organic carbon and concomitant sulfide production by sulfate- and sulfur-reducing bacteria. The reducing equivalents in sulfide therefore ultimately stem from carbon already fixed by oxygenic photosynthesis and capture of light energy by anoxygenic photosynthesis compensates for the loss of organic carbon in the anaerobic food chain. In turn, dense accumulations of phototropic sulfur bacteria can feed organic carbon (which would otherwise be lost) into the carbon cycle of toxic water or sediment layers. The role of purple sulfur bacteria in these globally important processes has so far been largely underestimated and not been well studied.