Biology of Green Sulfur Bacteria

Abstract Green sulfur bacteria have gained much attention because of a number of highly interesting features including unique structures of the photosynthetic apparatus and the presence of chlorosomes as very powerful light antenna that can capture minute amounts of light. This has important ecological consequences, because the efficient light‐harvesting determines the ecological niche of these bacteria at the lowermost part of stratified environments where the least of light is available. Furthermore, the strict dependency on photosynthesis to provide energy for growth and the obligate phototrophy of green sulfur bacteria together with their characteristic sulfur metabolism have provoked much interest in their physiology, ecology and genomics. The oxidation of sulfide as their outmost important photosynthetic electron donor involves the deposition of elemental sulfur globules outside the cells and separates the process of sulfide oxidation to sulfate into two parts. This is the basis for stable syntrophic associations between green sulfur bacteria and sulfur‐ and sulfate‐reducing bacteria in which the sulfur compounds are recycled. The green sulfur bacteria are distantly related to other bacteria and represent the phylum Chlorobi, though the known representatives are taxonomically treated as Chlorobiaceae with the generaChlorobium,Chlorobaculum,ProsthecochlorisandChloroherpeton. Key Concepts: Green sulfur bacteria depend on light for life due to their obligate phototrophic metabolism. Green sulfur bacteria perform a highly efficient photosynthesis due to the presence of light harvesting organelles, the chlorosomes, which are filled with special bacteriochlorophyll molecules. Green sulfur bacteria inhabit the lowermost part of the photic environments due to their efficient light capture. Green sulfur bacteria inhabit the lowermost part of the chemocline in the stratified environment due to their sensitivity to oxygen, their high sulfide tolerance and their dependence in light. Green sulfur bacteria are important drivers of oxidation of reduced sulfur compounds in the stratified, sulfide‐containing environment receiving low irradiation.