Diversity of Halophilic Sulfur-Oxidizing Bacteria in Hypersaline Habitats

The culturable diversity of halophilic obligately chemolithoautotrophic sulfur-oxidizing bacteria (SOB) in various aquatic hypersaline habitats, such as inland chloride–sulfate lakes, sea solar saltern and deep-sea salt brines, was found to be unexpectedly high. Six different groups of halophilic SOB belonging to the Gammaproteobacteria were found. Two groups of moderately halophilic strictly aerobic SOB dominated at 2 M NaCl, including representatives of the genus Halothiobacillus (at fully aerobic conditions) and the genus Thiomicrospira (at microoxic conditions). In a few cases, halothiobacilli also dominated at 4 M NaCl. Under denitrifying conditions at 2 M NaCl, moderately halophilic and facultatively anaerobic SOB capable of complete denitrification of nitrate were found. They are a member of a new genus, “Thiohalomonas”, with closest relatives among marine thiodenitrifyers. At moderate salinity and with thiocyanate as a substrate, a pure culture of moderately halophilic SOB capable of growth with thiocyanate and thiosulfate up to 4 M NaCl was obtained, and these SOB are a member of a new genus “Thiohalophilus” distantly related to the genus Thiomicrospira. Two groups of extremely halophilic SOB growing between 2 and 4 M NaCl with an optimum at 3 M NaCl dominated in enrichments at 4 M NaCl. The group of obligately aerobic extreme halophiles, members of a new genus “Thiohalospira”, are related to the Ectothiorhodospiraceae, and facultatively anaerobic nitrate-reducing extreme halophiles, members of a new genus “Thiohalorhabdus”, are distantly related to the genus Acidithiobacillus.