Hydrobiologia 522: 235–248, 2004. © 2004 Kluwer Academic Publishers. Printed in the Netherlands. 235 Prokaryotic communities of the north-eastern Mongolian soda lakes Dimitry Yu. Sorokin 1,4 , Vladimir M. Gorlenko 1 , Bair B. Namsaraev 2 , Zorigto B. Namsaraev 1 , Anatoly M. Lysenko 1 , Bulat Ts. Eshinimaev 3 , Valentina N. Khmelenina 3 , Yuri A. Trotsenko 3 & J. Gijs Kuenen 4 1 Institute of Microbiology, Russian Academy of Sciences, Prospect 60-let Octyabrya 7/2, 117811 Moscow, Russia Fax: (7095)1356530.E-mail: soroc@inmi.da.ru; D.Y.Sorokin@tnw.tudelft.nl 2 Buryat Institute of Biology, Siberian Branch of the Russian Academy of Sciences, Ulan Ude, Russia 3 Institute of Biochemistry and Physiology of Microorganisms, Russian Academy of Sciences, Pushchino, Moscow region, Russia 4 Kluyver Laboratory for Biotechnology, Delft University of Technology, Delft, The Netherlands Received 7 May 2002; in revised form 17 December 2003; accepted 17 December 2003 Key words: Mongolian soda lakes, hypersaline, haloalkaliphilic bacteria, methanotrophic bacteria, sulfur-oxidizing bacteria, phototrophic bacteria Abstract We have studied the activity and composition of several geochemically significant physiological groups of bacteria in more than twenty alkaline salt lakes of the north-east Mongolia steppe with water salinity from 3 to 390 g l −1 and pH values ranging from 9.0 to 10.6. Active and diverse microbial communities have been found in most of the lakes. The methanotrophic bacteria were represented by the Type I members. Among the culturable forms of sulfur-oxidizing bacteria obligately chemolithoautotrophic and haloalkaliphilic representatives of the genera Thioalkalimicrobium and Thioalkalivibrio were detected in the sediments at high numbers (up to 10 6 cells ml −1 ). The largest population of anaerobic phototrophic bacteria was represented by purple sulfur bacteria of the Ectothi- orhodospiraceae family. Salinity was the key factor in determining the activity and the composition of the microbial communities. The most diverse and active prokaryotic populations, including aerobic and anaerobic phototrophic, methanogenic, methanotrophic, sulfur-oxidizing, sulfate-reducing and nitrifying bacteria, were found in lakes with salinity less than 60 g l −1 . In hypersaline lakes with a salinity >100 g l −1 , the sulfur cycle remained active due to the activity of extremely halotolerant and alkaliphilic sulfur bacteria, while other important functional groups responsible for nitrification and methane oxidation processes were not detected. Overall, the prokaryotic communities of the Mongolian alkaline salt lakes represent an interesting new example of a diverse community of haloalkaliphilic bacteria well adopted to a broad salinity range. Introduction Saline soda lakes represent a specific type of salt lake, which contain a relatively high fraction of sodium car- bonate among the other, pH-neutral, salts. Like other salt lakes, the soda lakes are predominantly located in areas with a semiarid to arid climate, that allow salt accumulation in depressions. The main mechanism of their genesis is leaching of the sodium-containing rocks with high CO 2 -containing ground waters. One of the primary conditions for accumulation of sodium carbonates in water is low Ca and Mg concentrations (Eugster, 1970; Jones et al., 1977). The total salt con- tent and the sodium carbonate fraction in soda lakes varies depending on the local conditions. Despite their high salinity and alkalinity, many shallow soda lakes are highly productive (Melack & Kilham, 1974; Cloern et al., 1983; Javor, 1992). The extreme conditions limit the soda lake’s biota mainly to Prokaryotes. A typical microbial com- munity of hypersaline soda lakes includes haloalkali- philic nitrogen-fixing cyanobacteria (Oremland, 1990;