Microbial Diversity of Soda Lakes' Life in a Depression
Brian JONES*a, William GRANTb, Andrew DUCKWORTHb and Gerald OWENSONb
a Genencor International BV, PO BOX 642, 2600 AP, Delft, The Netherlands
b Department of Microbiology & Immunology, University of Leicester, University Road, Leicester LE1 9HN, UK
Soda lakes are highly alkaline extreme environments which form in closed drainage basins in arid regions exposed to high evaporation rates. The soda lakes of the East African Rift Valley are typical examples of permanent or semi-permanent standing bodies of water with pH's in the range 8 to > 12. Due to a scarcity of alkaline earth cations (Mg2+ and Ca2+) in the petrology of the water catchment areas surface waters are enriched in CO32- and C1-. The major dissolved anions are typically present in molar concentrations and may exceed saturation levels whereupon significant evaporite deposits composed of complex carbonate salts ('trona') are formed.
Photosynthetic primary production is probably the driving force behind nutrient recycling in soda lake systems. Contrary to general perceptions of restricted diversity in extreme environments, alkaline soda lakes harbour large communities and a rich diversity of microbes, including examples from many of the major taxa and representing all 3 domains of life. Systematic studies (1,2), have shown that the microbes are alkaliphilic or at least alkali-tolerant and many appear to represent separate alkaliphilic lineage's indicating they may have evolved separately within the alkaline environment. Cultivated diversity appears more restricted in the hypersaline, alkaline soda lakes than in the more dilute waters. Even in the more dilute lakes community structure seems to vary due to local conditions and seasonal variations leading to fluctuations in total salinity's, even though the pH remains fairly constant due to a complex buffering.
Although aquatic environments are the visually dominating aspect of the soda lake landscape, even to the casual observer it is obvious that other alkaline biotopes exist within their environs. Seasonal fluctuations in water level produce littoral muds and soils of varying water content due to capillary assisted evaporation. Such processes inevitably have a selective action resulting in a shift in microbial community structure. In particular, spore-forming species such as Bacillus may be subject to segregation. These organisms have enormous potential for biotechnology and the screening of Bacillus isolates has revealed new endo-cellulases which are useful detergent additives in domestic laundry applications.
