The alkaliphilic microbial community and its functional diversity

The hypothesis that soda lakes provide a refugium for relict microbial comm unities of epicontinental origin was put to test by studying the functional diversity of such communities. Alkaliphilic communities develop in water a lmost saturated with soda and contain representatives of major trophic grou ps. Primary organic matter producers are represented by various cyanobacter ia having optimal growth regions defined in pH-alkalinity-mineralization co ordinates. Eukaryotic microalgae occur merely as minor but persistent compo nents. Investigation of the anaerobic degradation pathway led to the descri ption of several new genera, Natroniella, Tindallia, and Natronoincola, whi ch participate primarily in acetogenesis. The hydrogen sink in the communit y is provided by alkaliphilic hydrogenotrophic sulfate reducers Desulfonatr onovibrio and Desulfonatronum. The anaerobic oxidative filter depends on a phototrophic Ectothiorhodospiraceae with a new genus Thiorhodospira and a n ew heliobacterium Heliospira. The aerobic filter comprises the H(2-)utilizi ng and sulfur-oxidizing bacterium Natronohydrogenobacter thiooxidans, heter otrophic and obligately autotrophic sulfur-oxidizing bacteria, methane-oxid izing Methylobacter alcaliphilus and alkaliphilic Methylomicrobium sp. able to oxidize methane and ammonia. With the addition of proteobacteria, natro no archaebacteria, spirochetes, and the alkaliphilic thermatogas recently d escribed, the alkaliphilic community attained sufficient diversity to repre sent all major functional trophic groups required to close up major biogeoc hemical cycles as well as the main branches of the phylogenetic tree of the prokaryotes. This is enough to correspond to an evolutionarily open commun ity able to represent a likely center of the diversification of inland micr oflora.