Microbial ecology of Kenyan soda lakes.

A combination of geological, geographical and climatic conditions; favour the evolution of stable alkaline environments, such as the soda lakes of East African Rift Valley. These lakes are often saline as well as alkaline. Soda lakes are potentially the best sources of alkaliphilic microoganisms, but they are virtually unexplored. The few studies done indicate that soda lakes have very high primary productivity, due to the dense population of phototrophic microorganisms, either cyanobacteria or phototrophic eubacteria or both. Soda lakes are therefore, likely to support a large population of organotrophic bacteria. A quantative study of major nutrients, cyanobacteria and organotrophic bacteria was undertaken from Oct. 1988 to Sept 1989 on Lakes Bogorla, Elmenteita, Nakuru and to a lesser extent Lake Magadi. The cyanobacteria, especially Spirulina platensis were the major primary producers in the less saline lakes (Bogorla, Nakuru and Elmenteita), while the photosynthetic bacteria probably Ectothiorhodospira are the major primary producers in the more saline Lake Magadi. The algal population was significantly limited by phosphate, nitrogen and conductivity levels. The population of alkaliphilic organotrophic bacteria in the Kenyan soda lakes was 10s CFU/ml (viable count) and total bacterial count of l0'.;-l0'.;bacteria/ml. The organotrophic bacterial populationwas also limited by total phosphate, total nitrogen and conductivity levels, and was determined more by the water chemistry than by organic carbon availability. Majority of the aerobic organotrophic bacteria were Gram-negative comprising of six distinct clusters when analysed using the procedures of numerical taxonomy. These Gram-negative alkaliphilic bacteria formed clusters distinct from any of the known gram-negative bacteria tested and are likely to be totally new organisms. Chemotaxonomic studies showed that these organisms possesed ubiquinones with 7 or 8 isoprene units, and the polar lipids lacked glycolipids but had large amounts of phosphclipids. The % mol G+C of the gram-negative alkaliphiles ranged from 53.1 to 65.0 (Tm). Organisms in the same cluster group were related at 60% homology by DNA-DNA hybridization; indicating that the majority of the Gram-negatives may belong to a large supraspecific grouping. The few Gram-positive isolates formed less distinct clusters, and two Gram-positive isolates N12 and N16 formed spores and closely resembled isolates WN11 and WN13 (Weisser & Truper, 1985) from Wadi Natrun which have been positively identified as Bacillus spp. Arcbaeobacteria were not isolated Lakes Bogoria, Elmenteita and Nakuru. In the more saline Lake Magadi, eubacterial haloalkaliphlies dominated at low conductivity and archaeobacterial haloalkaliphiles dominated at high conductivity. A new haloalkaliphilic archaeobacterium which has rod-shaped, gas-vacuolated cells and different polar lipid patterns from the 3 Natronobacterium species already described (Ross, et al, 1985) was isolated from Lake Magadi. A new species, 'Natronobacterium vacuolata' is proposed. A coccoid Gram-positive organism, MU5 was also isolated from Lake Magadi, and resembled the type species in the genus Natronococcus. However, polar lipid patterns and SDS-PAGE gels show that MU5 is not identical to SP4.