Obligately anaerobic alkaliphiles from Kenya soda lake sediments

During the month of December 1992, an expedition was undertaken to collect anaerobic sediment samples from alkaline lakes of the East African Rift Valley to Kenya. During this expedition, eleven samples were collected from the four non-saline, northern lakes (Lake Bogoria, Lake Nakuru, Lake Elmenteita and Lake Sonachi). A further five samples were obtained from the hypersaline Lake Magadi. Utilising an array of media, the isolation of alkaliphilic, obligate anaerobes, representing one of the major undescribed constituents of the trophic network, was attempted.;Extensive investigations into the sulphate-reducing bacteria (SRB) community of the sediment samples failed to provide pure culture isolates. However, successful enrichments utilising a range of substrates were obtained. Ethanol as a substrate resulted in the greatest number of positive enrichments, with representatives from each of the northern lakes visited. These are the first indications that ethanol may be used to enrich for alkaliphilic SRB. Lactate also performed well as a growth substrate, whilst acetate, butyrate, formate and fumarate also resulted in one or more positive enrichments. Contrary to previous findings, these data reveal the presence of alkaliphilic SRB capable of utilising a range of substrates. Although positive enrichments under hypersaline conditions were initially obtained using lactate, these cultures could not be maintained.;Using the substrates betaine, trehalose, starch, carboxymethyl cellulose, xylan and guar gum (galactomannan), a number of other organisms were isolated. Despite being enriched under anaerobic conditions, all the isolates were found to be facultatively anaerobic although exo-enzyme production appeared to take place only under anaerobic conditions.;Six alkaliphilic, obligate anaerobes were isolated from samples taken from two of the northern lakes (Lake Elmenteita and Lake Bogoria) using a complex medium with glucose. Phenotypic and taxonomic data indicated the presence of five species belonging to the Clostridium spectrum, although they were found to be phylogenetically distinct from previously described isolates. Each organism showed optimal growth at alkaline pH, and tolerated only low concentrations of NaCl (ca. 8% w/v).;Enrichment of three Lake Magadi samples resulted in the isolation of three haloalkaliphilic obligate anaerobes. These organisms had an obligate requirement for high NaCl concentrations (> 16% w/v), and showed a limited pH range for growth in the alkaline region (> pH 9.5). Phylogenetic analysis of 6S rDNA revealed these isolates also clustered within the Clostridium region of the low G+C Gram-positive bacteria, although they were also unrelated to any of the previously described species. Taxonomic proposals for a new genus and several new species are presented.