Gessesse Amare (PhD)Ali Yordanos2018-06-182023-11-182018-06-182023-11-182015-10http://etd.aau.edu.et/handle/12345678/1161Lake Chitu is a tropical crater lake located in the Ethiopian Rift Valley 287 km south of Addis. It is a small soda lake unusually with high primary productivity. The lake water has high pH, salinity, HCO3- + CO3 v2-, alkalinity, Na+ and Cl- ions and frequently limiting levels of inorganic nitrogen compounds due to high rate of denitrification. Microbial N2 fixation is expected to play an important role in the production of bioavailable nitrogen. To test this hypothesis the diversity, abundance and vertical distribution, of Lake Chitu diazotrophic microbial communities were investigated at 6 sampling sites including 0 m, 2.9 m, 5 m, 10 m, 14 m and 17 m depth using cultivation based and molecular methods. .A total of 120 isolates cultivated on nitrogen free medium cultures were analyzed to cultivation-based and molecular analysis. 19 isolates representative of each OTU were analyzed for NifH gene and 16S rRNA gene amplification and sequencing. PCR-DGGE analysis was also carried out from enrichment and environmental samples. 120 isolates clustered in to 9 operational taxonomic group (OTU).The presence of NifH gene was detected by PCR in 11 out of 19 representative isolates. The nifH gene sequences retrieved were closely related with more than 96% sequence similarity to nitrogenase (NifH) genes of uncultured microbial communities, retrieved from various sources. The results of the study indicate that alkaliphilic diazotrophic microbial communities were found along the whole water depth of the lakes including the surface water. However, the diversity of diazotrophic microbial communities was high at the bottom of the lake. The gene diversity depicted the presence of nitrogen-fixing bacteria affiliated phylum Proteobacteria, Firmicutes, and Actinobacteria. Keywords: Diazotrophs; Lake Chitu; nitrogenase; nitrogen fixation; Soda LakeenDiazotrophs; Lake Chitu; nitrogenase; nitrogen fixation; Soda LakeThesis