| Summary: | Lake Chitu is a highly productive soda lake found in the East African Rift Valley, where <i>Arthrospira fusiformis</i> (<i>Spirulina platensis</i>) is the main primary producer. High biomass accumulation requires an adequate supply of nitrogen. However, Lake Chitu is a closed system without any external nutrient input. A recent study has also demonstrated the presence of a diverse group of denitrifying bacteria, indicating a possible loss of nitrate released from the oxidation of organic matter. The aim of this study was to isolate culturable nitrogen-fixing alkaliphiles and evaluate their potential contribution in the nitrogen economy of the soda lake. A total of 118 alkaliphiles belonging to nine different operational taxonomic units (OTUs) were isolated using a nitrogen-free medium. Nineteen isolates were tested for the presence of the <i>nifH</i> gene, and 11 were positive. The ability to fix nitrogen was tested by co-culturing with a non-nitrogen-fixing alkaliphile, <i>Alkalibacterium</i> sp. 3.5*R1. When inoculated alone, <i>Alkalibacterium</i> sp. 3.5*R1 failed to grow on a nitrogen-free medium, but grew very well when co-cultured with the nitrogen-fixing alkaliphile NF10m6 isolated in this study, indicating the availability of nitrogen. These results show that nitrogen fixation by alkaliphiles may have an important contribution as a source of nitrogen in soda lakes.
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