Isolation of Potential Photosynthetic N\u3csub\u3e2\u3c/sub\u3e-Fixing Microbes from Topsoil of Native Grasslands in South Dakota

Nitrogen fertilizer is one of the most limiting factors and costly inputs in agriculture production. Current fossil fuel-dependent ammonia production is both energy intensive and environmentally damaging. An economically practical and environmentally friendly solution for the production of ammonia is urgently needed. Solar-powered N2-fixing cyanobacteria provide a unique opportunity and promise for applications in agriculture compared to all other N2-fixing bacteria that cannot use solar energy. Isolation of nitrogen-fixing microbes from the topsoil of native grasslands may have the potential to use them in crop fields as living ammonia factories. This may be a mechanism to free farmers from heavy reliance on fossil fuels-dependent chemical nitrogen fertilizers and to improve soil health for sustainable agriculture. To screen for solar-powered N2-fixing cyanobacteria in topsoil of native grasslands in South Dakota, we collected 144 topsoil samples from several native grasslands. Six photosynthetic microbial strains were isolated that are capable of growing well autotrophically in a nitrogen-free medium, suggesting that these six microbial strains have the ability to fix N2. They were assigned the names: Xu15, Xu81, Xu86, Xu111, Xu141, and WW3. Based on cell morphology and its 18S rRNA gene sequence that we obtained, strain Xu15 was reassigned as Chloroidium saccharophilum Xu15, a common terrestrial coccoid green alga. An acetylene reduction assay detected substantial ethylene production, suggesting nitrogenase activity occurrences in cultures Xu81 and Xu15. The other four are in the process of purification for testing their nitrogenase activity. Xu81, Xu111 and Xu141 are probably unicellular microalga, while WW3 and Xu86 are likely filamentous cyanobacteria. Future research will focus on developing these validated N2-fixing microbes as in situ living ammonia factories in crop fields