Exploring and Enriching for Nitrogen Cycling Microbes: Nitrogen Fixation, Ammonia Oxidizing Lithotrophy, and Anaerobic Nitrate Reduction

In this exercise, students enrich for soil-associated microbes that perform three key nitrogen cycle transformations: nitrogen fixation, aerobic ammonia oxidation, and anaerobic nitrate reduction. In terms of nitrogen fixers, they isolate Azotobacter and Rhizobium using adaptations of Beijerink's historic enrichments. In terms of aerobic oxidizers, they use ammonia-based media, indirectly observing for lithotrophs by assaying for the production of nitrate and nitrite. In terms of anaerobic reducers, they inoculate two different soils into standard nitrate tubes and test for denitrification and transformations to fixed nitrogen compounds.