Lawrence Berkeley National Laboratory

Microbialites are an example of complex and diverse microbial assemblages where several metabolic pathways are interconnected for biomass formation coupled to mineral precipitation. Lake Alchichica (Mexico) is an oligotrophic environment where nitrogen (N) and phosphorus alternately limit productivity, and massive microbialite growths are found along the lake's perimeter. Previous studies have described the importance of N2 fixation in these microbialites, although other pathways associated with the N cycle, including denitrification, nitrification and anaerobic ammonium oxidation (anammox), had not been evaluated. This study identified the genetic diversity associated with N cycling in both metagenomic DNA and RNA expression by targeting key genes for nitrogenase (nifH), ammonia monooxygenase (amoA), nitrite oxidoreductase (nxrA, nxrB), hydrazine oxidoreductase (hzo) and nitrite (nirS and nirK) and nitrous oxide (nosZ) reductases. While the genetic potential for N2 fixation, ammonia oxidation, anammox and denitrification was present in the microbialites of Lake Alchichica, the most transcribed pathway was N2 fixation.