Merging Genomics, Transcriptomics and Geochemistry to Assess Nitrogen Cycling in Terrestrial Hot Springs

A multi-faceted study was conducted to evaluate nitrogen cycling in Mound Spring and “Bison Pool,” two geochemically similar, alkaline hot springs of the Lower Geyser Basin in Yellowstone National Park. Downstream geochemical trends are supported by results of genomic and transcriptomic studies of Mound Spring and “Bison Pool” chemotrophic and phototrophic microbial communities. Nitrogen stable isotope trends of “Bison Pool” and Mound Spring biomass corroborate genomic and transcriptomic data and reflect topographically-driven differences in exogenous nitrogen input between the two hot springs. Results indicate that both chemotrophic and phototrophic communities at Mound Spring are genetically capable of fixing nitrogen, and nifH gene transcripts suggest downstream variability in nifH expression that may be a result of downstream changes in fluid chemistry and microbial community variation between sampling sites. Results of gene surveys targeting functional nitrification and denitrification genes indicate a marked difference between these processes at Mound Spring and “Bison Pool.” Nitrification potential is present only in the chemotrophic streamer biofilm communities at “Bison Pool,” yet the absence of amoA transcripts suggests that in situ nitrification was not occurring at the time of sampling. Genetic evidence of nitrification is absent from all Mound Spring chemotrophic and phototrophic communities sampled, and denitrification gene markers are relatively few. In contrast to Mound Spring, the microbial communities of “Bison Pool” appear to have widespread genetic capability of most denitrification processes, and the extensive presence of nirS gene sequences and transcripts suggests that nitrite reduction is an important metabolic process in the “Bison Pool” ecosystem.