Nitrogenase Genes in Urban, Agricultural, and Pristine Prairie Streams Exposed to Different Levels of Nitrogen Loading
R. Caton and M.A. Schneegurt
Ecosystem processes drive biogeochemical cycles that influence input and losses of nutrients in the biosphere. Through human activities the environment has been highly enriched with nutrients, especially nitrogen. In most ecosystems, nitrogen availability should be limited, but soils and aquatic ecosystems have been impacted by human activities. In streams, availability of nutrients, geochemical characteristics, hydrodynamics, and human activities influence the metabolic activities and structure of microbial communities. The aim of the current study is to compare the N2-fixing guilds in three different types of Kansas streams: urban, agricultural, and pristine. Nitrogen fixation activity was measured with acetylene reduction assays. The highest rates were found in leaf litter and algal biofilms. Samples of sediments and leaf litter were field-frozen for molecular analyses of the nitrogen-fixing microbial guild. Direct DNA extracts were examined by SYBR real-time PCR to determine the abundance of nifH genes. There was considerable variance in the dataset, and no significant correlation was observed between nitrogen fixation activity and nifH gene abundance. The construction and sequencing of PCR-based clone libraries is being used to assess the diversity of nifH genes in the different streams. Initial results suggest that methanogens, sulfate-reducers, and rhizobia are abundant members of the nitrogen-fixing guild in stream sediments. An understanding of the effects of nitrogen pollution on nitrogen cycling guilds in small streams will increase our ability to overcome the challenges of nutrient pollution. This work was supported by grants from Kansas NSF EPSCoR Ecological Genomics and NSF Graduate Fellows in K-12 Education.