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Small streams exert great influences on the retention and attenuation of nitrogen (N) within stream networks. Human land use can lead to increased transport of dissolved inorganic N compounds and downstream eutrophication. Microbial activity in streams is important for maintaining an actively functioning N cycle. Chronically high N loading in streams affects the rates of the central processes of the N cycle by increasing rates of nitrification and denitrification, with biota exhibiting decreased efficiency of N use. The LINXII project measured N-cycle parameters in small streams using 15 NO 3 - tracer release experiments. We concurrently measured N 2 fixation rates in six streams of three types (agricultural, pristine, and urban prairie streams) as part of this broader study of major N-cycle processes. Nitrogen fixation in streams was significantly negatively correlated with nitrate levels, dissolved inorganic N levels, and denitrification rates. Algal mat and leaf litter samples generally exhibited the highest rates of N 2 fixation. The abundance of nifH genes, as measured by real-time PCR, was marginally correlated with N 2 -fixation rates, but not to other N-cycle processes or stream characteristics. The nifH sequences observed were assigned to cyanobacteria, Deltaproteobacteria, Methylococcus, and Rhizobia. Seasonal changes, disturbances, and varying inputs may encourage a diverse, flexible, stable N 2 -fixing guild. Patchiness in the streams should be considered when assessing the overall impact of N 2 fixation, since algal biomass exhibited high rates of N 2 fixation.

The Great Salt Plains of Oklahoma is a natural inland terrestrial hypersaline environment that forms evaporite crusts of mainly NaCl. Previous work described the bacterial community through the characterization of 105 isolates from 46 phylotypes. The current report describes the archaeal community through both microbial isolation and culture-independent techniques. Nineteen distinct archaea were isolated, and ten were characterized phenetically. Included were isolates phylogenetically related to Haloarcula, Haloferax, Halorubrum, Haloterrigena, and Natrinema. The isolates were aerobic, non-motile, Gram-negative organisms and exhibited little capacity for fermentation. All of the isolates were halophilic, with most requiring at least 15% salinity for growth, and all grew at 30% salinity. The isolates were mainly mesothermic and could grow at alkaline pH (8.5). A 16S rRNA gene library was generated by polymerase chain reaction amplification of direct soil DNA extracts, and 200 clones were sequenced and analyzed. At 99% and 94% sequence identity, 36 and 19 operational taxonomic units (OTUs) were detected, respectively, while 53 and 22 OTUs were estimated by Chao1, respectively. Coverage was relatively high (100% and 59% at 89% and 99% sequence identity, respectively), and the Shannon Index was 3.01 at 99% sequence identity, comparable to or somewhat lower than hypersaline habitats previously studied. Only sequences from Euryarchaeota in the Halobacteriales were detected, and the strength of matches to known sequences was generally low, most near 90% sequence identity. Large clusters were observed that are related to Haloarcula and Halorubrum. More than two-thirds of the sequences were in clusters that did not have close relatives reported in public databases.