Distribution of Nitrogen-Fixing Microorganisms along the Neuse River Estuary, North Carolina.

Nitrogen fixation genes (nifH) were amplified and sequenced from DNA extracted from surface water samples collected from six stations along the length of the Neuse River Estuary, North Carolina, in order to determine the distribution of nitrogen-fixing organisms in the transition from fresh- to saltwater. Nitrogenase genes were detected in all samples by a nested polymerase chain reaction method, and the amplification products from the upriver, midriver, and downriver stations were cloned, sequenced, and used for phylogenetic analysis. The composition of nifH clone libraries from upriver, midriver, and downriver stations (each composed of 14 randomly selected clones) were very diverse (samples from upriver and midriver stations were composed of 14 unique sequences, downriver station composed of 7 unique sequences) and differed among the stations. Some phylotypes were found at more than one station, but were usually found in the upriver and midriver stations or in the midriver and downriver stations, indicating that the phylotypes were probably transported along the river. Cyanobacterial nifH were not found at the most upriver site, but were a large fraction of sequences (50%) recovered from the downriver station, where nitrate concentration was an order of magnitude lower and salinity was higher. In contrast, g proteobacteria nifH sequences were much more common at the midriver and upriver sites (58% and 64%, respectively), compared to the downriver site (14%). Results indicate that substantially different nitrogen-fixing assemblages are present along the river, reflecting differential watershed hydrological inputs, sedimentation, and environmental selection pressures, along the salinity gradient.