Patterns of ammonia and nitrite oxidizing microbial diversity along the salinity gradient of the Cape Fear Estuary

Background/Question/Methods

Nitrification, a two-step process involving the oxidization of ammonia to nitrite and then nitrite to nitrate by two chemolithotrophic microbes, is important for understanding the fate of nitrogen pollution in aquatic ecosystems. Nitrification provides a key linkage between the mineralization of nitrogen from organic matter and the formation of N₂ gas that can be released into the atmosphere. This study investigates the distribution and diversity of ammonia oxidizing bacteria (AOB), ammonia oxidizing archaea (AOA) and nitrite oxidizing bacteria (NOB) along the salinity gradient of the Cape Fear Estuary of North Carolina. DNA was extracted from sediments along the salinity gradient of the Cape Fear Estuary and the ammonia monooxygenase (amo) and nitrite oxidoreductase (nxr) genes were amplified, cloned and sequenced.  

Results/Conclusions

Although primer sets targeting nxr genes from both Nitrobacter and Nitrospira were used, only the Nitrospira primers yield a fragment of the correct size.  Analysis of approximately 600 sequences indicate that AOA and NOB were found to be distributed along the entire salinity gradient, while AOB were detected primarily in the higher salinity sites of the lower estuary. Of the three groups of sequences analyzed, amoA from AOA had the greatest sequence diversity. The spatial patterning of Operational Taxonomic Units (OTUs) demonstrates the presence groups whose distribution appears to be restricted by salinity, including both high and low salinity specific OTUs. In addition, preliminary results from a sediment transplant experiment provide insights into the adaptation and successional patterns of nitrifiers to high versus low salinity environments.  These patterns of nitrifier diversity and succession may be important to interpreting the biogeochemical cycling of nitrogen within coastal environments.