CHARACTERIZATION OF AMMONIA-OXIDIZING MICROBIAL COMMUNITIES ALONG THE SALINITY GRADIENT OF THE CAPE FEAR ESTUARY

This study investigates the distribution and diversity of ammonia oxidizing bacteria and archaea in the Cape Fear Estuary of North Carolina. Nitrifcation, the oxidization of ammonia to nitrite and nitrate by chemolithoautotrophic microbes, is an important process for understanding the fate of nitrogen pollution in aquatic ecosystems. Ammonia oxidation 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. The first step in nitrification is catalyzed by the enzyme ammonia monooxygenase (AmoA), which can be used as a genetic marker to study nitrifier diversity in the environment. DNA was extracted from sediments along the salinity gradient of the Cape Fear Estuary and amoA genes were applied using primer sets specific to bacterial and archaeal ammonia oxidizers. Ammonia-oxidizing archaea (AOA) were detected in 85% of the samples tested and were found to be distributed along the entire salinity gradient, suggesting they may be the dominant nitrifiers in the estuary. In contrast, ammonia-oxidizing bacteria (AOB) were detected in 62% of the samples tested, all from the high salinity zone of the lower estuary. Analysis of 321 AOA sequences and 184 AOB sequences shows a number of distinct taxonomic units in both classes of microorganisms, and their distribution appears to be correlated with salinity. This change in species composition of ammonia oxidizers along the estuarine salinity gradient may be important to understanding the rate of nitrification within this system and linkage to other functional groups of nitrogen cycling microbes (e.g. denitrifiers and anaerobic ammonia oxidizers) in the estuary.