Abstract:

The diversity and composition of ammonia oxidizing bacteria (AOB) were investigated in order to understand the magnitude and distribution of nitrification rates in natural aquatic environments. Oxidation of ammonia, the first step in nitrification, influences the form of inorganic nitrogen available to biological communities and mediates the substantial anthropogenic release of nitrogen into natural environments through linked nitrification-denitrification. Although the importance of this process is clear, the factors controlling the composition of AOB assemblages and rates of nitrification in the natural environments are not well understood. The goal of the research described in this dissertation was to assess the diversity and composition of AOB in aquatic environments and understand the ecological dynamics controlling their biogeochemical function, the oxidation of ammonia.

Culture independent molecular techniques were used to examine the distribution of AOB from a variety of aquatic samples including Monterey Bay, California and Chesapeake Bay, Maryland. Results from these studies indicate that one clade of Nitrosospira -like AOB dominates most marine environments, and that this group is not well represented by the culture collection. Changes in the diversity and composition of AOB were detected along estuarine gradients, where salinity appears to be an important factor influencing AOB composition. Enrichment cultures were used to isolate novel AOB strains from the Chesapeake Bay but none of the enrichments resulted in a culture of the dominant Nitrosospira -like AOB.

Experimental mesocosms were used to explore the influence of variable salinity and oxygen on nitrification rates. Salinity appears to be an important factor controlling the composition of AOB along estuarine gradients and significantly influences the nitrification rates of natural assemblages. AOB assemblages do not recover quickly from changes in salinity and regions impacted by tidal mixing are predicted to be more susceptible to the decoupling of nitrification-denitrification. This may have important implications for the cycling of nitrogen in coastal environments and negative feedbacks leading to eutrophication and prolonged periods of anoxia.