9410326 Capone Freshwater and marine primary production are often limited by the availability of N, yet the characteristics of N2 fixing populations in the two realms differ substantially. N2 fixing heterocystous cyanobacteria are prominent planktonic features of N limited freshwater lakes and rivers. In N-limited marine waters, if N2 fixation occurs at all, it is usually restricted to benthic environments. The reasons for the striking differences in N2 fixer distributions between marine and freshwater systems are not known, but could be due to physico-chemical constraints such as salinity, turbulence, or other factors, or they may reflect sources, diversity and expression (activity) of allochthonous seed populations advected into the marine realm. The contrast between the predominance of N2 fixing organisms in freshwater, and the apparent dearth of N2 fixing organisms in estuaries and oceans, provides the basis for the proposed research. Our primary objective is to identify and characterize the sources, diversity and distribution of N2 fixing genes in relation to environmental control of nitrogen fixation activity along a salinity gradient in an N-limited estuary of the Neuse River. The approach proposed here goes beyond the conventional analysis and interpretation of the presence or absence of the activity of N2 fixers by focusing on the fundamental relationships between the genetic potential for this activity and the environmental factors regulating species selection and expression of the N2 fixation apparatus. %%% A novel aspect of this research is that this investigation will examine the importance of ecological selection factors on microbial diversity with respect to how, why and when natural waters can respond to N limitation with development of a diazotrophic community and N2 fixing activity.
