Zehr 95-03593 The primary productivity of the oceans is intertwined with the biogeochemical cycling of nutrients required for autotrophic growth. Although there is not clear agreement on which nutrients limits ocean productivity, it is evident that the major nutrients nitrogen (N) and phosphorus (P) are present at low concentrations in the oligotrophic ocean gyres. The argument for N limitation has always been complicated by the fact that N deficiency could be alleviated by the biological reduction of atmospheric N2. Yet few N2 fixing organisms have been found in the open ocean. The development of DNA amplification and probe techniques for characterizing N fixation genes are powerful tools for assessing N2 fixation potential in the environment. Recent advances have demonstrated that this approach can be used to assay the expression of N2 fixation genes, which will be developed further as part of this program. The goals of this research will be to complete, substantiate and extend the conclusions from prior studies of N fixation in the context of physical, chemical and biological parameters of the Atlantic and Pacific Oceans. This research will answer the question of the presence of N2 fixing species in the oceanic environment and the factors limiting the distribution and expression of N2 fixation genes.
