Biological N2 fixation is a biogeochemically unique and potentially important "new" nitrogen input in chronically N deficient marine waters. Estimates of N2 fixation inputs (relative to ecosystem N demand) range from less than 2% in planktonic to at least 20-30% in certain benthic systems. A persistent problem with these assessments is our rudimentary knowledge of relevant and active microbial diazotrophs (cyanobacteria, bacteria), their location and interactions, the impacts of environmental constraints on and resultant spatial- temporal heterogeneity of N2 fixation. In prior work Dr. Paerl and co-workers applied immunochemical, electrochemical, cytochemical and microautoradiographic detection and characterization of N2 fixers in diverse benthic and planktonic habitats. Results indicate the presence and periodic preponderance of microheterotrophic diazotrophs. This poorly recognized and underassessed group exhibited relatively high rates of N2 fixation in oxygen-deplete surficial, aggregate- associated microzones, including detritus, mats, reefs, biofilms and epiphytic habitats. N2 fixation in these habitats appears to function in a consortial manner; that is, the presence and magnitudes of N2 fixation are reliant on specific intact microbial groups acting together in a highly coordinated and interdependent manner. In this research, Dr. Paerl will investigate the dynamics of and environmental controls on consortial N2 fixation. He will apply cell-specific immunochemical probing, microchemical sensing and microautoradiography to: 1) elucidate physical, chemical and biotic factors affecting the establishment and maintenance of consortial N2 fixation, and 3) evaluate environmental controls on consortial N2 fixation in relation to their impacts on "new" N inputs on larger scale ecosystem, and ultimately global, levels.
