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.