The productivity of the oceans is limited by the availability of nutrients, which has implications for the fluxes of carbon between the atmosphere and oceans. In a previous award the PIs found that previously unrecognized N2-fixing unicellular cyanobacteria are active and abundant in oligotrophic oceans. This finding has important implications for nitrogen cycling in the oceans and for the role of "new" nitrogen in carbon fixation. The PIs will address three major issues: First, there are at least two distinct groups of cyanobacteria that appear to be separated in space and time, due to unknown ecological variables. Second, the geographic distribution and factors controlling the distribution are unknown, so it is not clear how these organisms should be included in biogeochemical models. Finally, one of the groups of cyanobacteria appears to fix N2 during the day, which revives the enigma of simultaneous nitrogen fixation and photosynthesis that was previously limited to discussions of Trichodesmium.
