Nitrogen fixation rates at BATS and along the North Atlantic subtropical front

Nitrogen fixation is one of the major inputs of new N into the oligotrophic gyre of North Atlantic. In the Bermuda Atlantic Time-series Study (BATS), Trichodesmium spp. and diatom symbiont Richelia have been regarded as a major contributor to the newly fixed N. However, studies using nitrogenase gene (nifH) sequences have indicated a more diverse diazotroph assemblages including unicellular cyanobacteria in this oligotrophic system. Here, we report N2 fixation rates from diazotrophs of the size fraction that includes unicellular cyanobacteria and other bacteria. Samples were taken from BATS and several stations along the meridional transect from BATS southward to the area of the subtropical front (~27oN). Whole water samples were enriched with 15N-labelled N2, and incubated for 12-24 hours in simulated in situ conditions. Post-incubation filtration by using 20 micron size fractionation excluded Trichodesmium and potential diatom symbionts and constrained our rates to smaller non-filamentous N2-fixers. Sampling was conducted on two cruises during July and August 2012. 13C incorporation rates are also presented to show the photosynthetic activity. In addition, the 15N-enrichment of dissolved organic N (DON) was investigated in order to demonstrate the contribution of the diazotrophy to the regenerated nutrient and the potential link to the heterotrophic bacterial production.