Diazotrophic pico-cyanobacteria in the North Atlantic open ocean: their abundance and importance as a source of new nitrogen at the Azores Current Front
The increase in atmospheric CO2 within the last century and its effect on climate change has urged the need for more research on the processes leading to carbon sequestration. In oceanic biogeochemistry, the ‘Biological Pump' is the sum of a suite of biologically mediated processes transporting carbon from the euphotic zone to the ocean's interior. As a constituent of DNA and proteins, nitrogen is a vital element and its availability limits the growth of CO2-fixing biological actors over most of the global oligotrophic oceans. The largest N pool in the ocean, dissolved N2, is available to only a small number of planktonic microbial species producing the necessary enzymes for N2 fixation (one of which being encoded by the highy conserved NifH gene). These organisms, amongst which unicellular cyanobacteria (UCYN), introduce new nitrogen into the ocean, thereby contributing to carbon export. While the pace of research has steadily increased, the extent of the distribution and activity of N2-fixers in the ocean still remains poorly known. The present proposal aims at improving our understanding of the inter-linkages between the N2 and CO2 fixation processes mediated by unicellular cyanobacteria, in particular at the Azores Front/Current system in the North Atlantic Ocean. The front separates subtropical and temperate water masses, in each of which we will assess:
(1) the influence of nutrients, temperature, depth and light on N2-fixation, NifH gene diversity, UCYN distribution;
(2) the specific abundance of UCYN (using new fluorescence in situ hybridization techniques) and their contribution to N2-fixation;
(3) the transfer of fixed carbon into UCYN-specific biomarkers.
We expect to increase the pool of data on this group of organisms which might be key players in the atmospheric CO2 sequestration.
