The rate and fate of N(2) and C fixation by marine diatom-diazotroph symbioses

N(2) fixation constitutes an important new nitrogen source in the open sea. One group of filamentous N(2) fixing cyanobacteria (Richelia intracellularis, hereafter Richelia) form symbiosis with a few genera of diatoms. High rates of N(2) fixation and carbon (C) fixation have been measured in the presence of diatom-Richelia symbioses. However, it is unknown how partners coordinate C fixation and how the symbiont sustains high rates of N(2) fixation. Here, both the N(2) and C fixation in wild diatom-Richelia populations are reported. Inhibitor experiments designed to inhibit host photosynthesis, resulted in lower estimated growth and depressed C and N(2) fixation, suggesting that despite the symbionts ability to fix their own C, they must still rely on their respective hosts for C. Single cell analysis indicated that up to 22% of assimilated C in the symbiont is derived from the host, whereas 78–91% of the host N is supplied from their symbionts. A size-dependent relationship is identified where larger cells have higher N(2) and C fixation, and only N(2) fixation was light dependent. Using the single cell measures, the N-rich phycosphere surrounding these symbioses was estimated and contributes directly and rapidly to the surface ocean rather than the mesopelagic, even at high estimated sinking velocities (<10 m d(−1)). Several eco-physiological parameters necessary for incorporating symbiotic N(2) fixing populations into larger basin scale biogeochemical models (i.e., N and C cycles) are provided.