Biogeography of N2 Fixation in the Surface Ocean

Diazotrophs and N2 fixation are not homogenously distributed across the oceans, but have distinct patterns of rates, abundances and species composition. These patterns reflect the dynamic processes in the ocean and atmosphere and the distribution of nutrients, particularly P and Fe. The availability of non-N nutrients causes spatial variability in nutrient limitation that determines species composition. An important driver is the distribution of dissolved Fe which is largely driven by aeolian atmospheric deposition. Trichodesmium is one of the most widely distributed diazotrophic cyanobacteria in the tropical and sub-tropical ocean, but UCYN-A are also found there, but also in cooler and more nutrient-enriched coastal and polar waters. Non-cyanobacterial diazotrophs (NCDs) are widely distributed and may be important, but direct demonstration of N2 fixation by these diverse, uncultivated bacteria has not yet been obtained. There is seasonality of diazotrophs in the open ocean gyres. N2 fixation by heterocyst-forming cyanobacteria occur in estuaries and brackish water. Oxygen deficient zones (ODZs) could be habitats for N2 fixation and are characterized by presence of NCDs, but generally have low N2 fixation rates. UCYN-A is present and fixes N2 in the Arctic, although at very low abundances. Much remains to be learned how nutrients and environmental factors control the spatial patterns of oceanic N2 fixation and there are likely more surprises in store.