Nitrogen fixation in the sea

N cycle includes a gaseous phase (N2) which is fixed into biologically available ammonium by nitrogen fixation. The atmosphere provides a large potential reservoir of N. Generally N2 fixation rates are low and there are few N2 fixing organisms. In cyanobacteria nitrogenase is usually found in heterocysts (specialised cells, lacking PSII activity & hence anaerobic); or temporal separation of N2-fixation & photosynthesis. N2 -fixation genes are dispersed throughout the prokaryotic kingdom. Factors that might limit the genetic potential & expression of N2-fixation in the marine environment : i) the large energy & reductant requirement, ii) sensitivity of nitrogenase to inactivation by O2, iii) repression of nitrogenase activity by fixed nitrogen e.g nitrate, ammonia iv) the requirement for micronutrients such as molybdenum, vanadium & iron