A Review on Marine N 2 Fixation: Mechanism, Evolution of Methodologies, Rates, and Future Concerns

Investigations on marine N 2 fixation have gained momentum since 1960s with eventual establishments of relevant methodologies to identify species involved and quantify the rates. The evolution of various methodologies to understand N 2 fixation and to estimate its rates were underpinned by the constant efforts of pioneers in the ocean biogeochemical research field. Those efforts succeeded in introducing various methodologies that include experimental ( 15 N 2 bubble method and acetylene reduction method), geochemical (N* and P* method), mathematical modelling, and remote sensing techniques. However, the construction of an accurate N budget is still under progress due to inseparable issues associated with each method and difficulties in conducting the experiments onboard on a larger scale. Nevertheless, the contributions by each of the methodologies are significant and helped in forming basic ideas about N 2 fixation activities on a global scale. It is not only important to recognize the contributions made by the formation of various methodologies by marine research pioneers, but also vital to summarize what we have achieved in the marine N 2 fixation research area so far. Hence, this review is an attempt to brief on the various milestones achieved in research on the N 2 fixation mechanism, species involved, evolution of methodologies to estimate N 2 fixation rates, species identification, budgets, and future concerns.