It is proposed that measurements of dissolved molecular hydrogen, a known byproduct of nitrogenase activity, have the potential to open a new window on the study of marine nitrogen fixation which, although of prime importance to the marine carbon cycle and long a subject of study, remains inadequately quantified. Estimates of global ocean N2-fixation, derived from collections of snapshot isotopic rate measurements and indirect global geochemical evidence, range between 100 and 200Mtonnes N/yr (Carpenter & Capone, 2008).
Interest in the study of marine N2-fixation has been stimulated as recent applications of molecular biology techniques have revealed an increased number and diversity cyanobacterial species able to fix nitrogen. At the same time, the discovery of a major loss process (in addition to denitrification) for combined nitrogen in the oceans, through the anammox process, has focused more attention on the imbalances between N2-fixation and loss in the oceans as well as on the mechanisms acting to restore balance. Such imbalances are recognized as having the potential to significantly affect atmospheric CO2 levels by altering the capacity of the deep ocean to store CO2, possibly contributing to well known glacial-interglacial variations in atmospheric pCO2. Of particular interest are questions concerning how tightly coupled are these sources and sinks, and whether iron plays a controlling role in the distribution and intensity of nitrogen fixation. In a pilot study of saturation levels of molecular hydrogen and isotopically measured N2-fixation in surface tropical waters we have shown a correspondence between these two quantities. Now, in collaboration with international partners we propose to extend this work to intensive field studies which include the study of iron availability and characterization of the diazotroph community in the equatorial ocean. It is anticipated that H2 measurements, rapidly made from a moving vessel, have the potential to support the slower, more labour-intensive, but quantitative technique of N-15 uptake for quantifying N2-fixation on large spatial scales. In particular, it has potential for identifying local regions of fixation stimulated by deposition of iron in atmospheric dust.