On land, nitrogenase fixation of nitrogen by bacteria on leaves, has been discussed; see summation reference below.
In open ocean, far from land, upwelling, and currents, might any (?) bacteria with nitrogenase enzyme, in association with plankton’s surface, or intrinsic component of plankton membrane, allow for significant nitrogen fixation? Is this so for diatom and cyanobacteria? This would be in addition to any released fixed nitrogen from decaying plankton. So although such process might be insignificant on land, still in open ocean might it be a significant process for fixation of nitrogen for plankton, at top of food chain ? However rhizobium bacteria, in association with legumes, require an anaerobic environment for such nitrogenase activity. But some (?) cyanobacteria can sequester nitrogenase in heterocysts; hence avoiding oxygen environment.
Does current database of recent survey by Tara/Tara Ocean have a DNA library (archive), inclusive of nitrogenase sequence? If not, perhaps a more concentrated marine oceanic sample might confirm preliminary results as to possible nitrogen fixation at top of food chain vis a vis nitrogenase activity.
Pennisi Elizabeth, Science 22 May 2015, vol 348 issue 6237
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Limardo A.J., Worden A.Z., Exclusive networks in the sea, Nature vol 522, p.36, June 4, 2015;
also online, Nature May 26, 2015 at nature.com/newsandviews.
Zehr Jonathan P, Science Sept 11, 2015 vol 349 6253, p. 1163, and reference.
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Nitrogen Fixation in the Marine Environment
DOUGLAS G. CAPONE, EDWARD J. CARPENTER
Science 17 Sep 1982:Vol. 217, Issue 4565, pp. 1140-1142