Control of nitrogen fixation by oxygen depletion in surface-associated microzones

Abstract

Chronic deficiencies in biologically utilizable nitrogen control primary production in vast segments of the world's oceans^1–3. A priori, it would appear that these conditions offer selective advantages to N₂-fixing microorganisms (eubacteria and cyanobacteria). Most studies, however, concur that ecologically insignificant rates of N₂ fixation are found in such waters^4–6. We have addressed this paradox by identifying some environmental factors that may control the development and growth of marine N₂ fixers and experimentally manipulating them in nitrogen-depleted North Carolina coastal Atlantic Ocean water. The availability of phosphorus, iron and molybdenum, required for both synthesis and function of the N₂-fixing enzyme complex nitrogenase⁷, was sufficient to support the growth of N₂-fixers. However, colonizable inorganic and organic surfaces were important for the development and proliferation of N₂-fixing microbial consortia (mixtures of eubacteria, and cyanobacteria plus eubacteria). Surface-associated N₂ fixation potentials were much more dependent on organic than on inorganic nutrient enrichment, and were affected by the incidence and magnitude of localized O₂ depletion in the surface-associated micro-zones. As molecular O₂ is a potent inhibitor of N₂ fixation in many marine microorganisms, the extent to which biological nitrogen demands can be met by N₂ fixation depends on the presence and maintenance of O₂ depleted microzones.