Reports

Direct Measurement of O2-Depleted Microzones in Marine Oscillatoria: Relation to N2 Fixation

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Science  22 Jul 1988:
Vol. 241, Issue 4864, pp. 442-445
DOI: 10.1126/science.241.4864.442

Abstract

Among the nitrogen (N2)-fixing Cyanobacteria, the filamentous, nonheterocystous marine Oscillatoria spp. (Trichodesmium) appears enigmatic; it exhibits N2 fixation in the presence of oxygenic photosynthesis without structural protection of the N2-fixing apparatus (nitrogenase) from potential inhibition by molecular oxygen (O2). Characteristically, N2 fixation is largely confined to aggregates (bundles) of filaments. Previous work has suggested that spatial partitioning of photosynthesis and of N2 fixation occurs in the bundles as a means of allowing both processes to occur contemporaneously. The probing of freshly sampled bundles with O2 microelectrodes directly confirmed such partitioning by showing the presence of O2-depleted (reduced) microzones in photosynthetically active, N2-fixing bundles. Bundle size was directly related to both the development of internal reduced microzones and cellular N2 fixation rates. By enhancing microzone formation, bundles optimize N2 fixation as a means of supporting Oscillatoria spp. blooms in surficial, nitrogen-depleted tropical and subtropical waters.