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Summary: Oscillatoria spp. UCSB8 and UCSB25 are both capable of aerobic N2 fixation. The optimum temperature for C2H2 reduction was 22 ° for Oscillatoria sp. UCSB8 and 35 ° for Oscillatoria sp. UCSB25, whilst the optimum temperature for growth on N2 was 25 ° and 30 °, respectively. In Oscillatoria sp. UCSB25, but not in UCSB8, inhibition of N2 fixation may limit diazotrophic growth at temperatures above 35 °. When grown under alternating 12 h light and 12 h darkness, both isolates reduced C2H2 predominantly in the dark and both were capable of N2 fixation and photoheterotrophic growth in the presence of 20 μm-DCMU to inhibit photosystem II activity. Under these conditions, the best exogenous carbon source for Oscillatoria sp. UCSB8 was glucose, whilst that for Oscillatoria sp. UCSB25 was fructose. In Oscillatoria sp. UCSB8, exogenous glucose was catabolized mainly through the oxidative pentose phosphate pathway. Although cultures grown photoheterotrophically showed higher specific activities of nitrogenase than photoautotrophic cultures, they grew more slowly. Furthermore, cultures grown photoheterotrophically under alternating light and darkness reduced C2H2 both in the light and in the dark, but the highest rates of C2H2 reduction were observed in the dark. This cyclic pattern of N2 fixation was independent of photosystem II activity.

Nitrogen fixation by Oscillatoria spp. under autotrophic and photoheterotrophic conditions