Summary: spp. UCSB8 and UCSB25 are both capable of aerobic N fixation. The optimum temperature for CH reduction was 22 ° for sp. UCSB8 and 35 ° for sp. UCSB25, whilst the optimum temperature for growth on N was 25 ° and 30 °, respectively. In sp. UCSB25, but not in UCSB8, inhibition of N fixation may limit diazotrophic growth at temperatures above 35 °. When grown under alternating 12 h light and 12 h darkness, both isolates reduced CH predominantly in the dark and both were capable of N 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 sp. UCSB8 was glucose, whilst that for sp. UCSB25 was fructose. In 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 CH both in the light and in the dark, but the highest rates of CH reduction were observed in the dark. This cyclic pattern of N fixation was independent of photosystem II activity.


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