Algal nitrogen fixation in Californian streams: diel cycles and nocturnal fixation

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Abstract
Summary: Thirty‐six hour diurnal studies of Ng‐fixation by Nostoc in a rocky‐bedded stream were carried out during the peak of the seasonal cycle of growth on clear and cloudy days in 1971 and 1972. On both occasions an unexpected pattern of N2‐fixation occurred with maximum fixation rates in the light but also in the dark portion of the day, with lowest fixation periods in the early evening. I postulate that competition for reductant between nitrogenase and other processes, especially photorespiration, controls this unusual diel cycle rather than variations in the intracellular N‐pool. N2‐fixation rates on a cloudless May day in 1971 ranged from 0.2 to 4.8 nmoles C2H4 cm−2 h−1 and from 0.3 to 3.3 nmoles C2H4 mg−1 h−1 dry weight of Nostoc, depending on time of day and favourableness of site. On the same site on a cloudy, rainy May day in 1972 fixation ranged from 0.5 to 3.1 nmoles C2H4 mg−1 h−1 dry weight, and from 1 to 4.5 nmoles C2H4 mg−1 h−1 ash‐free dry weight of Nostoc. Since Nostoc is most abundant in unshaded areas, and since one‐third of each day's nitrogen i s fixed in the dark, future studies should take dark fixation into account.

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