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

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.