Diel variations in nitrogen and carbon dioxide fixation by the blue-green alga Rivularia in an upland stream

Abstract

Field and laboratory studies are described on nitrogen fixation (acetylene reduction) and ¹⁴CO₂ fixation during summer by a population of Rivularia growing in Red Sike, a stream in Upper Teesdale, northern England. An in situ diel study of carbon dioxide fixation showed a significant correlation (r = 0.84, P < 0.01) between fixation rates and light flux, but a relatively low correlation between nitrogen fixation and light flux (r = 0.23, P > 0.05). Although the rates of nitrogen fixation were much higher by day than night, between 8 and 16% of total activity occurred at night (5–8 h dark); a small nocturnal peak was evident in one of the three diel surveys. When colonies were shaded to reduce incident light to 15% of full irradiation, total daytime activity was doubled. This effect was also shown clearly in a laboratory study. Carbon dioxide fixation decreased with decreasing light intensity, whereas nitrogen fixation showed a bimodal response to light, with a low intensity peak at a photon flux density of about 10 μmol m⁻² s⁻¹. The highest ratio of nitrogen: carbon fixation occurred at this very low light level. Our hypothesis suggests that at high light fluxes nitrogen fixation may compete for energy with carbon dioxide fixation, whereas at low light fluxes it may use both most of the light-dependent energy as well as respiratory energy mobilizing glucan reserves synthesized under high light fluxes. This study showed that about 400 mol of CO₂ to 1 mol of N₂ was fixed by Rivularia during daylight. Nitrogen fixation may supply only a small percentage of the alga's nitrogen requirements, at least for the medium-sized (3–4 mm diameter) colonies studied.