RESUMO
Diel nitrogen fixation studies were conducted with assemblages of cyanobacteria sampled from surface blooms on Sanctuary Lake, Pa. The studies were conducted between July and September of 1982 to 1985 by using the acetylene reduction technique. Assemblages with the lowest cell concentrations (0.9 x 10 to 1.0 x 10 cells per liter) exhibited nitrogen fixation activity throughout the day, with maximum fixation rates occurring in mid to late afternoon; fixation proceeded throughout the night at rates equivalent to 23 to 28% of the afternoon maximum. In studies conducted with the highest cell concentrations (3.7 x 10 to 6.7 x 10 cells per liter), fixation rates reached maximum values in mid to late morning. The rates declined rapidly throughout the midday period and subsequently ceased from late afternoon until sunrise on the following day. The afternoon decline and cessation of fixation exhibited by high cell concentrations correlated with photosynthetically induced low total CO(2) and supersaturating O(2) concentrations. The midday decline could be prevented and partially reversed by experimentally lowering O(2) and increasing total CO(2) concentrations. Under experimental conditions which simultaneously prevented supersaturating O(2) concentrations and maintained high total CO(2) availability, nitrogen fixation continued throughout the solar day, with maximum rates occurring at midday. These observations indicate that temporal changes in photosynthetic activity may affect diel fluctuations in nitrogen fixation.
