Environmental and physiological controls on diel patterns of N2 fixation in epiphytic cyanobacterial communities

Epiphytic communities dominated by cyanobacteria on standing, dead Spartina alterniflora stems in a North Carolina salt marsh wore examined. Community composition changed over an annual cycle, with heterocystous cyanobacteria (Calothrix spp., Nostoc spp.) dominant in the spring, and nonheterocystous cyanobacteria (Lyngbya spp., Phormidium spp.) dominant in the fall. Diel patterns of N-2 fixation were investigated with acetylene reduction (AR) assays. Incubation conditions were varied in AR assays to determine physiological controls on N-2 fixation rates. AR assays conducted under aerial conditions exhibited significant daytime maxima only in the spring. In contrast, N-2 Fixation measured in submerged AR assays exhibited daytime maxima through October, although the diel pattern was more pronounced in the spring and summer. Anoxic (purged with N-2) incubations always exhibited daytime maxima in AR. Nighttime AR assays conducted under submerged or anoxic conditions exhibited lower rates of AR than nighttime aerial incubations. These data suggest that the N-2-fixing microorganisms in the epiphytic community rely on aerobic metabolism for nighttime N-2 fixation, presumably in order to provide energy and reductant to nitrogenase. In situ rates of nighttime N-2 fixation are, thus, partially controlled by tidal stage, as submergence promotes the development of anoxic conditions. AR rates were extremely low in epiphytic films held continuously in the dark, pointing to the dependence on photosynthate by N-2-fixing microorganisms.