Environmental and Physiological Controls on Diel Patterns of N2 Fixation in Epiphytic Cyanobacterial Communities

Abstract

Epiphytic communities dominated by cyanobacteria on standing, dead Spartina alterniflora stems in a North Carolina salt marsh were 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 N2 fixation were investigated with acetylene reduction (AR) assays. Incubation conditions were varied in AR assays to determine physiological controls on N2 fixation rates. AR assays conducted under aerial conditions exhibited significant daytime maxima only in the spring. In contrast, N2 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 N2) 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 N2-fixing microorganisms in the epiphytic community rely on aerobic metabolism for nighttime N2 fixation, presumably in order to provide energy and reductant to nitrogenase. In situ rates of nighttime N2 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 N2-fixing microorganisms.