Aspects of Nitrogen Fixation in Sargassum Communities Off the Coast of Florida

The magnitude and character of nitrogen fixation in Sargassum communities was studied for three different species, S. fluitans Borgesen, S. natans J. Meyen and S. filipendula C. Agardh. Nitrogen fixation activity was measured using the acetylene reduction technique. The character of epiphytic populations on the surface of Sargassum was investigated by scanning electron microscopy. All three species of Sargassum exhibited the potential for high levels of acetylene reduction. Mean rates of up to 7.1 μmol C2H4 produced·g−1 (Sargassum dry wt.)·h−1 were observed at one location. Nitrogen fixation activity was strongly light dependent. Saturation light intensity for nitrogen fixation was low, i.e. < 100 μE·m−2·s−1, and no photoinhibition was observed under full sunlight intensity (i.e. photon flux of ≈ 2000 μE·m−2·s−1). Results indicated that cyanobacteria were responsible for nitrogen fixation. Both Calothrix and LPP type cyanobacteria were commonly represented on the surface of the Sargassum. Activity associated with the benthic species S. filipendula exhibited significant seasonal variability. Nitrogen fixation activity in pelagic samples was variable but high throughout the year. The contribution of nitrogen fixation to the nitrogen budget of Sargassum communities appears to be particularly pronounced in the pelagic environment.