Growth, nitrogen fixation and occurrence of epiphytic algae at different pH in the cultures of two species of Azolla

Change in biomass, nitrogenase activity, chlorophyll and type of epiphytic algal contamination were determined for Azolla microphylla and A. pinnata, as a function of pH. These studies were done in a polyhouse at a constant temperature. The growth rate increased till pH 6 in both species, but sharply declined at pHs 7, 8, and 9. A. microphylla showed better growth than A. pinnata at all the pHs studied. A similar trend was found for chlorophyll content. Nitrogenase activity too showed variation with pH. It was maximum at pH 5 for both A. microphylla and A. pinnata but significantly higher for A. microphylla than for A. pinnata. Algal contamination was also different in the two species. Oscillatoria, Lyngbya, Calothrix, Tolypothrix and Nostoc could grow as epiphytic algae on A. microphylla, whereas only green algae and Lyngbya grew on A. pinnata.