Heterotrophic growth and nitrogen fixation in the filamentous blue-green alga Anabaena sp

Experiments were conducted to study the effect of five different organic substrates viz. lactose, fructose, mannose, xylose and sodium acetate on the heterotrophic growth characteristics with emphasis on growth, pigment composition, heterocyst frequency and nitrogen fixation of Anabaena sp. under light and dark conditions. Lactose was found to be the best carbon source to support the heterotrophic growth of the alga. Phycoerythrin pigments were synthesized in photoheterotrophic cultures but pigment production was not observed in dark-grown cultures. The alga fixed nitrogen to a limited extent in the dark in pure inorganic medium. But when suitable organic substrates were supplemented, appreciable nitrogen fixation was observed in dark heterotrophic as well as in photoheterotrophic cultures.