Comparative studies on calothrix isolates from Nepalese rice-fields

Algal samples were collected from three rice-fields in Nepal: A) Parwanipur (100 m), B) Khumaltar (1336 m) and C) Kakani (2064 m); the rice- field algal flora from 1982, 1983 and 1986 samples was recorded. Three Calothrix strains were isolated from 1986 samples, one from each of the three sites, as representatives of the rice-field flora. A study was carried out on morphology of these Calothrix isolates, in particular, the influence of phosphorus deficiency on morphology. Starting with hormogonium production, morphological changes were followed in one set of cultures without any further addition of phosphate and in another set, 10 mg 1(^-1) P was added to P-limited cultures. With increase in phosphorus deficiency, synthesis of hormogonia ceased in all the strains, with obvious increase in cyanophycin granules and decrease in polyphosphate granules. Formation of intercalary heterocysts in Calothrix D794 and D795 and separation discs in Calothrix D796 was followed by the formation of intra-thylakoidal vacuoles and false-branches in all, but no hair cells formed in any strain. The addition of further phosphate to P-limited cultures led to renewed synthesis of hormogonia within 18 h in all strains. The effect of light flux and nutrient concentration on morphology of three strains was tested. The effect of change in light flux on gas vacuole formation was examined in Calothrix D794. Gas vacuoles continued to form but at a lower rate when the alga was shifted to 170 pmol photon m(^-2)s(^-1) after initial period of incubation in dark. Gas-vacuolated hormogonia formation, release and disappearance of gas vacuoles in free hormogonia occurred within two hours of exposure to 170 photo m(^-2)s(^-1).