EFFECTS OF GROWTH-CONDITIONS ON EXOPOLYSACCHARIDE PRODUCTION BY CYANOSPIRA-CAPSULATA

Batch cultures of Cyanospira capsulata, a heterocystous cyanobacterium possessing a thick polysaccharidic capsule, were characterized by increasing viscosity owing to the continuous release of a soluble polysaccharide (EPS) into the culture medium. Both capsulated trichomes and solubilized EPS contributed to the flow properties of whole cultures. A typical pseudoplastic behaviour, the more marked the more aged were the cultures, was evidenced. The production of EPS was investigated under different growth conditions by changing some nutritional and physical parameters known to affect the synthesis of exopolysaccharides in algae and cyanobacteria. Among the factors tested (Ca2+, Mg2+ or PO4(3-) deficiencies, salinity and pH) only Mg2+ shortage caused a significant enhancement of the EPS production. Under continuous illumination in open ponds, the EPS productivity of batch cultures on standard mineral medium was about 5.8 g m-2 day-1, whereas under Mg2+ deficiency values of about 6.8 g m-2 day-1 were obtained, with a consequent increase of the cultures' viscosity.