Journal of Applied Phycology 5: 387-394, 1993. © 1993 Kluwer Academic Publishers. Printed in Belgium. 387 Exopolysaccharide production by a unicellular cyanobacterium isolated from a hypersaline habitat Roberto De Philippis, Maria Cristina Margheri, Elio Pelosi & Stefano Ventura Dipartimento di Scienze e Tecnologie Alimentari e Microbiologiche, Sez. Microbiologia Applicata, University, and Centro di Studio dei Microrganismi Autotrofi, CNR, Piazzale delle Cascine 27, 1-50144 Firenze, Italy Received 23 February 1993; revised 12 March 1993; accepted 13 March 1993 Key words: exocellular polysaccharide, cyanobacteria, Cyanothece, salinity, nutrient depletion, monosaccharidic composition Abstract The unicellular cyanobacterial strain 16Som2, isolated from a Somaliland saltpan and identified as Cyanothece sp., is characterized by cells surrounded by a thick polysaccharidic capsule, the external part of which dissolves into the medium during growth, causing a progressive increase in culture viscosity. In spite of this, the thickness of the capsule remained almost constant under all the culture conditions tested, demonstrating that the processes of its synthesis and solubilization occurred at a similar rate. The synthesis of carbohydrates was neither enhanced by increasing salinity (sea-water enriched with NaCI in the range 0 to 2.0 M) nor by Mg 2 , K or Ca 2 + deficiencies. In contrast, N-limitation and, to a lesser extent, P-limitation induced a significant enhancement of carbohydrate synthesis; in particular, N-deficiency stimulated the synthesis of all the carbohydrate fractions (intracellular, capsular and soluble). The soluble polysaccharide, separated from the culture medium and hydrolyzed with 2N trifluoroacetic acid, showed a sugar composition consisting of glucuronic acid: galacturonic acid: galactose: glucose: mannose: xylose: fucose in a molar ratio of 1: 2: 2.4: 6.8: 4.8: 2.9: 1.6. Cyanothece sp. culture subjected to nitrogen starvation synthesized polysaccharide with a mean productivity of 115 mg (EPS) 1- ld- 1, for the polymer solubilized into the medium, and of 15 mg (CPS) 1- 'd- ' for the capsular polysaccha- ride. Abbreviations: CPS = capsular polysaccharide, EPS = exocellular polysaccharide, PAR = photosynthetic active radiation Introduction available, on both unicellular and filamentous cy- anobacteria of freshwater or marine origin, con- The increased demand for natural biopolymers cerning either the chemical composition of the for various industrial applications :(Lewis et al., polysaccharides and the influence of different 1988; Linton et al., 1991) has led in recent years growth conditions on EPS production (Painter, to a renewed interest in exopolysaccharide pro- 1983; Panoffet al., 1988; Phlips et al., 1989; Vin- duction by cyanobacteria. Many studies are now cenzini et al., 1990a, b; De Philippis et al., 1991),