Exopolysaccharide production by a unicellular cyanobacterium isolated from a hypersaline habitat

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 NaCl in the range 0 to 2.0 M) nor by Mg2+, K+ or Ca2+ 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) l-1d-1, for the polymer solubilized into the medium, and of 15 mg (CPS) l-1d-1 for the capsular polysaccharide.