PHENOTYPIC VARIATION IN EXOPOLYSACCHARIDE PRODUCTION IN THE MARINE, AEROBIC NITROGEN-FIXING UNICELLULAR CYANOBACTERIUM CYANOTHECE SP

The aerobic nitrogen-fixing cyanobacterium, Cyanothece sp. BH68K produ ces non-mucoid variants defective in exopolysaccharide (EPS) productio n at a high frequency. The EPS-producing wild-type colonies (EPS(+)) h ave a characteristic smooth and shiny appearance which allows them to be easily distinguished from the EPS(-) variants. When grown on agar p lates lacking a source of combined nitrogen, the EPS(-) variants exhib ited a yellow phenotype typical of nitrogen starvation. These EPS(-) v ariants showed varying degrees of reversion back to the EPS(+) phenoty pe. After reversion, they exhibited normal diazotrophic growth on agar plates. Alcian blue and ruthenium red staining indicated that the EPS is an acidic polysaccharide, which is present as a loose network arou nd the cell, and which can be completely removed by low speed centrifu gation. The accumulation of EPS takes place mainly during the stationa ry phase. All EPS(-) variants failed to produce any EPS. Analysis of g rowth of wild-type and EPS(-) variants revealed that EPS production is beneficial for diazotrophic growth on solid medium, but not in liquid medium, In addition, EPS phenotypic alteration may have some advantag e in the dispersal of cells from one place to another in the natural e nvironment.