BACTERIAL-ALGAL INTERACTIONS IN POLYSACCHARIDE PRODUCTION

The effects of phosphate limitation and of the presence of marine bact eria during phosphate limitation on growth and polysaccharide producti on in the diatom Cylindrotheca fusiformis grown in batch cultures are described in this paper. Growth of C. fusiformis was inhibited under l ow Pi (inorganic phosphate) conditions, corresponding to an increasing N/P ratio, and higher amounts of polysaccharides were extruded in the medium, in particular during the stationary phase of growth. The pres ence of bacteria reduced phytoplankton cell density only when the phos phate added corresponded to 1/6 of the initial amount. Even when diato m cell growth was not affected, the presence of bacteria stimulated a higher polysaccharide production. These results are interpreted in the light of the fact that nutrient-stressed phytoplankton cells produced and released a higher amount of polysaccharides and, as bacteria exhi bited a better utilization of phosphate than algal cells, their presen ce accentuated the Pi depletion, resulting in a higher polysaccharide production.