STIMULATIVE AND INHIBITORY EFFECTS OF BACTERIA ON THE GROWTH OF MICROALGAE

Several examples of stimulative and inhibitory effects of bacteria on microalgal growth are introduced, and the importance of bacteria in al gal mass culture is investigated. Diatoms are often used as live food for planktonic larvae of sea urchin and bivalves. Monodispersed Chaeto ceros ceratosporum has been cultivated by using clean, high nutrient c ontent, deep seawater (DSW). However, the growth rate and cell yield o f diatoms fluctuated, to relatively large extent, with the season that DSW was collected. When some bacterial strains isolated from DSW were added to the culture, diatom growth was often stimulated and a relati vely constant cell yield was obtained. Another diatom species, C. grac ilis, was also stimulated by adding some bacterial strains to cultures . The positive effect of bacteria on diatoms was observed not only for planktonic species, but also on attached species. A benthic diatom, N itzschia sp., was stimulated by a bacterial film of Alcaligenes on the surface of the substratum. On the other hand, a strain of Flavobacter ium sp. isolated from natural seawater during the decline period of an algal bloom had a strong algicidal effect on the red tide plankton, G ymnodinium mikimotoi. Recent reports demonstrate that many bacterial s trains have significant algicidal effects on many species of red tide plankton. These results indicate that bacterial effects should be take n into account to obtain stable mass culture of food microalgae.