Several examples of stimulative and inhibitory effects of bacteria on microalgal growth are introduced, and the importance of bacteria in algal mass culture is investigated. Diatoms are often used as live food for planktonic larvae of sea urchin and bivalves. Monodispersed Chaetoceros ceratosporum has been cultivated by using clean, high nutrient content, deep seawater (DSW). However, the growth rate and cell yield of 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 relatively constant cell yield was obtained. Another diatom species, C. gracilis, 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, Nitzschia sp., was stimulated by a bacterial film of Alcaligenes on the surface of the substratum. On the other hand, a strain of Flavobacterium sp. isolated from natural seawater during the decline period of an algal bloom had a strong algicidal effect on the red tide plankton, Gymnodinium mikimotoi. Recent reports demonstrate that many bacterial strains have significant algicidal effects on many species of red tide plankton. These results indicate that bacterial effects should be taken into account to obtain stable mass culture of food microalgae.

The bacterial isolates from normal and diseased branches of Kappaphycus alvarezii and Eucheuma denticulatum in the Philippines were examined for possible role in the development of the ice-ice disease. The numbers of bacteria on and in ice-iced branches were 10-100 times greater than those from normal, healthy ones. Grampositive bacteria predominated in almost all branch sources, but with an increasing proportion of agar-lysing bacteria in branches suffering from the ice-ice disease. These agar-lysing bacteria were composed of yellow and non-pigmented, spreading colonies identified to the Cytophaga-Flavobacterium complex and the Vibrio group. Among isolates which mainly appeared on ice-iced branches, two strains, designated as P11 (Vibrio sp.) and P25 (Cytophage sp.), which showed pathogenic activity, were obtained. These strains caused early ice-ice whitening of K. alvarezii especially when subjecting branches to environmental stress, such as reduced salinity and light intensity, suggesting that these bacteria were occasionally pathogenic. This paper offers new evidence of bacterial role in the development of so-called ice-ice disease among farmed species of Kappaphycus.

To evaluate the role of fish larvae as a Link between the microbial loop and higher trophic levels, predation of protozoan zooplankton by young larvae was investigated. More than 400 individual fish larvae with total lengths of less than ca 10 mm in 52 different taxonomic groups were collected at different sampling times from several coastal regions, and the gut contents of larvae were exarnined under epifluorescence microscopy after staining with DAPI. Among numerous fragments of copepod nauplii, many flagellate-like cells with a size of 5 pm and ciliate-like cells with a size of 20 to 30 pm were frequently recognized. The number of protozoan cells varied significantly from one larva to another. Some individuals had more than 60 protozoa. while others contained none at all. The amount of protists contained in the gut of larvae depended on the fish species and did not show any trend with the body or rnouth sizes of larvae, nor the sampling site or season. Fish taxa were divided into 3 groups depending on the amount of protists in the gut: 'abundant', 'moderate', and 'none' The Acanthopterygii group contained the highest concentration of protozoa. Results of the present study suggested that fish larvae of some taxonon~ical groups were important predators of protozoa and may be an important link between the microbial loop and the grazing food chain.

A bacterium 5N-3 possessing a remarkable inhibitory effect on the growth of Gymnodinium nagasakiense was isolated from Uranouchi Inlet, Kochi. This bacterium was tentatively identified as Flavobacterium sp. The growth inhibiting effect of 5N-3 on G. nagasakiense was drastic in particular when the alga was in the logarithmic growth phase, and cell density decreased to less than 1% of the initial concentration within 4 days after inoculating 5N-3, indicating that the effect was algicidal.The effect was obtained when the density of the bacterium was more than 106 cells/ml. However, they grew very rapidly up to 108 cells/ml by using extracellular released organic carbon from various phytoplankton species. On the other hand, the algicidal effect of 5N-3 was only observed on G. nagasakiense but not on Chattonella antiqua, Heterosigma akashiwo, or Skeletonema costatum.These results indicate that the effect of 5N-3 was G. nagasakiense-specific and suggest that it grows to a level of cell density effective in inhibiting the alga in the field by using naturally occurring organic carbon from phytoplankton.

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