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Effects of algal excreted matters on epiphytic bacteria on the brown alga, Eisenia bicyclis



Effects of algal excreted matters on epiphytic bacteria on the brown alga, Eisenia bicyclis



Bulletin of National Research Institute of Aquaculture 0(28): 39-90



Preservation of seaweed beds is one of the most important factors for right management of coastal environment in Japan. Many seaweed beds are going to diminish or extinct recently due to "Iso-yake" (seaweed withering phenomenon on beach) or development of coastal area and contamination of seawater. It is an urgent issue to preserve or raise healthy seaweed beds. As a basic information to assess at seaweeds fitness, interactions between macroalgae and epiphytic bacteria should be considered. This study was done to clarify the relationship between a brown alga Eisenia bicyclis and its epiphytic bacterial communities, in stress with roles of algal excreted organic matters (EOM). E. bicyclis is a perennial macroalga distributed along the central Pacific coast of Honshu, Japan, and forms a dense marine forest on the rocky bottom. The results obtained are summarized as follows: SEM observation revealed that adhering cocci were dominated on blade surfaces of E. bicyclis. Ratios of respiratory active cells among these cocci were 32 - 60% on new blade surfaces where bacterial densities were low, and almost 100% on old blade surfaces where bacterial densities were high. The ratio of respiratory active cells became lower when a blade was incubated under light condition for several hours. The excretion of phenolic substances, which have antimicrobial activities, from E. bicyclis was larger from younger blade and it was also accelerated under light condition. From these results, it is suggested that activities of epiphytic bacteria on E. bicyclis blade surfaces were suppressed by excreted antibacterial substances such as phenols from the algae. Epiphytic bacterial communities on new blade surfaces of E. bicyclis needed algal EOM for their growth. A bacterial strain Moraxella sp. was isolated from a new blade surface of E. bicyclis and the effects of growth stimulation by EOM on this bacterium were examined. EOM included some substances which stimulated the bacterial growth in a small amount, simultaneously with other organic nutrients utilized by bacteria. The substance was thought to be a polysaccharide and its minimum effective concentration was 0.3 mu gC/ml as a dissolved organic carbon. The bacterium also could produce extracellular polysaccharide from mannitol, which is a major primary product of photosynthesis of brown algae. Extracellular polysaccharide usually promotes attachment abilities of bacteria. These results indicate that bacteria who colonized to blade surfaces of E. bicyclis early have a strong relationship with substrate macroalgae through algal excreted materials. Alginate degrading bacterial communities occurred on old blade surfaces of E. bicyclis were inhibited their alginate degrading activities by algal EOM. Some alginate degrading bacteria isolated from the old blade surfaces of E. bicyclis were inhibited their alginate degrading enzymatic activities but growth. This inhibition by EOM varies depending on the algal blade condition. EOM from healthy blades inhibited alginate degrading activity of these bacteria. On the other hand, the EOM from decomposing algal blades stimulated bacterial activities contrary. These results suggest that decomposition of algae by epiphytic bacteria was controlled by substrate macroalgae through algal excreted materials.

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Accession: 010544896

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