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In the marine environment, the bacteria that colonize the surface of a seaweed affect its growth and development. In the laboratory, several seaweeds do not develop normal foliose thalli in axenic cultures, but adopt an atypical morphology. Adding certain bacteria to the culture, however, can result in reversion to the normal morphology. The aim of the present study was to examine the effects of various algal-associated bacteria on the growth, morphology, and reproduction of the green alga Ulva fasciata. In axenic culture without added bacteria, U. fasciata grew into undifferentiated tissue initially and later developed into a tubular thallus. Of the 53 bacterial isolates we obtained from different species of Ulva and Gracilaria, only 5 were capable of inducing differentiation and subse- quent growth in U. fasciata cultured in axenic conditions. One bacterial isolate (which we later iden- tified as Marinomonas sp.) was more effective than the others in inducing morphogenesis and growth in U. fasciata. The physical association of bacterial cells with thalli was found to be crucial for the induction of U. fasciata foliose morphology, growth, and spore release. The culture filtrates from the 5 individual bacterial isolates and from a consortium of all 5 of them had less effect on morpho- genesis and growth of U. fasciata than the bacteria themselves had, but more effect than the control (no bacteria added). Analysis of partial 16S rRNA gene sequences from all 5 isolates with morpho- genesis-inducing ability led us to identify them as Marinomonas sp. and Bacillus spp. We found that the physical association of bacteria is essential for normal growth of the seaweed, suggesting a sym- biotic interaction between seaweed and bacteria.

Isolation of seaweed-associated bacteria and their morphogenesis-inducing capability in axenic cultures of the green alga Ulva fasciata