Four bacterial strains were isolated from larval cultures and collectors of the scallop Pecten maximus. They showed a high level of intragroup genomic relatedness (84-95 Yo) as determined by DNA-DNA hybridization. The cells were Gram-negative, strictly aerobic, motile, ovoid rods. They grew at temperatures from 15 to 37 "C and from pH 7.0 to 10, but did not grow in the absence of NaCl and required growth factors. They had the ability to use a wide variety of compounds as sole carbon source : D-mannose, D-galactose, D-f ructose, Dglucose, D-xylose, melibiose, trehalose, maltose, cellobiose, sucrose, mesoerythritol, D-mannitol, glycerol, D-sorbitol, meso-inositol, succinate, propionate, butyrate, y-aminobutyrate, DL-hydroxybutyrate, 2-ketoglutarate, pyruva te, f uma ra te, g I y c i ne, L-&-a Ian ine, &a I an ine, L-g I u tama te, L-arg in i ne, Llysine, L-ornithine and L-proline. They exhibited oxidase and catalase activities but no denitrif ication activity. The isolates did not contain bacteriochlorophyll a. The G+C content ranged from 57.6 to 58 molo/o. Phylogenetic analyses of the 16s rRNA sequence revealed that these isolates belong to the genus Roseobacter. On the basis of quantitative hybridization data, it is proposed that these isolates should be placed in a new species, Roseobacter gallaeciensis. The type strain is Roseobacter gallaeciensis BS1 07T (= CIP 1052103.

: A marine strain (BS107), identified as a Roseobacter species, was antagonistic to Vibrio species on agar plates. Results suggested that the inhibitory effect was displayed only in the presence of another bacterium. Quantification of the antibacterial activity showed that 48-hour-coculture supernatants from BS107 and another bacterial strain (V. anguillarum 408) reached the highest titers of bacterial inhibition. The antibacterial substance was also liberated when supernatants from V. anguillarum 408 were added to pure cultures of the inhibition-productive bacterium. The presence of a proteinaceous molecule may induce BS107 to display the inhibitory effect. The antibacterial substance was sensitive to trypsin (8000 U/ml) and stable at 100 degrees C. Cell extracts of the isolate BS107 (10(6) cells/ml) significantly enhanced scallop larval survival, thus being beneficial to the rearing process.