Polyphasic classification of 0.2 mu m filterable bacteria from the westernMediterranean Sea

The 0.2 mu m filtration of sea water samples from the Mediterranean Sea (Ba y of Calvi, Corsica), collected from 10 m and 35 m depth led to the isolati on of several Gram-negative bacterial strains able to grow on full-strength media as well as on diluted media. The analysis of the 16S rRNA gene seque nces and estimation of the phylogenetic relationships of these facultative oligotrophic bacteria indicated that they grouped into two phylogenetic bra nches. The strains RE10F/2, RE10F/5 (10 m depth samples) and RE35/F12 (35 m depth samples) were assigned to the gamma-subclass, while RE35F/1 (35m dep th sample) was assigned to the alpha-4-subclass of the Proteobacteria. The strains RE10/F2 and RE10/F5 were most closely related to species and strain s of the Pseudoalteromonas group, whereas the strain RE35F/12 placed adjace nt to the family Vibrionaceae. The phylogenetic analysis of strain RE35F/1 revealed that this bacterium clusters with marine strains and species of th e aerobic anoxygenic phototrophic bacteria Erythrobacter as well as Erythro microbium and more distantly to Sphingomonas spp. Supplementary to those ge notypic classifications the chemotaxonomic signatures including the major r espiratory lipoquinone systems, the cellular fatty acid compositions as wel l as the polyamine contents of the bacteria were investigated. The isolated organisms displayed differences in their physiological and biochemical pro perties to already described strains belonging to the same genera or famili es, as revealed by the comparative 16S rRNA analysis. Despite the fact that these bacteria were isolated from a 0.2 mu m filtrate, the cultured organi sms which were all rod-shaped, displayed width dimensions ranging from 0.4 up to 0.7 mu m, indicating that these bacteria were starvation forms at the time of isolation and not ultramicrobacteria as defined by TORELLA and MOR ITA (1981) Or by SCHUT et al. (1993). Because our isolated strains represen t potentially new taxa, this first investigation on 0.2 mu m filterable bac teria from the Western Mediterranean Sea supports the hypothesis that this bacterial fraction contributes to the diversity of marine bacteria.