Phylogenetic analysis of psychrophilic bacteria isolated from the Japan Trench, including a description of the deep-sea species Psychrobacter pacificensis sp nov.

Phylogenetic positions of psychrophilic bacteria isolated from the Japan Tr ench were determined by sequencing analysis of PCR-amplified bacterial smal l subunit (16S) rRNA genes. Between surface and deep-sea psychrophiles. dis tinct positions clearly differed within the gamma-Proteobacteria, In phylog enetic analysis using neighbour-joining, maximum-parsimony and maximum-like lihood, strains from surface seawater were inferred to be located in the Ha lomonas aquamarina-meridiana clade within the family Halomonadaceae, Strain s from deep seawater (5000-6000 m), however, formed a novel monophyletic cl ade within the Moraxella-Psychrobacter branch in the family Moraxellaceae, showing separation from terrestrial and Antarctic relatives. These deep-sea strains were also discriminated from other known Psychrobacter species in phenotype, e.g. limited growth in the absence of NaCl (optimum at about 3% NaCl), positive urease activity, acid production from xylose and arabinose, and the presence of multiple fimbriae, DNA relatedness values among six de ep-sea strains were >85% in DNA-DNA hybridization experiments and greater t han or equal to 98% in aligned 165 rDNA sequences. From this evidence, a ne w species, Psychrobacter pacificensis, is proposed for these deep-sea psych rophiles; the type strain of Psychrobacter pacificensis is strain NIBH P2K6 (T) (= IFO 16270(T)), Occurrence of psychrobacters in cold Japan Trench dee p seawater and at the Antarctic sea surface suggests that deep-sea bacteria l habitation and evolution have been mediated by global deep-ocean circulat ion linked to the sinking of cooled seawater in polar regions.