Characterization of two novel psychrophilic and piezotolerant strains, Shewanella psychropiezotolerans sp. nov. and Shewanella eurypsychrophilus sp. nov, adapted to an extreme deep-sea environment.

Three marine bacterial strains designated YLB-06, YLB-08 and YLB-09 were isolated under high hydrostatic pressure from deep-sea sediment samples collected from the Southwest Indian Ocean. They were Gram-stain-negative, oxidase- and catalase-positive, facultative anaerobic and motile. In addition, the strains were capable of growing at 0-20 °C (optimum 4-10 °C) and 0.1-40 MPa (optimum 0.1 MPa), were psychrophiles and piezotolerant, and could use trimethylamine N-oxide (TMAO), DMSO, elemental sulfur and insoluble Fe (III) as terminal electron acceptors during anaerobic growth. Strain YLB-06 could also use nitrate, and strains YLB-08 and YLB-09 could use nitrite as a terminal electron acceptor. Phylogenetic tree analyses based on 16S rRNA gene sequences and 400 optimized universal marker sequences indicated that the strains belonged to the genus Shewanella. The 16S rRNA gene highest similarity, together with the estimated ANI and DDH values for these strains with their related type strains, were below the respective thresholds for species differentiation. The ANI and DDH values between YLB-08 and YLB-09 were 99.9% and 91.8%, respectively, implying that they should belong to the same genospecies. The YLB-06 genome had duplicated genes, and multiple movement modalities, attachment modalities, biofilm synthesis systems, intercellular interactions and a strong antioxidant system, which were all beneficial for survival in an extreme deep-sea environment. The G + C contents of strains YLB-06, YLB-08 and YLB-09 were 45.1, 43.5 and 43.6 mol%, respectively. Based on polyphasic taxonomic properties, two novel psychropiezotolerant species are proposed, Shewanella psychropiezotolerans sp. nov. with YLB-06 (=MCCC 1A12715 = KCTC 62907) and S. eurypsychrophilus sp. nov with YLB-08 (=MCCC 1A12718 = KCTC 62909) as type strains.