Wall-less bacteria are broadly distributed in diverse habitats. They evolved from a common ancestor within the Firmicutes phylum through reductive evolution. Here, we report the cultivation, characterization, and polyphasic taxonomic analysis of the novel free-living wall-less bacterium, Hujiaoplasma nucleasis zrk29. We demonstrated that strain zrk29 had a strong ability to degrade DNA and RNA both under laboratory conditions and in the deep sea. We found that nucleic acids induced strain zrk29 to release chronic bacteriophages which supported strain zrk29 and other marine bacteria to metabolize nucleic acids without lysing host cells. We also showed that strain zrk29 tolerated high hydrostatic pressure via two pathways: (i) by transporting cations into its cells to increase intracellular osmotic pressure and (ii) by adjusting the unsaturated fatty acid chain content in its cell membrane phospholipids to increase cell membrane fluidity. This study extends our understanding of free-living wall-less bacteria and provides a useful model to explore the unique adaptation mechanisms of deep-sea microbes. IMPORTANCE The unique physiology and survival strategies of the Tenericutes bacterium-a typical wall-less bacterium-have fascinated scientists and the public, especially in extreme deep-sea environments where there is high hydrostatic pressure (HHP) and limited availability of nutrients. Here, we have isolated a novel free-living Tenericutes strain from deep-sea sediment and have found that it metabolizes nucleic acids with the support of chronic bacteriophages. This Tenericutes strain tolerates HHP stress by increasing intracellular osmotic pressure and the unsaturated fatty acid chain content of phospholipids in its cell membrane. Our results provide insights into the unique physiology of deep-sea free-living Tenericutes bacteria and highlight the significant role that chronic bacteriophages play in assisting wall-less bacteria to adapt to harsh conditions.
Keywords: chronic bacteriophage; deep sea; high pressure tolerance; nucleic acid degradation; wall-less bacteria.