| The γ-proteobacterial genus Halomonas is among the most abundant groups of microorganisms in hydrothermal-vent and pelagic habitats. Characterization of six novel Halomonas strains, including four new species isolated from hydrothermal plumes, low-temperature seafloor vents and sulfide rock, revealed traits consistent with other members of the genus, such as versatile heterotrophy, strong resistance to Cd 2+, and growth with 0.5 to ≥22% total salts. The strains shared more phenotypic traits with each other than with other Halomonas spp., including cold-shifted cardinal growth temperatures. Selected Halomonas spp. were also strongly resistant to CO2+, Cu2+ and Zn2+. The strains tested precipitated 55–98% of Cd2+ from solution, possibly as CdS or CdHPO 4, but only during static (versus agitated) growth. Amplification of Halomonas and Marinobacter 16S rRNA genes from low-temperature hydrothermal-vent and deep-sea samples delineated a distinct biogeography for four clades within these genera and revealed that certain cultured representatives cluster with environmentally relevant groups. The distribution patterns revealed that Halomonas sub-group 2A comprised a subseafloor population at Axial Seamount on the Juan de Fuca Ridge and suggested that other clades may include members that are cold-adapted or associated with metal-sulfide deposits. Growth curves produced using a range of hydrostatic pressures, temperatures and salinities confirmed that the Halomonas strains tested would grow well in cool to warm hydrothermal-vent and associated subseafloor habitats, but poorly or not at all under cold deep-sea conditions. Elevated salinity enhanced growth under certain high-hydrostatic-pressure and low-temperature conditions, highlighting a synergistic effect on growth for these combined stresses. Profiles of cytosolic and membrane proteins of H. hydrothermalis obtained at 30°C under high and low salinity and hydrostatic-pressure conditions indicated a variety of hydrostatic-pressure-salinity effects, including proteins whose expression was induced by either elevated salinity or hydrostatic pressure, but not by the combination of the two. The interplay between salinity and hydrostatic pressure on microbial growth and physiology suggests that a hydrostatic-pressure adaptation, not a salt adaptation, may explain the euryhaline phenotype of members of the genus Halomonas living in deep-sea environments. These versatile psychrotolerant bacteria may flourish in subseafloor habitats from which they could seed the overlying water column. |
