Deep under the ocean, at the boundaries between tectonic plates where the seafloor is geologically active, hydrothermal vents spew forth mineral-rich water.
When this hot water meets the cold, oxygenated surrounding seawater, it causes hydrothermal plumes to develop. They contain smoke-like particles of metal sulfide, and rise hundreds of meters above the seafloor and disperse thousands of kilometres away.
Bacteria are known to live and thrive around hydrothermal vents, but now new research has found evidence of an uncultured species of bacteria that lives happily in the billowing plumes that rise from them.
Named USulfurimonas pluma, it was discovered by genomic analysis of water samples from plumes in the Central Arctic and South Atlantic Ocean, according to a new study in Nature Microbiology.
Distinct from other species in its genus which use sulphide for their energy, these use hydrogen as an energy source.
“Obviously, they have found an ecological niche in cold, oxygen-saturated and hydrogen-rich hydrothermal plumes,” says first author Dr Massimiliano Molari from the Max Planck Institute for Marine Microbiology, Germany.
“That means we have to rethink our ideas on the ecological role of Sulfurimonas in the deep ocean – they might be much more important that we previously thought.”
Until now, gene sequences of bacteria in the genus Sulfurimonas had occasionally been detected in hydrothermal plumes – despite only being known to grow in low-oxygen environments.
“It was assumed that they were flushed there from seafloor vent-associated environments. But we wondered whether the plumes might actually be a suitable environment for some members of the Sulfurimonas group,” says Molari.
To investigate this, the scientists collected water samples and studied the composition and metabolism of bacteria found in them.
“We sampled plumes in extremely remote areas of ultraslow spreading ridges that were never studied before,” explains Professor Antje Boetius, group leader at the Max Planck Institute where this work was done..
“Collecting hydrothermal plume samples is very complicated, as they are not easy to locate. Sampling becomes even more difficult when the plume is located at depths of more than 2500 meters and below Arctic sea ice, or within the stormy zones of the Southern Ocean,” says Boetius.
By analysing the new bacterial species’ genome, they discovered it is reduced in size by 17-40%, compared to other Sulfurimonas strains. The species is missing genes typical for their relatives – including genes encoding for specific enzymes – while acquiring others that allow them to grow in pelagic oxygen-saturated environments.
“We think that the hydrothermal plume does not only disperse microorganisms from hydrothermal vents, but it might also ecologically connect the open ocean with seafloor habitats,” says Molari.
“Our phylogenetic analysis suggests that USulfurimonas pluma could have derived from a hydrothermal vent-associated ancestor, which acquired higher oxygen tolerance and then spread across the oceans. However, that remains to be further investigated.”
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