A genetic analysis suggests that petroleum-eating bacteria that live thousands of metres beneath the seafloor seep out into the ocean where they may then float along currents
26 August 2022
By James Dinneen
Heat-loving, petroleum-eating bacteria that live thousands of metres beneath the seafloor seep out into the ocean. They may then float around in a dormant state before coming back to life in other distant oil reserves.
As much as 45 per cent of all microbes live underground in hot rocks, subsisting on hydrocarbons and other chemical energy. This “deep biosphere” is the largest habitat on Earth, yet little is known about the ecology of all that lives there, says Casey Hubert at the University of Calgary in Canada. “You can’t put a radio collar on a microbe and follow it around like you would with a grizzly bear,” he says.
Hubert and his colleagues used a combination of acoustic surveys and genetic analysis to better understand how bacteria from the deep biosphere get around.
The researchers identified a section of the continental shelf south-west of Nova Scotia where petroleum was seeping out of cracks and into the ocean using an autonomous submarine equipped with sonar. They then took scoops of mud from 14 sites on the seafloor by lowering a tube from their research ship CCGS Hudson.
Heat-loving bacteria – called thermophiles – can become dormant spores when exposed to cold. To identify any thermophilic spores in the samples, the researchers heated them in the lab to 80°C (176°F). “That kills most of the stuff”, says Hubert. “But the spores wake up and start to have quite a party.” Some spores can remain viable in hostile environments for thousands and possibly millions of years.
The researchers sequenced portions of the DNA of the revived bacteria. The types of microbes in the samples that contained petroleum were distinct from those found in samples without petroleum. They also had genes enabling them to break down petroleum. The researchers then looked at drilling samples from petroleum reserves thousands of metres underground and found that they contained the same kinds of petroleum-eating microbes.
The finding is “very beautiful evidence” that the bacteria had been carried to the seafloor from deep in the earth within the petroleum seeps, a process which can take decades to hundreds of years, says Juan Höfer at Pontifical Catholic University of Valparaíso in Chile.
The bacteria’s journey probably doesn’t end there. Ocean currents could move the dormant bacteria elsewhere on the seafloor, with most of them ending up nearby but some travelling thousands of kilometres away, says Hubert.
Returned to the seafloor, the spores would then be buried in sediment, sinking to hotter depths over millions of years. If they were lucky enough to land on a petroleum deposit, the dormant spores might then come alive again, says Hubert. “They’re the toughest, most resilient life forms we know of.”
Rika Anderson at Carleton College in Minnesota says this “microbial dispersal loop” could help explain why bacteria found in petroleum reserves separated by thousands of kilometres share more genes than bacteria in different environments nearby. When dormant bacteria from one reserve made it to another, they could exchange genes.