Plunging down to a depth of about 11,000 m (36,000 ft), the Mariana Trench is the deepest part of the ocean, so it's no surprise that we don't really know what's down there. New species and strange sounds have turned up recently, and now researchers have discovered a new group of oil-eating bacteria.

For the new study, the team collected samples of the microbial population from the deepest part of the trench. After recreating those environmental conditions in the lab, the researchers discovered that some of these bacteria consume hydrocarbons – in fact, it turned out that nowhere on Earth had more hydrocarbon-degrading bacteria than the Mariana Trench.

"Hydrocarbons are organic compounds that are made of only hydrogen and carbon atoms, and they are found in many places, including crude oil and natural gas," says Jonathan Todd, an author of the study. "So these types of microorganisms essentially eat compounds similar to those in oil and then use it for fuel. Similar microorganisms play a role in degrading oil spills in natural disasters such as BP's 2010 oil spill in the Gulf of Mexico."

But just where are the bacteria getting all these hydrocarbons? To probe the mystery, the team took samples of water from the surface, as well as from depths of 2,000, 4,000 and 6,000 m (6,560, 13,120 and 19,685 ft). Large amounts of hydrocarbons were found every step of the way.

"We found that hydrocarbons exist as deep as 6,000 meters below the surface of the ocean and probably even deeper," says Nikolai Pedentchouck, an author of the study. "A significant proportion of them probably derived from ocean surface pollution."

But these hydrocarbons aren't only from human pollution – the team also found a previously-unknown natural source.

"To our surprise, we also identified biologically produced hydrocarbons in the ocean sediment at the bottom of the trench," says Pedentchouck. "This suggests that a unique microbial population is producing hydrocarbons in this environment. These hydrocarbons, similar to the compounds that constitute diesel fuel, have been found in algae at the ocean surface but never in microbes at these depths."

The team suggests that these hydrocarbons may be a survival mechanism by the microbes, in order to cope with the immense crushing pressure under that much water.

"They may also be acting as a food source for other microbes, which may also consume any pollutant hydrocarbons that happen to sink to the ocean floor," says David Lea-Smith, an author of the study. "But more research is needed to fully understand this unique environment."

The researchers will continue to study this alien environment, with a particular focus on quantifying just how much of our pollution is getting down there. The team also plans to try to identify which microbes are the natural culprits.

The research was published in the journal Microbiome.

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