Bacteria living more than 4,000 meters (2.5 miles) below the surface of the Pacific Ocean are absorbing an estimated 10 percent of the carbon dioxide that oceans remove from the atmosphere every year. 

The team "discovered that benthic bacteria are taking up large amounts of carbon dioxide and assimilating it into their biomass through an unknown process. This was completely unexpected,” said study author Andrew Sweetman in a statement. "Their biomass then potentially becomes a food source for other animals in the deep sea, so actually what we've discovered is a potential alternative food source in the deepest parts of the ocean, where we thought there was none."

Writing in Oceanography and Limnology, the researchers say benthic bacteria, rather than seafloor animals, could be the “most important organisms” consuming organic waste that floats down towards the ocean floor.

To examine the cellular processes of benthic organisms, the team analyzed sediment samples taken from an area in the eastern Pacific Ocean between Hawaii and Mexico known as the Clarion-Clipperton Fracture Zone (CCFZ), a deep-sea ecosystem completely void of light but for flashes of bioluminescence and with a surprisingly biodiverse seabed environment. Bacteria here “dominated the consumption” of organic waste over just one or two days. When scaling their results, that equates to about 200 million tonnes of carbon dioxide that could be fixed into biomass every year, making the region a potentially important fixture in the deep-sea carbon cycle.

"We found the same activity at multiple study sites separated by hundreds of kilometres, so we can reasonably assume this is happening on the seabed in the eastern CCFZ and possibly across the entire CCFZ,” said Sweetman.

The CCFZ is home to more than just deep-sea sponges, sea anemones, shrimps, and octopods. The clay-like muddy bottom is topped with trillions of potato-sized polymetallic nodules containing deposits of nickel, manganese, copper, zinc, cobalt, and other minerals, according to Pew Charitable Trusts. It’s an area so rich in minerals that the International Seabed Authority has awarded 16 exploration contracts for groups interested in conducting surveys for future seabed mining.

Assuming the results can be applied to the greater CCFZ, the authors say their findings could have implications for mineral extraction in this region.

"If mining proceeds in the CCFZ, it will significantly disturb the seafloor environment,” said Sweetman. “Just four experiments similar to ours have been conducted in situ in the abyssal regions of the oceans; we need to know much more about abyssal seafloor biology and ecology before we even consider mining the region.”