Life is bubbling to the seafloor with oil from the depths

The COVID-19 pandemic is a clear reminder that we move through a world made up of invisible life. Bacteria, viruses, and other microscopic organisms regulate Earth’s vital functions and resources, from the air we breathe to all of our food and most of our energy sources. An estimated one-third of Earth’s microbes are literally hidden, buried in sediments far below the ocean floor. Scientists have now shown that these “deep biosphere” microbes do not stay, but instead bubble to the ocean floor along with fluids from buried oil fields. These hitchhikers in oil leaks are diversifying the microbial community that thrives on the seafloor, impacting deep-sea processes, such as the carbon cycle, that have global implications.

“This study confirms that oil seeps are a conduit for transporting life from the deep biosphere to the sea floor,” says co-author Emil Ruff, a scientist at the Marine Biology Laboratory (MBL), Woods Hole. The study, led by Anirban Chakraborty and Casey Hubert of the University of Calgary, is published this week in procedures of the National Academy of Sciences.

The team analyzed 172 samples of sediments from the sea floor of the eastern Gulf of Mexico that had been collected as part of a 2011 survey for the oil industry. A fraction of these samples contained migrated gaseous hydrocarbons, the main components of oil and gas. These oil seeps at the bottom of the ocean harbored distinct microbial communities with bacteria and archaea that are known inhabitants of deep biosphere sediments.

“While sedimentation slowly buries microbial communities in the deep biosphere, these results show that it is more of a two-way street. Returning microbes offer a window to life buried below,” says Hubert. “These relatively accessible surface sediments give us an idea of ​​the vast underground kingdom.”

The study also adds a new dimension to understanding the metabolic diversity of microbial communities of oil seepage from the seafloor. “If it weren’t for the microbes that live in oil leaks, the oceans would be full of gas and oil,” says Chakraborty.

Co-authors Bernie Bernard and James Brooks of TDI-Brooks International obtained the 172 sediment cores from the Gulf of Mexico and performed geochemical tests on them, setting the stage for microbiology testing at the University of Calgary.

“One of the strengths of this study is the large number of samples analyzed, allowing robust statistical inferences from the microbes present in the oil leaks,” says Ruff. Because the seabed is so difficult to access, explorations of deep-sea ecosystems are often limited by the number and quality of samples. The team used metagenomic approaches to determine which microbes were present in the sediment samples and the genome sequencing of particularly interesting organisms to indicate what their activity might be underground.