May 12, 2020
Naturally occurring oil and gas seeps act as ‘elevators’ for microbial life in Earth’s deep sedimentary layers
Underwater oil and gas seeps are a channel for transporting life from the deep biosphere to the ocean floor, confirms a new study led by Faculty of Science researchers at the University of Calgary.
Postdoctoral fellow Dr. Anirban Chakraborty, PhD, and Dr. Casey Hubert, PhD, associate professor in the Department of Biological Sciences and Campus Alberta Innovation Program (CAIP) Chair in Geomicrobiology, report that microbial life is bubbling up to the ocean floor along with fluids from deep, naturally-occurring petroleum reservoirs.
Their findings were recently published in Proceedings of the National Academy of Sciences.
Deeply buried life occasionally rises to sea floor
Over the course of four years, Chakraborty, Hubert, and the team analyzed a set of over 170 seafloor sediment samples from the eastern Gulf of Mexico that industry partner TDI-Brooks International Inc. had collected as part of a 2011 survey for the oil industry.
“The Gulf of Mexico is widely known for natural petroleum seepage,” Chakraborty, the study’s lead author, explains. “It’s also a hot spot for petroleum exploration for oil companies, as petroleum seepage is one way to find out whether there might be oil and gas reservoirs beneath the sea floor.”
Chakraborty and Hubert’s research is interested in misplaced microbes — or microbes found in an environment where they are not known to originate. In this study, they discovered certain microbial groups in the sediment close to the sediment-water interface on the sea floor that normally exist well below it.
Co-authors Bernie Bernard and James Brooks of TDI-Brooks International performed geochemical testing on all 172 sediment samples, setting the stage for the Energy Bioengineering and Geomicrobiology Group (EBG) to carry out microbiology testing.
Within this sample set, they found evidence of oil seepage, gas seepage, and combined oil and gas seepage. “Our collaborators provided us with detailed hydrocarbon geochemistry data to support these claims, and categorize the sample set,” he says. “That allowed us to carry out our microbial assessments based on those characterizations.”
Gas seeps on the ocean floor showed distinct microbes that are well known to inhabit the deep sedimentary layers below the sea floor.
“These microbes are bubbling up from deeper layers in the subsurface, like riding an elevator,” Hubert explains. “Most of the time, you wouldn’t expect to see those bacteria at the sediment-water interface, but genomics shows us that when this buoyancy mechanism is present, the cells are hitching a ride and getting expelled up to the shallower layers.
"Whereas in normal seabed settings sedimentation processes bury microbial populations in surface sediments deeper and deeper over time, these results suggest that it’s more of a two-way street.”
Better understanding of organisms that are seldom seen
Chakraborty says microbial dispersal in the context of subsurface ecology is an enigmatic question, and scientists are still working to understand microbial life so far beneath Earth’s surface.
“Thanks to our partners, we were able to support our findings with statistics because we had such a large sample set,” he explains. “However, the marine subsurface is a difficult environment to access. You need research vessels fitted with expensive sediment coring equipment to really get good samples.”
Knowing that these life forms can now be found at shallower depths will help researchers conduct further work to build understanding of life forms below the ocean floor.
“Subsurface microbes being brought up to the surface contribute to the microbial diversity of the sea floor. They could also be potential indicator organisms for studying this kind of dispersal.”
The study also delivers a new perspective on the diversity and metabolic potential of seabed petroleum seep microbial communities that play “an important role in preventing the ocean from being full of seeping hydrocarbons,” Hubert says.
In addition to TDI-Brooks International, Inc, other collaborators also contributed to the study. Dr. Emil Ruff, PhD, from the Marine Biological Laboratory (MBL), Woods Hole, and Dr. Xiyang Dong, from Sun Yat-sen University, China. Both Ruff and Dong are former EBG postdoctoral fellows. Carmen Li is a research associate working with Chakraborty and Hubert, and Emily Ellefson, who worked on the project as a summer student, is now doing a master's in the Department of Geoscience.