Bacteria 'Feed' on Earth's Ocean-Bottom Crust

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The results of a recent study state that seafloor bacteria on ocean-bottom rocks are more abundant and diverse than previously thought and appear to "feed" on the planet's oceanic crust. The study’s results were reported in this week's issue of the journal Nature.

The findings pose intriguing questions about ocean chemistry and the co-evolution of Earth and life.

This is a region that was once considered a barren plain dotted with hydrothermal vents, the seafloor's rocky regions appear to be teeming with microbial life, say scientists from the Woods Hole Oceanographic Institution (WHOI) in Woods Hole, Mass., University of Southern California (USC) in Los Angeles, and other institutions.

Prior studies have detected that presence of seafloor microbes; however, this is the first time they have been quantified. Cara Santelli of WHOI, Katrina Edwards of USC, and colleagues used genetic analyses and found three to four times more bacteria living on exposed rock than in the waters above.

NOAA/WHOI Rocks made of basalt on and under the ocean bottom harbor surprising numbers of deep-sea bacteria. image:40515:2::0

"Initial research predicted that life could in fact exist in such a cold, dark, rocky environment," said Santelli. "But we really didn't expect to find it thriving at the levels we observed."

The scientists were surprised by the diversity they found and tested more than one site and arrived at consistent results, making it likely that rich microbial life extends across the ocean floor.

"This may represent the largest surface area on Earth for microbes to colonize," said Edwards.

"These scientists used modern molecular methods to quantify the microbial biomass and estimate the diversity of microbes in deep-sea environments," said David Garrison, director of the National Science Foundation (NSF)'s Biological Oceanography Program. NSF's Ridge 2000 program funded the research.

"We now know that this remote region is teeming with microbes, more so than anyone had guessed."

Santelli and Edwards also found that the higher microbial diversity on ocean-bottom rocks compared favorably with other life-rich places in the oceans, such as hydrothermal vents.
These findings raise the question of where these bacteria find their energy.

"We scratched our heads about what was supporting this high level of growth," Edwards said.

When they factored in the evidence that the oceanic crust supports more bacteria than overlying water, the scientists were able to hypothesize that the reactions with the rocks themselves might offer fuel for life.

In the lab, they calculated how much biomass could be supported by chemical reactions with the rocky basalt; then they compared this figure to the actual biomass measured.

"It was completely consistent," Edwards said.
This discovery supports the idea that bacteria survive on energy from Earth's crust, a process that could add to our knowledge about the deep-sea carbon cycle and the evolution of life.

Many scientists believe that shallow water, not deep water, is better suited for cradling the planet's first life forms. Before this project, dark, carbon-poor ocean depths appeared to offer little energy, and rich environments like hydrothermal vents were thought to be relatively sparse.

The newfound abundance of seafloor microbes makes it possible that early life thrived--and perhaps began on the seafloor.

"If we can really nail down what's going on, there are significant implications," Edwards said. "I hope that people turn their heads and notice: there's life down there."

In addition to Santelli and Edwards, the paper's co-authors are: Beth Orcutt of USC; Erin Banning of WHOI; Wolfgang Bach of WHOI and Universität Bremen; Craig Moyer of Western Washington University; Mitchell Sogin of the Marine Biological Laboratory; and Hubert Staudigel of the Scripps Institution of Oceanography.

The research was also funded by the NASA Astrobiology Institute and Western Washington University.