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.

On the deep ocean floor, microbial life is feeding on fresh volcanic rock and flourishing with greater abundance than even the most optimistic scientists thought possible. Scientists have found bacteria growing on oceanic crust in concentrations that are thousands- to ten-thousand times (three to four orders of magnitude) greater than what is found in the overlying waters.

Researchers from the Woods Hole Oceanographic Institution (WHOI), the University of Southern California (USC), and four other institutions collected and examined rock and water samples from the East Pacific Rise, the Nankai Trough, the Mid-Atlantic Ridge, the Sargasso Sea, and the seafloor near Hawaii. They used various molecular and genetic analysis tools (such as quantitative polymerase chain reaction and clone libraries) to quantify the abundance, richness, and diversity of communities of bacteria living on young ocean crust.

Bacteria form large undersea mats in some areas of the ocean's deepest realms. Credit: Ridge2000/National Science Foundation

What they found was a community of microbes—growing without light and eating rocks—that is as fertile and prodigious as what can be found in terrestrial farm soil. Such seafloor microbes have been detected before, but this is the first time that they have been quantified.

“We were truly shocked to find microbial life thriving at the levels we observed,” said Cara Santelli, a recent graduate of the MIT/WHOI Joint Program in Oceanography and now a postdoctoral scholar at Harvard University. Santelli conducted the research with geochemists Katrina Edwards and Wolfgang Bach while all three were in Woods Hole. Edwards is now at USC, while Bach has moved to the University of Bremen.

“Theoretical research by Edwards and Bach had suggested that life could exist in such a dark, cold, and rocky environment,” said Santelli, lead author on the Nature paper. “The objective of our project was to provide tangible evidence. Not only was the biomass in seafloor lava greater than in the overlying seawater, but the bacterial diversity was significantly greater than we could have imagined.”

Biologist Katrina Edwards discusses her research on seafloor bacteria on ocean-bottom rocks.

The findings open up new questions about the evolution of Earth and of seafloor life. With evidence that the oceanic crust supports more bacteria than overlying water, the scientists hypothesized that reactions with the rocks themselves might offer fuel for life.

“We are just beginning to scratch the surface,” Santelli said. “What role does this microbial community play in global ocean chemistry? What sort of metabolism is needed to live in and dominate this environment? And what exactly are these microbes doing down there?”

This discovery lends support to 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. Up until now, dark, carbon-poor ocean depths appeared to offer little energy, and rich environments like hydrothermal vents were thought to be relatively sparse.

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

The research by Santelli and Edwards, et al, was funded by the National Science Foundation’s Ridge 2000 program, the NASA Astrobiology Institute, and Western Washington University.

Nature: Abundance and diversity of microbial life in ocean crust

Oceanus: Living Large in Microscopic Nooks

Formation of Lava Bubbles Offers New Insight into Seafloor Formation