'Barren' seafloor teeming with microbial life

Life at rock bottom great for bacteria

Once considered a barren plain with the odd hydrothermal vent, the seafloor appears to be teeming with microbial life, according to a paper being published May 29 in Nature.

“A 60,000 kilometer seam of basalt is exposed along the mid-ocean ridge spreading system, representing potentially the largest surface area for microbes to colonize on Earth,” said USC geomicrobiologist Katrina Edwards, the study’s corresponding author.

While seafloor microbes have been detected before, this is the first time they have been quantified. Using genetic analysis, Edwards and colleagues found thousands of times more bacteria on the seafloor than in the water above.

Surprised by the abundance, the scientists tested another Pacific site and arrived at consistent results. This makes it likely that rich microbial life extends across the ocean floor, Edwards said.

The scientists also found higher microbial diversity on the rocks compared with other vibrant systems, such as those found at hydrothermal vents.

Even compared with the microbial diversity of farm soil—viewed by many as the richest—diversity on the basalt is statistically equivalent.

“These scientists used modern molecular methods to quantify the diversity of microbes in remote deep-sea environments,” said David L. Garrison, director of the National Science Foundation’s biological oceanography program.

“As a result, we now know that there are many more such microbes than anyone had guessed,” he added.

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 when the organic carbon content is pretty darn low,” Edwards recalled.

With evidence that the oceanic crust supports more bacteria compared with overlying water, the scientists hypothesized that reactions with the rocks themselves might offer fuel for life.

Back in the lab, they calculated how much biomass could theoretically be supported by chemical reactions with the basalt. They then compared this figure to the actual biomass measured. “It was completely consistent,” Edwards said.

This lends support to the idea that bacteria survive on energy from the crust, a process that could affect our knowledge about the deep-sea carbon cycle and even evolution.

For example, many scientists believe that shallow water, not deep water, cradled the planet’s first life. They reason that the dark carbon-poor depths appear to offer little energy, and rich environments like hydrothermal vents are relatively sparse.

But the newfound abundance of seafloor microbes makes it theoretically possible that early life thrived—and maybe even began—on the seafloor.

“Some might even favor the deep ocean for the emergence of life since it was a bastion of stability compared with the surface, which was constantly being blasted by comets and other objects,” Edwards suggested.