Deep Down, Life Thrives

Once considered a barren plain with an odd hydrothermal vent, the seafloor appears to be teeming with microbial life, according to a paper to be published 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 similar results. This makes it likely that rich microbial life extends across the ocean floor, said Edwards, associate professor of biological sciences and earth sciences in USC College.

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.

The 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.

Still, current knowledge of the deep biosphere can fit on the head of a pin, Edwards said. Most seafloor bacteria uncovered in this study show little relation to those cultivated in labs, which makes experimentation difficult.

Rather than bringing bacteria to the lab, however, Edwards plans to bring the lab to bacteria — with a microbial observatory 15,000 feet below sea level.

Thanks to a $3.9-million grant awarded in March by the Gordon and Betty Moore Foundation, Edwards and more than 30 colleagues will continue to study seafloor bacteria, but also will study their subseafloor cousins that cycle through the porous rock.

The first expedition of its kind, the drilling operation will penetrate 100 meters of sediments and 500 meters of bedrock.

Besides experiments aimed at learning how precisely these bacteria alter rock, the scientists will measure the diversity, abundance and relatedness of microbes at different depths.

This will shed light on whether the bacteria evolved from ancestors that floated down from above or from some as yet unknown source deep in the crust.

The Nature study provides a crucial base of comparison between the seafloor and subseafloor microbes, both completely unknown until just recently.

The decadelong undertaking will further bridge the earth and life sciences, a key goal in the emerging field of geobiology, described by Edwards as the co-evolution of Earth and life.

The deep biosphere is uniquely suited for a geobiological approach, Edwards said, since a proper understanding requires genomics, analysis of microbe-rock chemical interactions and a timescale in the millions of years.

USC recently hosted the fifth annual Geobiology Symposium, co-organized by USC postdoctoral student Beth Orcutt, the second author of the Nature paper.

Edwards believes that most people don’t realize how much life thrives in the watery depths.

“If we can really nail down what’s going on, then there are significant implications,” she said. “It is my hope that people turn their heads and notice that there’s life down there.”

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