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Microbial diversity and abundance has long been thought to be highest in soil environments. Now soil has a potential rival: the basalt rocks that line the ocean floor. These are carbon-poor but oxygen-rich habitats where organisms obtain energy for metabolic processes from inorganic elements such as nitrogen, iron or sulphur. Katrina Edwards at the University of Southern California in Los Angeles and her colleagues studied oceanic crust from two locations in the Pacific and found it to be surprisingly rich in microbial life (see page 653). Edwards tells Nature that the microbes hold important clues to global phenomena, such as carbon cycling.

Why look for microbial life in basalt rocks?

We know that chemical reactions occur between rocks and seawater, and wanted to determine to what degree microbes participate in or take advantage of those reactions. Little was known about what types of microbe can colonize rock. We expected it to be a sparse habitat, so finding 60 million microbial cells per gram of rock was a shock.

Did the microbes you found resemble organisms from other known habitats?

Most of those that we examined are not closely related to organisms that have previously been cultivated in the lab. As a result, we can't immediately describe their physiology. We did find a few groups that are closely related to cultivated organisms — for example, some involved in nitrogen cycling. A clear message from our survey is that active nitrogen cycling occurs on deep-sea rock.

What do you want to learn about these communities?

The organisms seem to be slow-growing ones that take a long time to colonize the rock. We are developing techniques to properly cultivate them so that we can study their physiology. We also want to compare the two basalt systems in this study — one around Hawaii and one at a mid-ocean ridge — with other ocean basins and the subsea floor.

Do these organisms affect the carbon cycle?

Yes. The bacteria are able to use rock and minerals as fuel to generate an estimated 500 billion grams of carbon each year — enough to be relevant to the deep-sea carbon cycle. Soon, we plan to drill into active, geologically young [less than 10,000 years old] Atlantic Ocean rock to determine how far down in the subsurface rock these processes occur.

What do you hope this work accomplishes?

I'd be excited to have more people understand that there is a 'bottom' to the ocean and that it does harbour life, which fundamentally controls the long-term chemistry of oceans.