As much as 70 percent of the microbes alive on Earth reside on and just below the ocean floor, two new studies suggest.

The seafloor was once thought to be a barren expanse of muck dotted with an occasional thriving ecosystem near a hydrothermal vent. More recently, however, scientists have discovered that microorganisms can fuel their metabolisms by taking advantage of the chemical energy stored in various minerals, including those that make up the ocean crust.

Even relatively fresh rocks formed by oozing lavas at the mid-ocean ridges are home to many microorganisms, says Katrina J. Edwards, a biogeochemist at the University of Southern California in Los Angeles.

Edwards and her colleagues recently conducted a biological census of rocks that formed during the past 20,000 years at a spot along the East Pacific Rise, a mid-ocean ridge segment that lies off the northwestern coast of South America.

While the deep water in that area only contained between 8,000 and 90,000 microorganisms per cubic centimeter, each gram of the seafloor basalt held between 3 million and 1 billion microbes in its pore spaces. “Bacteria were jam-packed on the rocks,” Edwards says.

The team’s genetic analyses suggest that the seafloor ecosystem is populated with many types of microorganisms, including two of the main types, bacteria and archaea. While deep water in the region contained, on average, just 12 distinct types of microorganisms, the ocean-bottom basalt was home to about 440 different types. For comparison, farm soil — long thought to be one of the richest microbial ecosystems on the face of the planet — hosts more than 1,400 distinct types of microorganisms.

Similar tests on seafloor samples taken near the Hawaiian Islands, several thousand kilometers to the northwest, confirmed the abundance and diversity of microbes that reside in ocean-bottom sediments. “This makes it likely that rich microbial life extends across the ocean floor,” Edwards says. She and her colleagues report their findings in the May 29 Nature.

Apparently, freshly formed basalt isn’t the only seafloor ecosystem where bacteria can thrive. Another study indicates that microorganisms can prosper in sediments that were deposited millions of years ago but now sit hundreds of meters below the ocean bottom.

R. John Parkes, a microbiologist at Cardiff University in Wales, and his colleagues analyzed nine samples of sediment drilled off the shore of Newfoundland at depths between 860 and 1,626 meters beneath the North Atlantic seafloor.

Their analyses suggest that each cubic centimeter of those sediments, on average, holds around 1.5 million microorganisms in pore spaces.

About 60 percent of those cells are alive and could reproduce, the team’s tests suggest. Also, microscopy reveals that many of the cells found in the North Atlantic samples — in one sample, nearly 12 percent — were caught in the act of dividing, the researchers reported in the May 23 Science.

Although the numbers of microbes found in the deep sediments are much thinner than the 1 billion or so that live in each cubic centimeter of sediment near the surface of the seafloor, “these are still very significant cell populations,” Parkes says.

The temperatures of rocks at the depths the team drilled range from 60° Celsius to more than 100° C, he says. Known types of microbes can survive in temperatures up to around 120° C, which corresponds to a sediment depth of about 4 kilometers, Parkes says. If microorganisms thrive throughout seafloor sediments above that depth, the material could house about 70 percent of the microbes now alive on Earth, Parkes says.

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