Even after 100 million years buried in the seafloor, some microbes can wake up. And they’re hungry.

An analysis of seafloor sediments dating from 13 million to nearly 102 million years ago found that nearly all of the microbes in the sediments were only dormant, not dead. When given food, even the most ancient microbes revived themselves and multiplied, researchers report July 28 in Nature Communications.

Scientists have pondered how long energy-starved microbes might survive within the seafloor. That such ancient microbes can still be metabolically active, the researchers say, just goes to show that scientists are still fathoming the most extreme limits to life on Earth.

The microbes’ patch of seafloor lies beneath a kind of ocean desert, part of a vast abyssal plain about 3,700 to 5,700 meters below sea level. Researchers, led by microbiologist Yuki Morono of the Japan Agency for Marine-Earth Science and Technology in Kochi, examined sediments collected in 2010 from part of the abyssal plain beneath the South Pacific Gyre. That region of the Pacific Ocean contains few nutrients that might fuel phytoplankton blooms and thereby support a cascade of ocean life. As a result, very little organic matter makes its way down through the water to settle on the seafloor.

The extremely slow accumulation of organic material and other sediments in this region does allow oxygen in the water to seep deep into the sediments. So Morono and colleagues wondered whether any aerobic, or oxygen-liking, microbes found there might be revivable. After “feeding” microbes from the collected sediments with nutrients including carbon and nitrogen, the team tracked the organisms’ activity based on what was consumed.

The aerobic microbes in the sediments turned out to be a highly diverse group, consisting mostly of different types of bacteria belonging to large groups such as Alphaproteobacteria and Gammaproteobacteria (SN: 9/14/17). Nearly all the microbes responded quickly to the food. By 68 days after the experiment’s start, the total number of microbial cells had increased by four orders of magnitude, from as little as about 100 cells per cubic centimeter to 1 million cells per cubic centimeter.

Those increases weren’t just among the youngest microbes. Even in the sediment sample containing the most elderly — about 101.5 million years old — up to 99.1 percent of the microbes were revived.