A research team in the South Pacific recently ventured 18,700 feet below the surface of the ocean, and then a further 246 feet below the sea floor. What they found was a community of life forms that had been sitting dormant there for 100 million years. They managed to resurrect them, and they are currently multiplying at a rate that has even experts astonished.

Researchers take samples from sediment cores below the deep ocean floor. Image credit: IODP JRSO

Microbes that have been buried under the sea floor for more than 100 million years are still very much alive, according to a new study published in Nature Communications. When brought back to the lab and fed, they started to multiply. The microbes are oxygen-loving species that somehow exist on what little of the gas diffuses from the ocean surface deep into the seabed.

The discovery raises an intriguing possibility. As one of the scientists involved put it, the microbes could have been sitting in the deep sea floor dormant – or at least slowly growing without dividing – for eons. That’s a long time.

Yes, according to the new findings, microbial life is that persistent – and often finds ways to survive even in these conditions.

The study has great significance because it shows how life can survive in places scientists previously thought uninhabitable. All this stuff increasingly points towards the likelihood of life elsewhere in the solar system, or the universe. If the surface of a particular planet does not look promising for life, the subsurface could still be suitable.

Bacteria isolated from 100-million-year-old clay. Image credit: JAMSTEC

Researchers have been aware that life exists beneath the ocean floor for more than 15 years. But geomicrobiologist Yuki Morono of the Japan Agency for Marine-Earth Science and Technology wanted to know more. He wanted to find the limits of such life forms.

Microbes are known to live in very hot or toxic environments, but can they survive where there’s hardly any food? To find out, Morono and his colleagues drilled numerous sediment cores 100 meters below the seafloor and nearly 6,000 meters below the ocean’s surface. The drilling expedition took place aboard the research drillship JOIDES Resolution in the South Pacific Gyre, an intersection of ocean currents east of Australia that is considered the deadest part of the world’s oceans, almost completely lacking the nutrients needed for survival.

The expedition confirmed the samples did indeed contain some oxygen, a sign that there was very little organic material for bacteria to eat.

Morono’s team then carefully extracted small clay samples from the centers of the drilled cores and placed them in glass vials, adding simple compounds of acetate and ammonium containing some heavier forms – or isotopes – of nitrogen and carbon that can be detected in living microbes.

For 557 days after the team first “fed” the mud samples with the compounds, bits of clay were extracted from the samples and dissolved to spot any living microbes that may be there despite the lack of food in the clay.

Seafloor mud typically contains at least 100,000 cells per cubic centimeter. Yet in these samples, there were only up to 1000 bacteria found in the same amount of sediment. That rendered the work very challenging.

The researchers had to develop specialized techniques such as using chemical tracers to detect whether any contaminating seawater got into the samples. Then, they had to develop a way to analyze miniscule amounts of cells and isotopes.

The added nutrients woke up a variety of oxygen-using bacteria. In the 101.5-million-year-old samples, the microbes increased by four orders of magnitude to more than 1 million cells per cubic centimeter after 65 days.

The genetic analysis of the microbes revealed they belonged to more than eight known bacterial groups. Many of these varieties of bacteria play important roles in breaking down organic matter, suggesting to scientists that learning to survive under conditions of extreme energy limitation is a widespread ability, one that may have evolved early, when there was not much for microbes to feed on. So it could have been a handy survival trick.

The researchers are not sure what the gyre microbes have been doing all these millions of years. Most of the species they found do not form spores, and exist in an inactive life stage that some bacteria enter in unfavorable conditions. This means the bacteria could have been dividing very slowly all this time, and the ones isolated in this study could be the distant descendants of milion-year-old ancestors.

The bottom line, at least at the sea bottom, is that there’s so little food in the deep-sea sediments that any microbes subsisting there could not do much more than repair any damaged molecules.

“If they are not dividing at all, they are living for 100 million years, but that seems insane,” says Steve D’Hondt, an oceanographer at the University of Rhode Island, Bay Campus, co-author of the study.

The bottom line is, again, that low food and energy seem not to set the ultimate limit for life on Earth. Or any other planet, for that matter.