Above is a fluorescence microscopy image of 13 million-year-old microbes after separation from subseafloor sediment and sorting.
CNN  — 

Humans can go without food for about three weeks before the effects of starvation begin to kill them.

Some microbes deep underneath the seafloor have us beat: They can survive with barely any sustenance for more than 100 million years.

These microorganisms live more than 18,000 feet underneath the ocean surface — in an area so deep it’s called the subseafloor, below the seafloor.

These sparse microbial populations exist in the slowly accumulating oxygen-filled sediment of the South Pacific Gyre, located within the South Pacific Ocean and bound by the equator, Australia, the Antarctic Circumpolar Current and South America, according to a study published Tuesday in the journal Nature Communications.

Marine microbes are tiny, single-celled microorganisms that live in the ocean and account for more than 98% of the total mass of organisms living within the ocean.

The area, part of the Earth’s system of rotating ocean currents, doesn’t have a lot of food to feed almost anything. It’s relatively low in plant nutrients but contains abundant oxygen in the deeper parts of the subseafloor.

Because the center of the South Pacific Gyre is the site on Earth farthest from all land and productive ocean regions, it’s called the “oceanic pole of inaccessibility” and is regarded as Earth’s largest oceanic desert.

There are five major gyres, which are large systems of rotating ocean currents. The ocean churns up various types of currents. Together, these larger and more permanent currents make up the systems of currents known as gyres.

It’s not a spot where most life would thrive, although microbes below the seafloor were known to be present in the South Pacific Gyre sites. Luckily for the microbes, their population wasn’t limited by the availability of nitrogen and iron or other dissolved major inorganic nutrients necessary for the growth of living things.

Life beneath the seabed

Up until now, there hasn’t been much evidence for how these starved microbes function and their survival status in such a food-scarce setting, the study said. That’s because before a cell can grow, divide into more cells or keep up the energy needed to complete basic metabolic functions, it has to consume and use carbon.

So the researchers went looking for sediment samples from approximately 12,140 to 18,700 feet below sea level, during a 2010 Integrated Ocean Drilling Program expedition.

The sediment was deposited over a period from 13 million to 101.5 million years ago, and it contains small amounts of carbon and other organic material.

Pictured is a fluorescence microscopy image of the samples of 13 million-year-old sediment before cell purification processes. There were highly abundant sediment particles seen.

Then within a lab setting the researchers fed these multimillion-year-old samples with carbon and nitrogen substrates — materials from which an organism gets its nourishment — to test whether the cells were capable of feeding and dividing into more cells.

Most of the nearly 7,000 ancient cells analyzed readily ate up the carbon and nitrogen foods within 68 days of the incubation experiments.

Researchers incubated 95.4 million-year-old subseafloor sediment samples with carbon and nitrogen substrates (nutrients for the microbes).

They also rapidly divided and increased their total numbers more than 10,000 times.

That was a growth rate the researchers didn’t expect since there wasn’t much to eat, said Yuki Morono, first author of the study and a geomicrobiologist and senior scientist at the Japan Agency for Marine-Earth Science and Technology.

Pictured is a fluorescence microscopy image of the samples of 13 million-year-old microbes after cell separation (before cell sorting). Still there were many sediment particles remaining.

After 101.5 million years in food-scarce conditions, the dormant microbes retained their abilities to stay alive, eat and divide. These microbes dominate the microbial communities housed in the sediment in the abyss of the ocean, the study said.

“The microbes are almost completely trapped in the sediment, surrounded by grains, not allowed to move” and kept there for millions of years, Morono said.

“In addition, their nutrients are very limited, almost at the state of ‘fasting,’” Morono added in an email. “So it is surprising and biologically challenging that a large fraction of microbes could be revived from a very long time of burial or entrapment in extremely low nutrient/energy conditions.”

Microbes only accounted for less than 0.01% of the sediment samples, so the techniques used allowed microbes imperceptible to the human eye to be visible and recognizable by humans, Morono said.

The study is “of global importance but is difficult to extrapolate to the other (ocean sediments), because sediments are complex and vary from one site to another,” said Yohey Suzuki, an associate professor in the department of earth and planetary science at The University of Tokyo, who wasn’t involved in the study.

The authors also discovered various types of bacteria with high tolerance of extreme environmental conditions.

How they survived

The revived microbes were trapped in subseafloor sediment for up to 100 million years without food, and the researchers have yet to discover how the microbes could have survived such extreme scarcity.

In addition to determining how the microbes were able to survive for millions of years, the authors are also “looking forward to seeing the limits of the subseafloor,” Morono said.

“We now know there is no age limit (to organisms), but there should be the end of the biosphere in somewhere of the subseafloor,” he said. “We want to see the extent of the habitable space in our Earth and know the life limiting condition in detail.”