Scientists have discovered a 100-million-year-old bacteria living under the South Pacific seafloor.

When the microbes were brought back to the lab and fed, they started to multiply again.

It appears that they had existed in a dormant state, living on tiny amounts of gas diffusing from the ocean surface deep into the seabed.

The discovery raises a mind-bending prospect - that the microbes have been slowly growing without dividing for eons.

Geomicrobiologist Yuki Morono from the Japan Agency for Marine-Earth Science and Technology led the study, originally intended to learn more details about how microbes live where there is little food to eat.

They ran a drilling expedition in the South Pacific Gyre, where ocean currents intersect east of Australia.

It is considered a sort of ‘dead zone’ - almost completely lacking the nutrients needed for survival.

The team extracted cores of clay and other sediments from down to 5,700 meters below sea level, and found that the samples contained some oxygen - a hint of organic material for bacteria to eat.

Clay samples were taken from the drilled cores, placed in glass vials, and fed simple compounds, such as acetate and ammonium.

They then waited up to 557 days for some samples, before extracting bits of clay from the samples and dissolving them to spot any living microbes.

Biologists normally expect to find at least 100,000 cells per cubic centimetre of seafloor mud, but these samples contained no more than 1000 bacteria in the same amount of sediment.

This meant that specialised techniques had to be developed just to detect the very small amounts of cells and isotopes.

The nutrients that the scientists introduced appeared to wake up a variety of oxygen-using bacteria.

Astoundingly, samples from a 101.5-million-year-old layer saw microbes increase by four orders of magnitude to more than 1 million cells per cubic centimetre after 65 days.

Genetic analysis revealed the microbes belong to several bacterial groups which are known to play important roles in breaking down organic matter.

It suggests that learning to survive under conditions of extreme energy limitation is a widespread ability, which may have evolved in the early days of the Earth, when there was less for microbes to feed on.

The researchers do not know what the gyre microbes have been doing for the millions of years they laid under the seafloor.

They speculate that the bacteria may have been dividing very slowly all this time, meaning the examples isolated in this study are the direct descendants of ancestors millions of years old.

But with so little food in the deep-sea sediments, experts also say the microbes would not have been able to do much more than repair any damaged molecules.

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

He suggests there may be some unrecognised source of energy - perhaps radioactivity - that allows slow division by the bacteria.

The full study is accessible here.