Bacteria from millions of years ago

Scientists studying sediments from the Pacific Ocean east of New Zealand have found bacteria that appear to have survived since the time of the dinosaurs.

“They’re violating our sense of the [microbial] world as we know it,” says Steven D’Hondt, an oceanographer at the University of Rhode Island. And it’s a finding that might be relevant to the search for life on Mars or the moons of Jupiter and Saturn.

These bacteria, D’Hondt says, are survivors of microbial communities that lived on the deep seabed as much as 101.5 million years ago – the age of the oldest sample studied – then buried.

This ancient seabed now lies 3700-5700 metres beneath a region known as the South Pacific Gyre, which, due to the vagaries of ocean-current circulation, is extremely nutrient poor. That’s important, because it means that sediments there accumulate at an extremely low rate – less than a metre per million years – and are themselves extremely nutrient poor.

“Our main question was whether life can exist at such a nutrient-limited environment, in almost absence of food,” says Yuki Morono of the Japan Agency for Marine-Earth Science and Technology, the lead author of the team’s paper in the journal Nature Communications.

When they were first buried, D’Hondt says, these sediments would have contained about one million cells per cubic centimetre. “What’s left are about 1000 cells per cubic centimetre, but they’ve been living under very challenging conditions for up to 100 million years.”

He continues: “Basically, they’re only getting enough energy to repair their molecules as they break”, with none left over to grow and divide.

When brought into the lab and given more nutrient-rich diets, however, these bacteria prove to be not just alive, but able to revive, grow and multiply, exactly like normal bacteria.

How they can do that, D’Hondt says, is a mystery. Either the individual cells are somehow surviving for “ridiculous lengths of time” or they “are reproducing with less energy than we thought possible”. But one way or another “they are starvation artists”.

One implication is that if Mars once had life, remnants might still exist, not just as fossils or biosignatures, but as living microorganisms that could be revived and studied. “If these things can survive 100 million years, maybe they can survive a billion or three billion,” D’Hondt says.

He notes, however, that there would come a point where even the most miserly of starvation artists would eventually exhaust their food supplies.

The new find also means that life might be even more possible than we’d previously thought in other low-productivity environments, such as subsurface oceans in the moons of Jupiter and Saturn, where the basic elements of life might be possible, but the energy to renew their food sources might be extremely limited.

What we don’t need to worry about, D’Hondt adds, is that digging into these old seabeds might unleash a 100-million-year-old plague here on Earth – an issue of particular relevance in the time of COVID-19.

To begin with, he says, if such a plague were possible, it would probably already have been produced by offshore drilling, which has long been stirring up similar sediments on a much larger scale. But the reality is that bacteria in the deep seabed aren’t something we need to worry about.

“Pathogens are common in harbors,” D’Hondt says, noting that these are places easily contaminated by human waste. “But they’re not common in the deep ocean or the sediment. It’s just the wrong environment for them.”

Related reading: Ancient forest microbes live 2.5km under sea