Pushing Limits

Ancient microbes in the "deadest" part of Earth redefine boundaries of life

“There’s probably not a limit to life anywhere."

Imagine you're running a 5K race — but instead of a route that loops, you run the distance in a straight line from start to finish. Now, conceptualize that, instead of traveling horizontally, you're running straight down, burrowing into the Earth.

That's how far beneath the ocean's surface scientists dug to explore whether life can persist in ancient sediment — a region that was previously believed to be lifeless. They found not only life but whole communities of microbes that have lived for more than 100 million years.

A team of Japanese and American scientists reports this discovery in a study published Tuesday in the journal Nature Communications.

The researchers collected and analyzed sediment samples from beneath the South Pacific Gyre on an abyssal plain — a flat, wide area of sediment deep in the ocean. Samples came from 3,700 to 5,700 meters, or more than 3.5 miles, beneath the ocean's surface.

Previously, researchers believed that these plains in the ocean are void of life beyond a few meters deep. The sediment has limited organic material, making microbes' food sparse. The microbes they found were essentially starving, the oldest community languishing in sediment dated at 101.5 million years old.

Once researchers in lab experiments fed the ancient microbes, the tiny bacteria and archaea were able to grow, reproduce, and absorb nutrients.

Diagram of an abyssal plain.

Study co-author Steven D’Hondt, a professor at the University of Rhode Island, was a part of the expedition team and collected samples. The goal was to “understand what life is like in the deadest marine sedimentary community we could think of," D'Hondt tells Inverse.

The community, it turned out, was far from dead. Biologist Yuki Morono, who led the study, conducted a series of tests over two years to determine that the unearthed microbes — 10 types of bacteria and traces of archaea — could thrive.

“As soon as Yuki gave them a better world, they were able to take advantage of it,” D'Hondt says.

Life in a dead zone — The new findings upend previous understandings about energy-poor, deep-ocean zones.

Although the deepest sediment came from miles below the ocean's surface, the sediment sampled comes from a layer that's only about 250 feet thick. It rests atop an igneous volcanic rock, accumulating slowly — about 75 centimeters every million years. Mostly, the sediment is dust. There isn't much life in the area ocean above it, and animal remains that do float down mostly dissolve as they descend.

That means, for the microbes that inhabit the sediment, there isn't much energy available. The microbes also can't exactly search for food — they're trapped within the layers of marine dust.

“It’s like they’re stuck in a very fine-grained sponge," D'Hondt says.

Yet, the communities survived. Because researchers don't know how fast microbes can reproduce with such little energy, they don't know how old the individual microbes actually are. Rather than a single set of microbes living for 100 million years, “it could be the great-great-grandparents of those microbes” dredged up by scientists.

The researchers discuss their results.

The findings beg the question: How did these microbial communities manage to survive all this time?

Largely, the answer remains a mystery.

“We don’t know how they lived for such a long time under those extraordinarily energy-limiting conditions,” D'Hondt says.

But there are several possibilities. On idea is that the microbes may be finding a way to reproduce despite the circumstance, continuing their communities for millions of years. Or, the tiny organisms are finding a yet-undetected source of energy. It's also possible that individual cells are living for an incredibly long amount of time.

"My gut feeling, is down the road, we’re going to find out it’s all three things," D'Hondt says.

In the meantime, the discovery hints at new possibilities for life in general — suggesting that, perhaps, it can exist on other planets, too. On Mars, or Europa, D'Hondt posits, microbial communities established hundreds of millions of years ago, or even longer, could feasibly persist.

“With just a slight prickle of energy, they might still be there,” D'Hondt says, "barely eking out survival.”

If ancient life can persist in the deadest parts of the Earth, the boundaries of survival disappear.

“There’s probably not a limit to life anywhere," D'Hondt says.

Abstract: Sparse microbial populations persist from seafloor to basement in the slowly accumulating oxic sediment of the oligotrophic South Pacific Gyre (SPG). The physiological status of these communities, including their substrate metabolism, is previously unconstrained. Here we show that diverse aerobic members of communities in SPG sediments (4.3‒101.5 Ma) are capable of readily incorporating carbon and nitrogen substrates and dividing. Most of the 6986 individual cells analyzed with nanometer-scale secondary ion mass spectrometry (NanoSIMS) actively incorporated isotope-labeled substrates. Many cells responded rapidly to incubation conditions, increasing total numbers by 4 orders of magnitude and taking up labeled carbon and nitrogen within 68 days after incubation. The response was generally faster (on average, 3.09 times) for nitrogen incorporation than for carbon incorporation. In contrast, anaerobic microbes were only minimally revived from this oxic sediment. Our results suggest that microbial communities widely distributed in organic-poor abyssal sediment consist mainly of aerobes that retain their metabolic potential under extremely lowenergy conditions for up to 101.5 Ma.
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