Half of Earth's microbial life may be hiding beneath the surface

In the depths of planet Earth, a hidden microbial world thrives under extreme conditions that are beyond the reach of our regular perception.

This extraordinary and invisible ecosystem harbors an abundance of life forms that flourish in environments such as gold mines, aquifers, and deep boreholes in the seafloor.

But how do these below-the-surface microbiomes compare to the vibrant ecosystems bathed in Earth’s sunlight?

Discovering the microbial deep dwellers

Researchers have now investigated this question in a first-of-its-kind comprehensive endeavor undertaken at the Marine Biological Laboratory (MBL).

What they found was incredibly rich microbial diversity in certain subsurface habitats, which extended up to 491 meters (1,610 feet) below the seafloor and 4,375 meters (14,300 feet) below ground level in terrestrial habitats.

This unprecedented revelation hints at vast, unexplored, subsurface sources of diversity, and presents possible opportunities for finding new compounds and medicines.

The study holds potential for identifying how cells adapt to extremely energy-scarce environments. The findings may also be useful in the search for extraterrestrial life.

Microbial diversity below the surface

The study’s lead scientist, MBL associate scientist Emil Ruff, addressed some of the common assumptions about life below the Earth’s surface.

“It’s commonly assumed that the deeper you go below the Earth’s surface, the less energy is available, and the lower is the number of cells that can survive,” noted Ruff.

“Whereas the more energy present, the more diversity can be generated and maintained – as in tropical forests or coral reefs, where there’s lots of sun and warmth.”

However, the study’s findings tell a different story. In some subsurface habitats, microbial diversity can easily match or surpass the diversity on the surface.

This was found to be especially true for marine environments and for microbes in the Archaea domain.

A tale of two realms

The study, which took almost a decade to complete, was also one of the first to compare marine and terrestrial environments in terms of microbial diversity and community composition.

While the experts found that the marine and terrestrial microbiomes differed greatly in composition, their level of diversity was similar. This finding gives rise to a fundamental ecological principle.

“There’s a very clear divide between life forms in the marine and in the terrestrial realms, not just in the surface, but also in the subsurface. The selective pressures are very different on land and in the sea, and they select for different organisms that have a hard time living in both realms,” explained Ruff.

Microbes in low-energy environments

The existence of a vast reservoir of microbes kilometers deep – both under our feet and beneath the seafloor – was first broadly recognized by scientists in the mid-1990s.

Now, scientists estimate that between 50 and 80 percent of Earth’s microbial cells live in the subsurface sediments, where energy availability is significantly lower than on the sunlit surface.

The study suggests that potentially half of Earth’s microbial diversity can be found in subsurface zones.

In addition, the findings point towards a fascinating phenomenon: in these low-energy environments, life appears to be slowed down to an absolute minimum. Some subsurface cells are estimated to divide only once every 1,000 years.

These microbes live on completely different timescales and could provide intriguing insights about aging.

Implications for finding extraterrestrial life

If planets like Mars had liquid water at some point in their history, the rocky ecosystems below their surface could resemble those currently on Earth.

“The energy would be very low; the organisms’ generation times would be very long. Understanding deep life on Earth could be a model for discovering if there was life on Mars, and if it has survived,” said Ruff.

Microbiomes of diverse locations on Earth

Past studies of microbial life on Earth’s surface and subsurface aren’t scarce, but synthesizing the data was challenging due to inconsistencies in methodology across different research groups.

However, in 2016, a breakthrough method allowed for a consistent dataset to compare more than 1,000 samples from 50 marine and terrestrial ecosystems.

Samples from around the world were sent to MBL, where scientists used identical routines to sequence and analyze the microbial DNA present.

This consistent methodology allowed for the first direct comparison of microbiomes from vast and diverse locations.

Ultimately, the research serves as a foundation for exposing the rich tapestry of life forms that inhabit the invisible worlds below the Earth’s crust.

As we continue to explore these subsurface habitats, we stand to learn much about the adaptability and resilience of life in its myriad forms.

The full study was published in the journal Science Advances.

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