Hydrogen-Eating Bacteria in Earth’s Crust

For decades, biologists believed that almost all life on Earth relied on the sun. We thought the food chain started with photosynthesis. However, recent discoveries deep beneath the surface have shattered that assumption. Miles underground, scientists have found a thriving ecosystem of bacteria that survive not on sunlight, but on hydrogen gas produced by rocks. This “deep biosphere” changes everything we know about how life survives in extreme environments.

The Hidden World Beneath Our Feet

When you think of the Earth’s crust, you likely imagine solid rock, intense heat, and a sterile environment. Yet, researchers from the Deep Carbon Observatory (DCO) have revealed that the subsurface is actually teeming with life. This subterranean ecosystem is massive.

Estimates suggest that the deep biosphere occupies between 2 and 2.3 billion cubic kilometers of the planet’s subsurface. That is almost twice the volume of all the world’s oceans. Within this space, scientists estimate there are between 15 and 23 billion tonnes of carbon based life. To put that in perspective, the mass of these microscopic organisms is hundreds of times greater than the combined mass of every human on Earth.

This is not a small pocket of life. It is a dominant feature of our planet that we are just beginning to understand.

How Rocks Feed Bacteria: The Science of Radiolysis

The most critical question regarding these deep-dwelling organisms is simple: What do they eat? Surface life relies on plants converting solar energy into food. Down in the dark, that is impossible.

Instead, these bacteria are “lithoautotrophs,” which translates roughly to “rock-eating self-feeders.” They rely on a process called radiolysis. Here is how the process works in the deep crust:

• Radioactive Decay: Rocks deep underground contain trace amounts of radioactive elements like uranium, thorium, and potassium.
• Splitting Water: As these elements decay, they release radiation. When this radiation hits water molecules trapped in the rock fractures, it splits the water ($H_2O$) apart.
• Hydrogen Release: This splitting creates free hydrogen gas ($H_2$) and oxidants.
• Dinner is Served: The bacteria consume the hydrogen gas as an electron donor to fuel their metabolic processes.

This cycle creates a self-sustaining ecosystem that is completely independent of the surface world. Some of these colonies may have been isolated from the sun for millions, or even billions, of years.

The "Bold Traveler": A Species in Isolation

One of the most fascinating examples of this hydrogen-eating life was found in the Mponeng gold mine near Johannesburg, South Africa. At a depth of 2.8 kilometers (1.7 miles), researchers discovered a bacterium named Desulforudis audaxviator.

The name comes from a passage in Jules Verne’s Journey to the Center of the Earth and translates to “bold traveler.” This organism is unique because it represents a “single-species ecosystem.” usually, an ecosystem requires many different species creating a food web (producers, consumers, decomposers).

Desulforudis audaxviator does it all alone. It has everything it needs in its DNA to:

1. Consume hydrogen produced by radioactive rocks.
2. Fix nitrogen directly from the environment.
3. Protect itself from viruses.
4. Form spores to survive when resources are low.

This discovery proved that life can exist in total isolation, fueled strictly by the geology around it.

The Kidd Creek Mine Discovery

Another major breakthrough occurred at the Kidd Creek Mine in Ontario, Canada. This site is distinct because it sits on the Canadian Shield, one of the oldest continental crusts on Earth.

Geochemist Barbara Sherwood Lollar and her team found water flowing from fractures 2.4 kilometers (1.5 miles) underground. By analyzing the noble gases dissolved in the water, they determined that this fluid had been trapped underground for at least 2 billion years.

Despite the water’s extreme age and isolation, it contained cells. These microbes were living in complete darkness, surviving on hydrogen and sulfate generated by the interaction between the water and the rock. This discovery pushed back the timeline for how long life can survive disconnected from the surface.

Implications for Life on Mars and Beyond

The existence of hydrogen-eating bacteria in Earth’s crust has massive implications for astrobiology. If life can thrive deep inside Earth without sunlight, it could theoretically do the same on other planets.

• Mars: The Martian surface is bombarded by radiation and is likely too harsh for life. However, the Martian subsurface might be warm enough to support liquid water. If the rocks on Mars contain radioactive elements (which they do), radiolysis could be generating hydrogen there just like it does here.
• Europa and Enceladus: These moons of Jupiter and Saturn have icy shells covering liquid oceans. Hydrothermal vents or rock-water interactions at the bottom of these oceans could produce hydrogen, fueling similar microbial life.

NASA and other space agencies are now looking at “subsurface analogues” on Earth to design equipment for detecting life on these other worlds.

Rethinking Carbon and Energy

Understanding these bacteria also helps us understand Earth’s carbon cycle. These organisms trap huge amounts of carbon underground. As they grow and die, they influence the chemistry of the deep earth.

There is also interest in the potential for “gold hydrogen.” This refers to naturally occurring hydrogen gas trapped underground. If bacteria are not consuming it all, there could be reservoirs of natural hydrogen that humans could tap into as a clean energy source. Companies and researchers are currently mapping areas where geologic hydrogen might accumulate faster than microbes can eat it.

Frequently Asked Questions

Are these deep-earth bacteria dangerous to humans? Generally, no. These organisms are adapted to extreme pressure, heat, and specific chemical environments found miles underground. They usually cannot survive the oxygen-rich, low-pressure environment where humans live. They are specialized for the deep biosphere.

How deep can life go? We have not found the bottom yet. Life has been found as deep as we have drilled. The theoretical limit is likely determined by temperature. At a certain depth (likely around 122°C or 252°F), proteins break down faster than cells can repair them. This limit is likely several miles deeper than current drill sites.

Do these bacteria grow fast? No. Life in the deep biosphere moves at a glacial pace. Because energy resources like hydrogen are limited and sporadic, these bacteria reproduce very slowly. Some estimates suggest cell division might happen once every few years or even centuries, rather than the minutes or hours seen in surface bacteria.

How do scientists study them? Scientists access the deep biosphere through deep mines (like gold or copper mines) and scientific drilling projects (like those on drill ships in the ocean). They collect water and rock samples from pristine fractures to ensure they are not contaminated by surface air or equipment.