Deep beneath the Earth's surface, a diverse array of life thrives, holding the secrets to aging (photo).

As early as the mid-1990s, scientists made an astonishing discovery that transformed our understanding of life on Earth: vast reservoirs of microbes are hidden beneath the planet's surface. These microbes often reside several kilometers deep in rock formations and without sunlight. Now, in a new study, researchers have estimated that between 50% and 80% of all microbial cells on Earth actually live underground, reports IFLScience.

In this new research, scientists from the Marine Biological Laboratory (MBL) in Woods Hole have developed a global atlas of microbial diversity that inhabits the depths of the Earth. As a result, they have begun to unveil the secrets of this fascinating hidden world.

The groundbreaking work of the scientists includes the analysis of over 1,000 microbial samples, including bacteria and archaea, collected from 50 marine and terrestrial ecosystems around the globe. In total, it took the researchers 8 years to study the samples, discovering 31,000 unique types of archaea and over 377,000 unique bacteria. These are also known as ASVs (amplicon sequence variants), which serve as a kind of "barcode" identifying the composition of microbial communities.

The scientists' findings indicate that subsurface microorganisms are actually incredibly diverse, which is quite surprising given the harsh and extreme environments in which they live. According to the lead author of the study, Emile Raff, it is believed that the deeper one goes beneath the Earth's surface, the less energy is available, and consequently, the fewer cells can survive. At the same time, the more energy that is available, the greater the diversity that can be created and maintained — much like in tropical forests or coral reefs, where there is plenty of sunlight and warmth.

However, the results of the new study show that this is not entirely the case. In fact, in some subsurface environments, diversity can easily rival, if not exceed, that found on the surface. This is particularly true for marine environments and microbes in the domain of archaea.

Researchers also found a significant difference between marine microbes and their terrestrial counterparts, which holds true for other forms of life on Earth as well. According to Raff, he and his colleagues now believe this to be a universal ecological principle: there is a clear distinction between life forms in marine and terrestrial spheres, not only on the surface but also beneath it.

Subsurface life forms likely follow their own rules. Most living organisms on the planet's surface can trace their energy back to photosynthetic cells that utilize light to produce organic molecules. In the absence of sunlight, underground microbiomes must rely on alternative methods.

Some will scavenge tiny amounts of organic power found in desert environments, while others will derive energy through the radioactive decay of rocks. Another tactic is chemosynthesis, which produces nutrients through inorganic chemical reactions.

The authors of the study note that their discovery of subsurface microbiomes challenges previous assumptions about life elsewhere in the Solar System. The scientists also believe that the rich microbial life hidden beneath the Earth's surface may hold secrets to aging.