The Earth's surface is teeming with life: this is a defining characteristic of our planet that scientists believe makes it unique. However, researchers now think that we have likely underestimated how deeply life can thrive beneath our feet, as reported by Science Alert.
In a new study that lasted 8 years, an international team of scientists discovered a remarkable diversity of microbes living beneath the surface of our planet, deeper than anything previously found.
According to co-authors of the study, microbiologist Emil Raffa from the Woods Hole Oceanographic Institution in the U.S. and bioinformatician Isabella Krabe de Angelis from the Max Planck Institute for Chemistry in Germany, they and their team analyzed samples from over fifty locations worldwide. The samples were collected from above and below the Earth's surface, as well as from caves and deep-sea hydrothermal vents forming the surface threshold.
Surface samples were taken from soils, sediments, or water columns, while subsurface samples were extracted from wells, mines, aquifers, and fluids from fracking. The study revealed life forms at depths of up to 491 meters below the ocean floor and even deeper: up to 4375 meters down. This suggests that the diversity of life forms found follows an unexpected pattern.
Raffa stated that it was previously assumed that the deeper one goes beneath the Earth's surface, the less energy is available, and thus the number of cells capable of surviving is also lower. However, the new research shows that in some subsurface environments, diversity can easily compete with, and possibly even exceed, that of surface life.
The authors of the study note that this is particularly true for microbes in marine environments as well as archaea: the deeper the scientists descended, the more genetically rich the life became. The bacterial diversity in the ocean depths turned out to be unexpectedly high compared to surface ecosystems.
Life in the subterranean worlds, as discovered by the researchers, operates on a completely different timescale than that on the surface. Without sunlight, energy is scarce, so it must be derived from surrounding materials and their chemical reactions: hydrogen, methane, sulfur, serpentinization, dead (or living) bodies of neighboring microbes, and even radioactivity.
Equally intriguing was the speed at which these ecosystems operate. For instance, scientists calculated that some cells in the deep biosphere divide only once every thousand years. According to Raffa, this makes sense: to be as energy-efficient as possible, it is necessary to minimize energy and power needs while optimizing each individual part of metabolism.
During the study, the researchers assessed microbial diversity among 478 archaea and 964 bacteria, as well as 147 metagenomes from various marine and terrestrial locations. The results indicate that surface and subsurface life did not actually exist as two separate ecosystems with a clear division. However, there were differences in how marine and terrestrial ecosystems were composed, regardless of whether they inhabited surface or subsurface environments.
The team notes that there was too much data for certain environments, such as the ocean surface, while data was scarce or lacking for other environments like caves or soils.
According to Raffa, if life can exist so deeply beneath the planet's surface, it likely could have ventured even further.