Under the bottom of the ocean, in the hydrated oceanic crust,a huge thriving ecosystem , one of the largest in the world and virtually unknown, is what the study says of a team of French researchers from the Institute of Earth Physics of Paris and Italian researchers from the University of Modena and Reggio Emilia. This giant ecosystem probably plays an important role today and in addition, could host the first forms of life.
Earth mantle rocks, called peridotites, brought to outcrops on the ocean bottom by the interaction of tectonic environments which spurs the deep carbon cycle. These rocks, unstable in the presence of water, have the remarkable ability to generate large amounts of hydrogen by the hydration of silicates that comprise them.
This hydrogen, reducing CO2 from the seawater or the mantle, may lead to the formation known as the “abiotic” of methane and hydrocarbons light . These products of hydration of peridotites provide, in addition, the metabolic energy required for the development of microbial communities in depth, away from all sources of photosynthetic energy.
Abnormalities in H2 and CH4, supporting autotrophic ecosystems have been demonstrated on the ocean floor, hydrothermal fields at low temperatures, as the spectacular Lost City site located on the Atlantis massif near the Mid-Atlantic Ridge. But what exactly is it deep? Until the recent study published in Nature Geoscience, there were no direct evidence of the existence of deep-sea ecosystems fed by volatiles derived from the Earth mantle into the depths of the oceanic lithosphere.
Detection of carbon of biological origin
This study was conducted on samples of hydrated peridotite (altered or “serpentinized”), collected by dredging along the Mid-Atlantic Ridge (4-6 ° N). They host strings of hydrogrenats (hydrated garnet crystals) whose heart is strongly affected by dissolution. Investigations by electron microscopy and Raman spectroscopy have allowed to detect endogenous accumulations of organic carbon of biological origin.
Such hydrogrenats seem to serve as a substrate for microorganisms chemoautotrophs that would use the derived products Serpentinite to develop. The rate of thermal maturation of the carbonaceous material is used to determine the temperature range (between 80 and 100 ° C) where the process took place, a depth of occurrence in the first two kilometers of the oceanic lithosphere serpentinized.
Greatest microbial habitat in the world
Two-thirds of the lithosphere created along 60,000 km of ocean ridges are mainly composed of peridotites that ridges in slow and ultralente (expansion rate <60 mm / year), flush with the seafloor and thus are serpentinized. In addition, the seawater circulates at depth of several kilometers. Considering these two aspects, it may be that these environments are the largest microbial habitat on Earth.
This raises the question of the role of deep-sea ecosystems in carbon sequestration, the rate of primary productivity associated and physicochemical factors that limit production. Neglected so far in the global models, this life seems intraterrestrial play and have played a key role in the evolution of our planet as a mediator between lithosphere elementary streams, oceans and atmosphere.
The origin of life?
These new signatures of life given in a context reminiscent of the environment of our Earth Hadean (4.5 – 3.8 billion years), also open up interesting perspectives about the emergence of life on our planet. For the first living cells may arise from CO2, rocks and water, a steady source of energy is needed. Serpentinization, now viewed with increasing attention, appears to be a prime candidate.
Natural source of chemical energy, it could have provided the first biochemical pathways that underlie the emergence and development of microbial ecosystems, exploiting, rather than causing, existing geochemical processes. In this perspective, hydrogrenats have therefore have been a more than favorable prebiotic environment.