Microbes Fix Massive Amounts of Carbon without Sunlight in Earth’s Deep Subsurface Biosphere

May 11, 2021. By Kolemann Lutz

Researchers sampled and compared the bacterial community composition of hot springs with the deep-subsurface subduction geochemistry in Costa Rica. Metagenomic DNA analysis suggests that vast underground forests of chemolithoautotrophs and microbial ecosystems fix and transform CO2 into biomass without any sunlight via chemical redox reactions. These subsurface microbes are estimated to sequester up to 1.4 × 10^10 mol of carbon per year, which would decrease the total carbon delivered to the mantle by up to 22%.

Subduction zones are the regions where tectonic plates interact and move carbon between the interior and exterior of Earth. Because of the extremely high pressures and temperatures involved, this process is thought to occur without microbes. In the early 2020’s, scientists learned that microbes extend far deeper into Earth's crust than previously thought.

Deeply sourced hydrogen and methane in fluids are though to be important energy and carbon sources supporting the largest microbial habitat on Earth, the deep subsurface biosphere. Microbes in the deep biosphere, where temperatures are around 122 °C, can reside at least 5 km below the continental surface and 10.5 km below the sea surface.

On Earth, the carbon cycle is at equilibrium on the surface through a combination of tectonic processes that buries carbon, volcanism which releases it, and biology to mediate carbon transfer. As subducting slabs can range from 20–150 km in depth, volatiles and elements mobilized from the descending slab and mantle can be altered by interactions with the deep-subsurface biosphere on their trek to the Earth’s surface.

Developed over billions of years of evolution, chemolithoautotroph bacteria sequester vast quantities of carbon primarily from their unique metabolism and diet, which allows them to make energy without sunlight. Chemolithoautotrophy bacterial communities can be viewed as vast underground dynamic forest. Researchers hypothesize that the upward mobility of deeply sourced fluids may connect microorganisms such as chemolithoautotrophs to the deep tectonic processes below.

A team of 46 scientists from 19 institutions demonstrate that a vast microbial ecosystem primarily eats the carbon, sulfur, and iron chemicals produced during the subduction of the oceanic plate beneath Cos