Microbes near deep-sea hydrothermal vents propagate via "electrosynthesis" instead of "photosynthesis"

A group including Japan Agency for Marine-Earth Science and Technology (JAMSTEC) demonstrated that microorganisms living near deep-sea hydrothermal vents in Okinawa utilize electrical energy to create organic matter from carbon dioxide (CO₂) and propagate. This is a synthetic reaction where the "light" in "photosynthesis" is replaced with "electricity," meaning that even in environments with little light or organic matter, life activities can be maintained by utilizing weak electricity.

At deep seafloors where boundaries of plates, the rock slabs covering Earth, exist, seawater that seeps in is heated by geothermal energy and released as high-temperature water, creating hydrothermal vents. Around the vents, "chimneys" are formed where cooled and precipitated deposits accumulate like smokestacks.

It has been known that among microorganisms cultured in laboratories, some "electrosynthesize" organic matter from CO₂ using electricity taken into their cells as energy. However, whether microorganisms conducting electrosynthesis existed in the natural environment remained unclear.

JAMSTEC Associate Senior Researcher Masahiro Yamamoto, a microbiology specialist and his colleagues demonstrated in 2017 that discharge phenomena occur around chimneys, creating a potential difference of approximately 600 millivolts (about 40% of a standard 1.5-volt battery). The group thought that if they reproduced an equivalent environment in the laboratory and confirmed that microorganisms in rock fragments collected near deep-sea hydrothermal vents increased even in the absence of organic matter, they could prove that electrosynthetic microorganisms inhabit the natural environment.

Soaking rock fragments in artificial seawater prepared as culture solution, electricity was passed under air with a water temperature of 15℃ and a CO₂ concentration of 10%. The types and cell propagation of microorganisms in the culture solution were observed every week. As a result, rod-shaped bacterium Thiomicrorhabdus sp. strain SREC-4 increased and came to account for the highest proportion among surviving microorganisms.

To investigate whether the increased strain SREC-4 was truly taking CO₂ into cells and converting it into organic matter, strain SREC-4 was fluorescently stained so that its locations could be identified. Using a two-dimensional high-resolution secondary ion mass spectrometer (NanoSIMS) to compare the locations where isotope-labeled CO₂-derived carbon was taken into cells, the staining locations matched. This indicates that strain SREC-4 takes CO₂ into cells and converts it into organic matter.

As an energy source supporting life activities in nature, photosynthesis, by which plants create organic matter from sunlight, is familiar. However, at "redox boundaries" where electron transfer readily occurs, electrosynthesis becomes possible. Yamamoto stated: "Redox boundaries are unexpectedly close to us, such as places where white sand changes to black sand as you dig during clam digging, or mines where metallic minerals are extracted. While deep-sea hydrothermal vents are the prime example of redox boundaries, I would like to search for other bacteria in natural environments that conduct electrosynthesis in the future."

The research was conducted jointly with Yokohama City University and The University of Tokyo and published online in the British scientific journal The ISME Journal on June 23.