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Primitive life hidden in the crevices of rocks reveals the mystery of the birth of life

Yohei Suzuki, Associate Professor, Department of Earth and Planetary Science

The search for the origin of life has entered an era in which the answers are being sought beyond the Earth. In particular, research on the formation process of macromolecules (nucleic acids, amino acids, etc.), which are the building blocks of life, is being conducted in addition to experiments using state-of-the-art synthesis techniques. The search for extraterrestrial life forms is also underway, including plans to search for life on Mars and ice satellites, since the discovery of such life forms will place significant constraints on the origin of life. Certain substances are thought to have played an important role in the synthesis of macromolecules, the partition between the inside and outside of the cell, and metabolism and heredity within the cell during the development of life on Earth. These substances are clays and metal sulfides formed by the reaction of rocks and water, and are the main targets in the detection of life from extraterrestrial materials.

We define a "life birthplace" as a place where life-driving materials such as clays and metal sulfides exist in an environment similar to that at the time of the birth of life on Earth. The time of the birth of life is assumed to be the period before the birth of photosynthetic organisms and the current earth environment where photosynthetic organisms and their metabolic products, such as organic matter and oxygen, do not reach. Our research has focused on the subsurface, where photosynthetically derived organic matter and oxygen do not reach, and has led to the discovery of microorganisms living in clay-filled rock cracks at a density as high as that of the human gut (introduced in Faculty of Science News 2020, Vol. 52, No. 2 "Research Frontlines"). On the other hand, metal sulfides universally form in deep-sea bottom hydrothermal vents, forming chimney-like structures composed of metal sulfides. These structures are called metal sulfide chimneys, and the hypothesis that life originated within them is considered to be a textbook-level hypothesis.

Since oxygen-rich deep seawater permeates the present-day Earth, there have been concerns that the conditions for a "birthplace of life" in a metal sulfide chimney, which was anoxic immediately after its formation, have not been met, and that primitive life hidden in the crevices of rocks, which is the mystery of the birth of life, has not yet emerged. Although the deep sea lacks organic matter derived from photosynthetic organisms, it was a technical challenge to prove whether the interior of the chimney was maintained in an anoxic condition. We collected metal sulfide chimneys from deep-sea bottom hydrothermal vents in the southern Mariana Trough using an unmanned submersible (top image), and observed the narrow crevices inside the chimney, which were just barely accessible to microorganisms, with an electron microscope. As a result, we succeeded in capturing a dense population of microscopic copper-encrusted microorganisms (cell size: 100 nm = 1/10,000 of 1 mm) (middle figure). Phylogenetic classification of microorganisms living inside chimneys based on DNA sequences showed that the microscopic microorganisms living in grain boundaries diverge at the root of the universal phylogenetic tree, indicating that the microorganisms originated at the beginning of the evolution of life. The microorganisms inhabiting the grain boundary are direct descendants of life forms that emerged at the beginning of the evolution of life (as described in "Mysteries of Science, " Faculty of Science News, Vol. 52, No. 5, 2021 ).

We are currently collecting detailed biological information through genome sequencing and listening to the stories of primitive microorganisms about the environment of the "birthplace of life" and the evolution of early life.

The results of this research were published in H. Takamiya et al. , Frontiers Microbiology ( 2022).

Figure: Photograph of metal sulfide chimney formed at a deep-sea floor hydrothermal vent (top), electron micrograph of microscopic microorganisms found inside the chimney (middle), and illustration of microscopic microorganisms living inside the chimney (bottom)

(Press release, June 7, 2022)

Published in the September 2022 issue of Faculty of Science News

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