LAUSR

Life is a chemical reaction. For 4 billion years (1), all microbial cells have required a source of carbon, energy, and electrons to power energy-releasing reactions forward toward growth. How did the first cells grow, where did they emerge, and from what chemical reactions did they arise? Some modern habitats might hold clues. In PNAS, Colman et al. (2) report an ancient lineage of anaerobic bacteria that live in the darkness of desert wells fed by hydrothermal vents. These bacteria obtain their carbon, energy, and electrons solely from the products of geochemical reactions. Their chemical environment might resemble the habitat where the first microbes arose, and the anaerobic lineage might reflect the lifestyle of microbes on early Earth. Their findings mesh well with the theory that life arose in hydrothermal vent environments (3), which, if true, would mean that life’s origin was not powered by sunlight, but by chemical energy generated within Earth itself. The findings impact our views on early microbial evolution and the search for life beyond Earth. The paper reports acetogens from serpentinized environments (2). For background, serpentinization is a geochemical process (4) involving the reaction of water, drawn by gravity into the crust, with rocks that are rich in Fe. The Fe-rich minerals react with water by giving up electrons. Fe minerals are thereby converted to Fe minerals, while H2O is converted to hydrogen gas, H2, that is carried by hydrothermal effluent to the surface. Oxygen atoms from H2O remain in the crust as iron (III) oxides such as magnetite (Fe3O4). Serpentinization also generates iron and magnesium hydroxides, which makes the vent fluid alkaline. The water in the report has pH 11 and contains 3 mM H2, orders of magnitude more than microbes need to grow (2). The process is named for its final product, serpentinite (Fig. 1), used by stonemasons for centuries. Serpentinization has been going on in submarine crust since there was first water on Earth (1, 4). Over geological time scales, it sometimes happens, however, that pieces of submarine crust are thrust up onto the continental surface at continental margins. Such surface-exposed slabs of crust are called ophiolites. They can host serpentinization. One example is the Samail ophiolite found on the east coast of Oman. This is where Colman et al. (2) employed metagenomic tools to identify microbes that live in H2-rich and hyperalkaline hydrothermal effluent. They found genomes of bacteria that can live from chemicals made by serpentinization: acetogens. Acetogens are strictly anaerobic bacteria that can produce acetate as the sole end product of their energy metabolism (5). Some acetogens have an ability that puts them almost in a class by themselves and, furthermore, in the focus of theories for microbial origin: They can grow on H2 and CO2 as their source of carbon, energy, and electrons. The only other organisms known that share that ability are methanogens (6), strictly anaerobic archaea from the opposite side of the tree of life. This shared ability to live from H2