Deep-sea vent near the Mariana Arc
At deep-sea vents like this one in the western Pacific near the Mariana Arc, highly concentrated slurries of minerals are released from the vents, mix with seawater, and encourage the growth of microbes. In some cases, the reducing environment can support organisms that actually release energy when they form, instead of requiring it. Credit: Pacific Ring of Fire 2004 Expedition, NOAA Office of Ocean Exploration; Dr. Bob Embley, NOAA PMEL, Chief Scientist, CC BY 2.0
Source: Journal of Geophysical Research: Biogeosciences

The deep-sea neighborhoods around hydrothermal hot spots are a party of productivity, especially compared to the majority of the seafloor. The heat, minerals, dissolved gases, and pressures found in these hot spots provide rich environments in which microbial communities can thrive.

In these harsh conditions with swirling chemicals, organic compounds can form from inorganic materials, releasing energy along the way, which is the opposite of more familiar conditions on Earth’s surface, in which energy is consumed to form organic materials. Bacteria gain energy by reducing carbon dioxide with hydrogen to make methane and water—a process called autotrophic methanogenesis.

In a new study by Dick and Shock, the researchers looked into where else energy might be released in ultramafic, hydrothermal ecosystems and what that might mean for life in these complex biogeochemical environments. They looked at hydrothermal vents in the Mid-Atlantic Ridge (the vent field called Rainbow that is hosted in ultramafic rocks) and a vent on the Juan de Fuca boundary in the Pacific (a basalt-hosted vent field called Endeavour). The researchers looked at nearly 1,800 proteins for Methanocaldococcus jannaschii, a member of the Archaea found in hydrothermal vents, and parsed out autotrophic methanogenesis reactions and overall amino acid synthesis reactions in both vent locations.

They found that methanogenesis was driven by the large disequilibrium of chemicals that result from the mixing of hydrothermal fluids and seawater. The team discovered that in ultramafic systems, energy is released in protein synthesis over a wide range of temperatures. However, the same was not found for basalt-hosted vents, where temperature ranges were smaller for methanogenesis and protein synthesis doesn’t release energy.

Considering these findings, the researchers note that particular hydrothermal systems are hot spots for microbial proliferation. They note that in highly reduced systems, the way proteins are synthesized and energy is released can tell researchers much about how biogeochemical cycles could have driven the emergence of life in the deep sea. (Journal of Geophysical Research: Biogeosciences, https://doi.org/10.1029/2021JG006436, 2021)

—Sarah Derouin, Science Writer

Citation: Derouin, S. (2022), Hydrothermal microbes can be green energy producers, Eos, 103, https://doi.org/10.1029/2022EO220020. Published on 7 January 2022.
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