Hot Subseafloor Sediments

An international team of researchers has discovered microbial life, in particular bacterial vegetative cells, in up to 1.2-km-deep and up to 120 degrees Celsius hot sediments in the Nankai Trough subduction zone off Cape Muroto, Japan.

A microbial cell (center of the picture) detected from a sediment core sample at the depth of 1176.8 m at 120 degrees Celsius. Scale bar - 20 μm. Image credit: JAMSTEC / IODP.

A microbial cell (center of the picture) detected from a sediment core sample at the depth of 1176.8 m at 120 degrees Celsius. Scale bar – 20 μm. Image credit: JAMSTEC / IODP.

“Water boils on the (Earth’s) surface at 100 degrees Celsius, and we found organisms living in sediments at 120 degrees Celsius,” said team member Dr. Arthur Spivack, a scientist in the Graduate School of Oceanography at the University of Rhode Island.

In October 2020, a team of researchers announced that microbial diversity below the seafloor is as rich as on Earth’s surface.

They discovered 40,000 different types of microorganisms from core samples from 40 sites around the globe.

Dr. Spivack and colleagues focused on the Nankai Trough off the coast of Japan, where the deep-sea scientific vessel, Chinkyu, drilled a hole 1,180 m deep to reach sediment at 120 degrees Celsius.

“Only a few scientific drilling sites have yet reached depths where temperatures in the sediments are greater than 30 degrees Celsius,” said Professor Kai-Uwe Hinrichs, a researcher in the Center for Marine and Environmental Sciences (MARUM) at the University of Bremen.

“The goal of the T-Limit Expedition, therefore, was to drill a thousand-meter deep hole into sediments with a temperature of up to 120 degrees Celsius — and we succeeded.”

“Surprisingly, the microbial population density collapsed at a temperature of only about 45 degrees Celsius,” said Dr. Fumio Inagaki, a researcher in the Research and Development Center for Ocean Drilling Science and Kochi Institute for Core Sample Research at the Japan Agency for Marine-Earth Science and Technology (JAMSTEC).

“It is fascinating — in the high-temperature ocean floor, there are broad depth intervals that are almost lifeless.”

“But then we were able to detect cells and microbial activity again in deeper, even hotter zones — up to a temperature of 120 degrees Celsius.”

While the concentration of vegetative cells decreases sharply to a level of less than 100 cells per cm3 of sediment at over 50 degrees Celsius, the concentration of endospores increases rapidly and reaches a peak at 85 degrees Celsius.

Endospores are dormant cells of certain types of bacteria that can reactivate and switch to a live state whenever conditions are favorable again.

“Some specialist types are able to adapt to these severe conditions and persist over geological time spans in a sort of deep sleep,” Dr. Inagaki said.

“The findings of our expedition are surprising,” said Dr. Verena Heuer, a researcher of MARUM.

“They show that at the lower boundary of the biosphere lethal limits coexist with opportunities for survival. We didn’t expect that.”

“And this new understanding would not have been possible without the strong interdisciplinary team and its dedicated spirit of cooperation.”

“We found chemical evidence of the organisms’ use of organic material in the sediment that allows them to survive,” Dr. Spivack said.

“This research tells us that deep sediment is habitable in places that we did think possible.”

While this is exciting news on its own, the research could point to the possibility of life in harsh environments on other planets.

“Like the search for life in outer space, determining the limits of life on the Earth is fraught with great technological challenges,” the scientists said.

The findings were published in the journal Science.

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Verena B. Heuer et al. 2020. Temperature limits to deep subseafloor life in the Nankai Trough subduction zone. Science 370 (6521): 1230-1234; doi: 10.1126/science.abd7934

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