Researchers discover life in deep ocean sediments at or above water's boiling point

"Water boils on the (Earth's) surface at 100 degrees Celsius, and we found organisms living in sediments at 120 degrees Celsius," said URI Professor of Oceanography Arthur Spivack, who led the geochemistry efforts of the 2016 expedition organized by the Japan Agency for Marine-Earth Science and Technology and Germany's MARUM-Center for Marine and Environmental Sciences at the University of Bremen. The study was carried out as part of the work of Expedition 370 of the International Ocean Discovery Program.

The research results from a two-month-long expedition in 2016 will be published today in the journal Science.

The news follows an announcement in October that microbial diversity below the seafloor is as rich as on Earth's surface. Researchers on that project from the Japan marine-earth science group, Bremen University, the University of Hyogo, University of Kochi and University of Rhode Island, discovered 40,000 different types of microorganisms from core samples from 40 sites around the globe.

The research published in Science today focused on the Nankai Trough off the coast of Japan, where the deep-sea scientific vessel, Chinkyu, drilled a hole 1,180 meters deep to reach sediment at 120 degrees Celsius. The leader of the study is Professor Kai-Uwe Hinrichs of MARUM.

Spivack, who was joined by recent Ph.D. graduates, Kira Homola and Justine Sauvage, on the URI team, said one way to identify life is to look for evidence of metabolism.

"We found chemical evidence of the organisms' use of organic material in the sediment that allows them to survive," Spivack said. The URI team also developed a model for the temperature regime of the site.

"This research tells us that deep sediment is habitable in places that we did think possible," he added.

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

According to the study, sediments that lie deep below the ocean floor are harsh habitats. Temperature and pressure steadily increase with depth, while the energy supply becomes increasingly scarce. It has only been known for about 30 years that, in spite of these conditions, microorganisms do inhabit the seabed at depths of several kilometers. The deep biosphere is still not well understood, and this brings up fundamental questions: Where are the limits of life, and what factors determine them? To study how high temperatures affect life in the low-energy deep biosphere over the long-term, extensive deep-sea drilling is necessary.

"Only a few scientific drilling sites have yet reached depths where temperatures in the sediments are greater than 30 degrees Celsius," explains study leader Hinrichs of MARUM. "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."

Like the search for life in outer space, determining the limits of life on the Earth is fraught with great technological challenges, the research study says.

"Surprisingly, the microbial population density collapsed at a temperature of only about 45 degrees," says co-chief scientist Fumio Inagaki of 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."

Spivack said the project was like going back to his roots, as he and David Smith, professor of oceanography and associate dean of URI's oceanography school, where they were involved in a drilling expedition at the same site about 20 years ago, an expedition that helped initiate the study of the deeply buried marine biosphere.

As for the current project, Spivack said studies will continue on the samples the team collected. "The technology to examine samples collected from the moon took several years to be developed, and the same will be true for these samples from deep in the ocean sediments. We are developing the technology now to continue our research."