International Ocean Discovery Program’s (IODP) Expedition 370: T-Limit of the Deep Biosphere off Muroto targeted many of the science goals of the Deep Carbon Observatory, including mapping the abundance and diversity of subsurface marine microorganisms and their interactions with deep carbon.
The state-of-the-art D/V Chikyu traveled to the central Nankai Trough, ~120 kilometers off the coast of Japan where the ocean is 4.7 kilometers deep, and drilled a further 1.2 kilometers beneath the ocean’s floor to collect sediment and rock cores. The total distance from the ocean’s surface to the target sample depth is equivalent to the height of 18 Eiffel Towers.
The Nankai Trough off Cape Muroto offers unique conditions where temperatures can approach ~130°C as cores were collected. Based on previous data from deep-sea hydrothermal vents, researchers expected the upper limit of life in the seafloor near 120°C. The hottest life currently catalogued on Earth includes Geogemma barossii, a single-celled organism thriving in hydrothermal vents on the sea floor. Its cells, tiny microscopic spheres, grow and replicate at 121°C. Scientists found the organism, also known as strain 121, in a sample collected during a 2003 expedition to the Juan de Fuca Ridge off the northwest USA coast and named it after Deep Carbon Observatory scientist John Baross (University of Washington, USA), who provided the sample.
The Trough is located on the Eurasian plate, where heat flow is particularly high, near its boundary with the subducting young, hot Philippine Sea tectonic plate. At the targeted site, the geothermal gradient is about four times steeper than elsewhere in the Pacific Ocean. Reaching these temperatures in other areas would require collecting cores from ~4 kilometers below the seafloor, rather than the 1.2 kilometers target for the T-Limit Expedition.
Researchers found the site in Nankai Trough 15 years ago but returned in 2016 with a renewed purpose and more advanced technologies, coupled with analysis capabilities at top laboratories around the world. In their sample analyses, investigators also are considering existing data to learn how cell concentrations drop to zero, or if life continues at a slower pace or with different forms of microbes with different energy needs.
A unique aspect of this endeavor involved investigations occurring simultaneously onboard the Chikyu and on land at the Kochi Core Center in Kochi, Japan. For the first time, helicopters transported fresh core samples from the Chikyu to the state-of-the-art research facilities in the Kochi Core Center. There, members of the shore-based science team analyzed samples to determine the geochemical and microbiological characteristics of the sediments, and painstakingly counted minuscule and sparse cells.
The team used next-generation DNA sequencing technology to determine which organisms can survive in deep subseafloor sediments and their ancestry. Genetic data like these provided clues showing how resident microbes adapt to such extreme environments.