Just off the coast of the continents, thousands of mud volcanoes are erupting from the seafloor, spouting methane and ancient sediments. Previous studies have suggested that the majority of methane bubbling from mud volcanoes is thermogenic, meaning that it comes from the breakdown of deep organic matter at high temperatures. A new multidisciplinary study, however, finds that the gas in some mud volcanoes comes from the stimulation of long-buried microbes, which produce surprising amounts of methane.
In a new paper in Science Advances , an international team of researchers combines geological, geochemical, and microbiological analyses to explore the geosphere-biosphere interactions occurring within a submarine mud volcano off the coast of Japan. DCO Deep Energy and Deep Life Community members Akira Ijiri, Fumio Inagaki, Tatsuhiko Toshino, Hiroyuki Imachi, Shinsuke Kawagucci, Yuki Morono, Ken Takai, (all at Japan Agency for Marine-Earth Science and Technology, Japan), David T. Wang, Shuhei Ono, (Massachusetts Institute of Technology, USA), Marcos Y. Yoshinaga, Kai-Uwe Hinrichs, (both at University of Bremen, Germany), Jens Kallmeyer (University of Potsdam, Germany), Mark A. Lever (Aarhus University, Denmark), and Victoria J. Orphan (California Institute of Technology, USA) collected and analyzed sediment cores from up to 200 meters inside the mud volcano. The researchers discovered that the geogenic fluids were stimulating a small but active community of the deeply buried microbes, which contributed to vast deposits of methane. The study suggests that methane within mud volcanoes likely represents a larger piece of the global carbon budget than anticipated.
“The volume of methane is roughly 10 times larger than we expected, and it was mainly produced by microbes,” said Inagaki, a member of the Deep Life Scientific Steering Committee. “This means that we have to reconsider the piece of the total carbon budget stored in global mud volcanoes and to what extent microbes contributed to methane production.”
The material erupting from mud volcanoes offers a rare window into the geological, geochemical, and microbial processes occurring deep beneath the seafloor. In 2009 and 2012, researchers collected cores from the mud volcano, located in the Kumano Basin of the Nankai Trough off Kii Peninsula of Japan, during two expeditions aboard the Japanese drilling vessel Chikyu. They used a high-pressure drilling tool that maintains the pressure within the cores, so that they could determine the exact gas content inside the mud volcano’s sediment.
Ono’s group analyzed the methane from within the mud volcano and showed evidence that it originates from microbes. His team used a next-generation infrared laser spectrometer that Ono developed with DCO funding. Ono’s group has shown previously that this technique can determine a methane sample’s origin by detecting small differences in the abundance of methane molecules that carry more than one rare isotope (isotopes are atoms of an element that have different numbers of neutrons). Their analysis showed that 90% of the methane from the mud volcano comes from microbes. “We can tell this methane is really, really different from methane from some other mud volcanoes. Without the information provided by this new instrument, we could have thought it was conventional thermogenic methane,” said Ono.
The microbial communities responsible for this methane have been surviving for up to millions of years beneath accumulated seafloor sediments, under increasing pressure and with limited carbon and energy sources. These microbes hit the metabolic jackpot, however, when faults in the underlying rock began supplying them with water squeezed from clay minerals and hydrogen produced by geological processes, which stimulated their growth.
Hinrichs’ team analyzed the lipids that make up the cell membranes of microbes from the sediments to estimate the population’s size and function. “The lipids we found are typical for methanogens. Their concentrations suggest a surprisingly small population compared to the inferred rates of methane produced. This requires that the small community must be quite active and that the high methane levels have accumulated over geological timescales, “ said Hinrichs, a member of the Deep Life Scientific Steering Committee.
Researchers also analyzed DNA extracted from the sediments and cultured a microbe that makes methane. They found it grows best in warm, low-salt cultures, similar to the environment predicted to exist at the bottom of the mud volcano, based on geochemical and seismic profile data.
“This methane likely sits trapped within a network of ice crystals, called gas hydrates, located between shallow depths to 570 meters beneath the mud volcano’s summit,” said Ijiri. The researchers estimate the deposit holds 3.2 billion cubic meters of methane, which is an order of magnitude larger than they had expected.
Not all mud volcanoes likely hold this much biogenic methane, however, as many mud volcanoes are hotter, and may be uncomfortably warm for microbial life. To pinpoint the hottest temperatures that subseafloor life can survive, many of the DCO scientists on this project are already hard at work analyzing the results of the “T-Limit of the Deep Biosphere off Muroto” project. This International Ocean Discovery Program initiative will further our understanding of where microbial life may be active beneath the seafloor.