Researchers have discovered a previously-unrecognized biological sink for a potent greenhouse gas: methane-breathing microbes living within rocky mounds on the seafloor. By using methane for respiration, these rock-dwelling microbes remove large amounts of the greenhouse gas from the ocean before they escape into the atmosphere. The findings were published in Nature Communications this week.
Methane-consuming microorganisms are known to live near cold seeps -- ocean floor areas where methane seepage naturally occurs -- as well as in thin layers of sediment on the surface of huge, rocky outcroppings of calcium carbonate surrounding seep sites. These tall structures are better known as foundations for coral and sponges and homes for rockfishes, clams, and crabs. Finding active methane-consuming microbes in the interior of carbonate rocks extends their known habitat and introduces a new ecological niche for key methane consumers.
"Methane is a much more powerful greenhouse gas than carbon dioxide, so tracing its flow through the environment is really a priority for climate models and for understanding the carbon cycle," Caltech’s Victoria Orphan says in a university statement. Her team previously found that two microorganisms that survive without oxygen work together to consume methane using sulfate from seawater: single-celled creatures called anaerobic methanotrophs and their bacteria partners. Until now, this two-microbe system has only been observed oxidizing methane at seeps.
“No one had really examined these rocks as living habitats before,” Andrew Thurber of Oregon State says in a news release. They were just assumed to be inactive, serving as passive recorders of methane oxidation over time. “This goes to show how the global methane process is still rather poorly understood.”
Using manned and robotic submersibles, the team collected rock samples from active cold seeps as well as carbonate mounds that appeared to be dormant at three sites: the tectonic plate boundary near Costa Rica (right), Eel River basin off the coast of northwestern California, and Hydrate Ridge off the Oregon coast (above). The rocks range in depth from 600 to 800 meters below the surface and in size from small pebbles to carbonate pavements stretching for dozens of kilometers.
Back at the surface, the carbonates were cracked, and a series of tests confirmed that the rocks did indeed host anaerobic methanotrophs and sulfate-reducing bacteria. Genetic analysis showed how they were related to methane-munchers previously characterized in seafloor sediment.
"The carbonate-based microbes breathed methane at roughly one-third the rate of those gathered from sediments near active seep sites," Caltech’s Jeffrey Marlow explains. The team used radiolabeled carbon-14 methane tracer gas to quantify the rates of methane consumption. "However, because there are likely many more microbes living in carbonate mounds than in sediments, their contributions to methane removal from the environment may be more significant."
Up to six percent of methane in the atmosphere comes from marine sources, and that number is so low because marine microbes consume up to 90 percent of the methane that would otherwise escape.
Images: Victoria Orphan