Our planet is literally full of life, as can now be seen once again: Researchers have found bizarre biotopes in basalt rock more than 100 meters below the sea floor: amazingly dense bacterial communities live in tiny cracks lined with clay minerals. They apparently live on organic material or methane, which is transported by fluid movements. The finding suggests that comparable organisms could also exist on other planets with similar underground conditions, such as on Mars.
Studies in recent years have even shown that life has even penetrated to the depths of the earth’s crust: Researchers have found microbes in drill cores at a depth of several kilometers. Microbes have also been discovered in the subsoil of the oceans. These were unicellular organisms in comparatively low population densities, which often derive their energy from reactions with chemical substances that occur in the deep rocks, including methane, hydrogen or metals. These microbes of the oceanic earth’s crust have so far been found in comparatively young volcanic rocks with an age of 3.5 and 8 million years. However, 90 percent of the oceanic crust is significantly older.
Sampling in the oceanic subsurface
For this reason, researchers led by Yohey Suzuki from the University of Tokyo have now examined samples of 13.5 million, 33.5 million and 104 million year old rocks from the marine subsurface. They came from drilling at three locations in the Pacific. First, the drill sank 5.7 kilometers to the bottom of the sea. There he milled himself into the subsoil more than 100 meters deep and obtained samples with a diameter of about 6.2 centimeters each. The first 75 meters below the seabed were mud sediments, below which there was the solid basalt rock, the researchers report.
After initially failing to detect microbes in the samples, they finally used a method that pathologists use to examine body tissues when diagnosing diseases: ultra-thin sections. Suzuki and his colleagues embed the rock samples in epoxy resin to support their natural shape so that they do not crumble when cut. They then treated the fine disks with marking substances that only stain certain substances – such as the DNA of microbes.
Lifelines are emerging
“I thought it was a dream when I saw what was shown,” says Suzuki: Bacteria appeared as bright green balls that sat tightly in shimmering orange tunnel structures, which in turn were surrounded by the black basalt rock. As the researchers explain, the tunnel structures were cracks up to a millimeter wide that once formed when the volcanic rock cooled. Over the course of millions of years, clay minerals then accumulated on the surfaces of these fine cavities. They caused the orange staining in the samples, the scientists explain. “These cracks are very life-friendly. Clay minerals are like a magical material – wherever you can find them, you can almost always detect microbes because they offer them attractive living conditions, ”says Suzuki.
This was also clearly evident in the cracks in the basalt lined with clay: the researchers estimate that the fine cracks harbor a bacterial community that is similarly dense as the human intestinal flora: around ten billion bacterial cells per cubic centimeter. In contrast, the average density of bacteria living in the sludge sediment on the sea floor is estimated at 100 cells per cubic centimeter.
From the oceanic underground to Mars
The researchers then carried out genetic tests to learn more about the bacteria. As it turned out, there are different types, the composition of which differed slightly at the three locations of the sample name. According to the analysis results, it is bacteria that use oxygen and organic nutrients, but also microbes, whose basis of life is methane. Suzuki and his colleagues speculate that the cracks lined with the clay minerals concentrate these nutrients. This would explain why the density of the bacteria in the rock cracks is so amazingly high.
In addition to the exciting information for earthly biology, the results now also direct our gaze into space. Because similar beings as in the oceanic underground of the earth could also exist on other celestial bodies, in which there are porous basalt rock, liquid water and clay minerals below the surface. In particular, Mars is now being targeted. There is evidence that the three components and also methane as an energy source are on our neighboring planet. Suzuki has already contacted NASA to develop detection strategies for microbes in rocks during Mars missions. “This discovery of life where nobody would have expected it in solid rock beneath the sea floor could change the search for organisms in space,” says Suzuki.
Source: University of Tokyo, technical article: Communications Biology, doi: 10.1038 / s42003-020-0860-1