Microbes can live pretty much anywhere. Bacteria, archaea, and primitive eukaryotes have been found at the tops of volcanoes and at the bottom of the oceans, in natural habitats and man-made situations. And for a long time it has been thought that these hardy cells could survive within Earth’s crust.
Far beyond the reach of the Sun’s rays, and therefore unable to use photosynthesis to fix carbon, such deep life, if it existed, would need to use different chemical and metabolic pathways to survive. Several groups around the world have used the chemical signatures of these alternate pathways as a proxy for microbial activity within the crust . Others have looked for patterns that life might leave behind, such as unique textures and chemical deposits in deep oceanic glass . But finding definitive, direct evidence of life beneath the ocean floor has proven extremely difficult.
The most persuasive line of evidence for deep life would be the capture and subsequent growth of an organism in a controlled, laboratory environment, but for a number of reasons this experiment has proven extremely difficult to perform. However, in a recent issue of Science, the results of such an experiment are described .
An international group of researchers, including scientists from the Deep Carbon Observatory, analyzed material from a borehole drilled over half a kilometer into the seafloor of the Juan De Fuca Ridge in the Northern Pacific. They found genetic evidence of Methanosarcinales, anaerobic archaea known to metabolize methane. Further experiments showed that microbes have affected the chemical signature of sulfur in the host basalt, suggesting they could harness energy from the breakdown of sulfates.
But the crucial question of whether there were living microbes still present in the rock still needed an answer. Samples of the rock were carefully processed to ensure that no external contaminants, such as bacteria from the drill fluid or the sediment above the basalt, were introduced. Next, they were added to equally sterile media in the lab, and the cultures were watched carefully for any sign of life. After seven years it was confirmed: The cultures contain methanogenic archaea that are actively metabolizing, albeit incredibly slowly.
These data confirm what deep life scientists have long suspected; that Earth’s crust plays host to life. While that life may not be massively abundant, nor prolific, it represents a crucial part of our planet’s ecosystem. Indeed, at several kilometers thick and accounting for around 60% of the surface area, the upper oceanic crust is the largest habitat on Earth. Moreover, that life can exist in such an environment has enormous implications for extraterrestrial exploration, hinting at the potential for unique biospheres in places where scientists might not previously have thought to look.
Photo courtesy of William Crawford of the IODP