Deep Within A Gold Mine, A Wealth Of “Microbial Dark Matter” Is Unearthed

By biomass, microbes under Earth’s surface outweigh all life in the ocean.

To illuminate the “microbial dark matter” that lives deep within Earth, scientists took a journey deep within a gold mine and returned with samples detailing hundreds of different microbial species. This, however, is just a drop in the ocean of all the life that lives beneath our feet.

The amount of microbial life below our planet’s surface is immense. If scooped up and placed on a giant scale, the microbes living deep within Earth’s crust would outweigh all of the biomass from the world’s oceans. Despite this ubiquity, scientists know very little about them.

To get a glimpse of this subterranean world, scientists at Northwestern University studied samples taken from the Deep Mine Microbial Observatory, a former gold mine in the Black Hills, South Dakota.

“The deep subsurface biosphere is enormous; it’s just a vast amount of space,” Magdalena Osburn, lead study author and an associate professor of Earth and planetary science at Northwestern’s Weinberg College of Arts and Sciences, said in a statement.

“We used the mine as a conduit to access that biosphere, which is difficult to reach no matter how you approach it. The power of our study is that we ended up with a lot of genomes, and many from understudied groups. From that DNA, we can understand which organisms live underground and learn what they could be doing. These are organisms that we often can’t grow in the lab or study in more traditional contexts. They are often called ‘microbial dark matter’ because we know so little about them,” Osburn added.

The team sequenced the microbial DNA held within the samples and identified nearly 600 genomes from 50 distinct phyla and 18 candidate phyla.

Within this diverse collection, it appeared that almost all of the microbes fell into one of two roles: “minimalists” that have limited but specialized jobs, or “maximalists” that will readily utilize any resource they come across.

“Man[y] of the microbes we found were either minimalists: ultra-streamlined with one job that it does very well alongside a close consortium of collaborators, or it can do a little bit of everything,” Osburn said. “These maximalists are ready for every resource that comes along. If there is an opportunity to make some energy or transform a biomolecule, it is prepared. By looking at its genome, we can tell it has many options. If nutrients are scarce, it can just make its own.”

This research may even hold some implications for the search for extraterrestrial life. When looking for signs of life elsewhere in the solar system, we tend to focus on water or evidence on the surface. These kinds of discoveries, however, are a reminder that life is also perfectly capable of living below a moon or planet’s surface.

“I get really excited when I see evidence of microbial life, doing its thing without us, without plants, without oxygen, without surface atmosphere. These kinds of life very well could exist deep within Mars or in the oceans of icy moons right now. The forms of life tell us about what might live elsewhere in the solar system,” added Osburn.

The study has been accepted by the journal Environmental Microbiology. You can read an early version of the manuscript right here.