Miles beneath our feet, Earth’s rocky crust may seem a cold, dead place. On closer inspection it’s anything but.
Microbes have been making a home on and in rocks since…well, since the beginnings of life, some 3.5 billion years ago. The traditional view of rock-dwelling microbes is one of sparse, energy-starved survivalists eking out an existence on a hostile frontier. These critters are at the mercy of their environments, their numbers dictated by physical factors – things like temperature and pH.
A study published in Geomicrobiology Journal, led by graduate student Aaron Jones at the University of Texas at Austin, challenges the assumption that subsurface microbes are passive survivalists. In a series of lab experiments, Jones demonstrates that mineral-dwelling microbial communities are uniquely adapted to their environments on the scale of individual mineral grains. Moreover, his research suggests microbes and minerals share a partnership that stretches back through geologic time.
Jones was interested in how mineralogy- the composition of the minerals that form rocks- influences the abundance and diversity of microbes. In particular, he was interested in sulfur oxidizers- bacteria that strip electrons from sulfur to acquire energy. Sulfur oxidizers are common in other subsurface environments including deep ocean vents. Their metabolism generates strong acids that eat away at certain minerals, forming cracks, pores, even caves.
In the lab, Jones set up a series of “bioreactors”- tanks in which liquid of a specific chemical composition flows over thin sheets of rock. He used a range of rocks that differ in their mineralogy. These included several rocks composed of calcium carbonate (limestone, dolostone and calcite) and several made of silica-rich minerals (albite, microcline and quartz). Jones inoculated his bioreactors with samples of a sulfur-metabolizing biofilm collected from a Wyoming cave spring.
After three weeks of incubation, he measured bacterial growth, extracted DNA and used pyrosequencing to examine the composition and diversity of the microbial communities formed on different rock types.
Different rocks, different microbes
Growth patterns differed substantially across the carbonate and silicate rocks. On carbonates, bacteria grew in thick, filamentous biofilms. By contrast, growth was much sparser on silicates, with bacteria either accumulating in small clumps or spreading out individually.
Community composition also differed among different rocks. The bacterial phyla Alphaproteobacteria dominated on most carbonates. Sulfur oxidizers that thrive at near-neutral pH were common, representing 17-45 % of the community.
A different picture emerged on the silicate rocks. Members of the phyla Firmicutes and Actinobacteria- also known as Gram-positive bacteria– were found here. While community composition was relatively similar among the carbonates, communities that grew on quartz were very different than those found on albite or microcline. Jones observed fewer sulfur oxidizers on silicates, and those he found tended to be more acid-tolerant.
Perhaps the most obvious conclusion here is that microbes do not select rocks at random. Different growth patterns among rocks tell us this. Better microbial growth on carbonates probably reflects a higher density of essential nutrients, particularly phosphorus. Moreover, two of the silicate rocks, albite and microcline, contain high concentrations of aluminum, a toxic element that restricts bacterial growth.
More subtly, however, there appears to be “selective relationship” between mineralogy and microbial ecology. In other words, the microbes making a living on any given rock are uniquely adapted to taking advantage of, or protecting themselves from, the specific chemical properties of that rock.
This relationship bears itself out in many ways. For one Jones found more acid-tolerant microbial communities on silicate rocks. Remember, acid is a byproduct of sulfur metabolism. Carbonate rocks have a high “buffering capacity”, meaning their minerals react with and neutralize acid. By contrast, silicate rocks, especially quartz, are pretty unreactive. Rather than eating away at minerals, acid accumulates at silicate surfaces. The presence of acid-loving bacteria on quartz suggests a feedback loop between sulfur metabolism, mineralogy and microbial community structure.
The presence of Gram-positive bacteria on silicates also underscores the relationship between microbial community composition and the mineral environment. The cell walls of Gram-positives possess unique chemical properties that promote bonding with silicate surfaces. This microbe-mineral binding, known as adhesion, is a key element of survival for most rock-dwelling bacteria. By adhering to a solid surface, bacteria can mine nutrients and form protective biofilms.
Even microbes living on the undesirable aluminum-rich rocks may possess unique adaptations. While few bacteria grew on albite or microcline, these sparse populations were highly diverse. The unique challenges presented by a toxic environment may serve to maintain competition, preventing any single group of organisms from gaining dominance.
Our planet’s rocky subsurface is the largest microbial habitat, but we still know very little about its inhabitants. Most microbial ecologists prefer to study environments they can see, feel and intimately relate to. Jones’s research points to complex ecological interactions between microbes and minerals in the deep, dark depths of the world. Rocks shape microbial communities, but microbes may also shape rocks in profound ways. This study lends evidence to a deep synergy between the geological and biological evolution of our planet.
Jones, A., & Bennett, P. (2014). Mineral Microniches Control the Diversity of Subsurface Microbial Populations Geomicrobiology Journal, 31 (3), 246-261 DOI: 10.1080/01490451.2013.809174