Urban endoliths: analyzing the incidental microbial communities occurring within ordinary concrete

Concrete is the most abundant engineered material on Earth. Billions of tons of concrete are made each year using a variable mixture of loose aggregates (usually sand and gravel) and an agent that binds them into solid structures (usually water-activated Portland cement). Hardened concrete is like natural rock in that it exists as a rigid, mineralized matrix that is largely impermeable to water, air, and light. These rock-like features make concrete a popular building material, but they also seem to render concrete as physically unchangeable, chemically inert, and inhospitable to life. However, the studies presented here test the emerging “concrete endolith hypothesis,” which states that concrete functions as habitat for endolithic microbes (microbes that live inside solid rock). We first conducted a life detection survey, which determined if and how often endolithic microbes are incidentally present inside various types of ordinary concrete. We gathered concrete samples from pre-existing structures from the city of Lubbock, Texas, USA and isolated the internal portion (endolithic zone) of each sample. We subject each prepared sample to a panel of life detection that involved culture tests and biomolecules assays. These tests revealed that ordinary concrete is commonly inhabited by viable and diverse microbes including bacteria, fungi, and archaea. Moreover, by correlating microbiological test results with the physicochemical characteristics of our concrete samples, we found concrete endolith communities are highly variable (as variable as the concrete materials they inhabit). After learning that viable endolithic microbes are present in ordinary concrete, we hypothesized that concrete endoliths, as living organisms, help store or release carbon out of their concrete-based ecosystems. In other words, concrete endoliths may mitigate or exacerbate the carbon profile of global concrete industry. We formally estimated that the global stock of concrete, which amounts to several hundred gigatons of new and old concrete, holds thousands, perhaps millions of tons of microbial biomass, which means that concrete endoliths around the world represent a large but overlooked pool of organic carbon. We attempted to measure microbial-induced carbon dioxide (CO2) emissions from solid concrete (“concrete respiration”), but only found limited evidence that only the oldest and most weathered concrete structures contain endoliths that are active enough to release measurable amounts of carbon. Finally, we evaluated concrete endoliths in the broader contexts of urban ecology and ecosystem engineering. We argued that concrete endoliths are at the intersection of three important phenomena: (1) global environmental change, (2) the vast production of concrete building materials, and (3) the mysterious ecological guild of endolithic organisms. A large component of global change is urbanization and the general proliferation of built environments. We often modify environments by building, and we often choose to build with concrete (a type of rock). So, it follows that our built environments unintentionally and unknowingly contain a plethora of niches for rock-dwelling microbes (i.e., concrete endoliths). We call for additional studies that would elucidate how concrete endoliths (and other cryptic urban species) affect the sustainability of built environments.

Embargo status: Restricted until 06/2024. To request the author grant access, click on the PDF link to the left.