Corona Chen's research while affiliated with Massachusetts Institute of Technology and other places

There is great interest to understand and reduce the massive carbon footprint of the concrete industry. Recent descriptions of microbes incidentally living inside concrete materials ("concrete endoliths") raised questions about how much carbon is either stored in or released from concrete by these microbes. We generated preliminary global estimates of how much organic carbon is stored within the living biomass of concrete endoliths (biomass-carbon) and much CO2 is released from respiring concrete endoliths. Between 2020–2022, we collected widely varying samples of Portland cement-based concrete from Lubbock, Texas. After quantifying endolith DNA from 25 concrete samples and estimating the current global mass of concrete, we calculated that the global concrete endolith biomass-carbon as low as 5191.9 metric tons (suggesting that endoliths are a negligible part of concrete's carbon profile) or as high as 1141542.3 tons (suggesting that concrete endoliths are a pool of carbon that could equal or offset some smaller sources of concrete-related carbon emissions). Additionally, we incubated concrete samples in air-tight microcosms and measured changes in the CO2 concentrations within those microcosms. Two out of the ten analyzed samples emitted small amounts of CO2 due to the endoliths. Thus, "concrete respiration" is possible, at least from concrete materials with abundant endolithic microbes. However, the remaining samples showed no reliable respiration signals, indicating that concrete structures often do not harbor enough metabolically active endoliths to cause CO2 emissions. These results are preliminary but show that endoliths may alter the carbon dynamics of solid concrete and, thus, the carbon footprint of the concrete industry.

Concrete is now a prevalent type of synthetic rock, and its production and usage have major environmental implications. Yet, assessments of ordinary concrete have rarely considered that concrete itself is potential habitat for a globally important microbial guild, the endolithic microbes, which live inside rocks and other mineralized substrates. We sought evidence that many common concrete structures harbor endolithic microbial communities and that these communities vary widely depending on the conditions imposed by the concrete. In Summer 2022, we obtained samples from various concrete structures found throughout Lubbock, Texas, USA and subjected the internal (non-surface) portions of each sample to controlled microbial life detection tests including culture tests, DNA quantifications, DNA amplification tests, and ATP assays. The great preponderance of positive life detection results from our concrete samples suggests that most modern concrete hosts cryptic endolith communities composed of bacteria, sometimes co-occurring with fungi and/or archaea. Moreover, many of these microbes are viable, culturable, and identifiable via genetic analysis. Endolith signatures varied widely across concrete samples; some samples only yielded trace evidence of possibly dormant microbes while other samples contained much more microbial biomass and diversity, on par with some low-biomass soils. Pre-cast masonry units and fragments of poured concrete found underwater generally had the most endolith signatures, suggesting that concrete forms and environmental positioning affect endolithy. Endolith biosignatures were generally greater in less dense and less alkaline concrete samples. So, concrete endolith communities may be as ubiquitous and diverse as the concrete structures they inhabit. We propose further research of concrete endoliths to help clarify the role of modern concrete in our rapidly urbanizing biosphere.