Endolithic microbes may alter the carbon profile of concrete

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.