Of Bacteria and clouds: when microbial substances trigger cloud formation in Earth’s atmosphere

This presentation reports the discovery that substances produced by microorganisms might trigger the formation of cloud in the atmosphere, at least under certain conditions.The Cloud Condensation Nuclei (CCN) efficiency of substances produced by microorganisms (bacteria, fungi, micro-algae …) that are common at Earth’s surface and in the oceans were studied. Their Kohler curves were determined experimentally by surface tension and osmometry measurements and found to have much lower critical supersaturations than any material studied so far, including inorganic salts.The presence of these substances was evidenced in aerosols from four different origins (coastal, marine, temperate forest, and Amazonian forest) by LC/MS/MS analyses and by their unique signature on the surface tension. These substances lowered the surface tension of the aerosols below 40 mN/m, allowing them to be activated into cloud droplets before inorganic particles.Microorganisms would thus be able to control cloud formation in Earth’s atmosphere under certain conditions. This would explain many previous observations such as correlations between algae bloom and cloud cover. Most importantly, this work identifies a potentially important component of Earth’s hydrological cycle and a new direct link between biosphere and climate.