Critical role of agar in bacterial cloud activation

Science

Ice nucleating particles (INPs) are relatively rare atmospheric particles that affect cloud properties and precipitation. Biological INPs, including bacteria and their fragments, have been of great interest due to the special ability of some to form ice crystals even at modestly cold temperatures. U.S. Department of Energy scientists and their international colleagues investigated the ability of two strains of cultivated bacteria and their fragments to nucleate cloud droplets and ice crystals. They found that smaller particles composed of bacterial fragments mixed with agar growth media activated as cloud droplets, which freeze to form ice crystals. In contrast, larger intact bacteria cells did not activate as cloud droplets and thus could not initiate ice nucleation by droplet freezing.

Impact

Most laboratory studies on how bacteria affect cloud formation use cultivated bacteria to make measurements that are then extrapolated to atmospheric conditions. This work clearly indicates that the unavoidable presence of agar or other growth media in cultivated bacteria samples can make particles composed of bacterial fragments mixed with growth media into efficient cloud nuclei. This effect needs to be considered when interpreting laboratory data that will be used in atmospheric climate models.

Summary

Laboratory studies that aim to elucidate the ability of bacteria to form ice clouds are designed to generate parameters for atmospheric models. These experiments are conducted on aerosolized solutions of cultivated bacterial cells that inevitably yield two particles types—a small fraction of intact or whole bacterial cells and a large number of smaller particles that are composed of bacterial fragments mixed with agar. The results of this study show that agar plays a critical role in the cloud activation of bacteria and bacterial fragments, an effect that must be considered when relating laboratory measurements to ice formation in atmospheric clouds.

After inducing cloud formation, cloud droplets and ice crystals were separated using a special pumped counterflow virtual impactor inlet. The size and composition of cloud droplets and ice crystal residuals were then characterized using a state-of-the-art, single-particle mass spectrometer, miniSPLAT, and compared to those of the aerosol particles in the cloud chamber before and after cloud formation. This approach makes it possible to identify particles that activated as cloud droplets and ice crystals. The results of this study clearly indicate that the size distributions and mass spectra of cloud droplet and ice crystal residuals closely matched those of bacterial fragments mixed with agar. In contrast, the larger intact bacteria that contain very small amounts of agar and are therefore less hygroscopic were not observed in cloud droplet and ice crystal residuals. This study demonstrates the importance of agar in transforming bacterial fragments into effective cloud nuclei by enhancing their hygroscopicity. It also implies that drop freezing and other bulk immersion freezing techniques bypass the issue of limited hygroscopicity by submerging intact bacteria in a droplet or well of water, thereby artificially overcoming the barrier to droplet activation.