Survival of ice nucleating bacteria in freezing cloud droplets

The widespread plant pathogenic bacteria Pseudomonas Syringae are one the most efficient ice nucleating organisms found in the atmospheric clouds. Various strains of P. Syringae have been identified not only in agricultural regions and on the plant leaves, but also in the samples of cloud water and in fresh snow and rain collected far away from the ecosystems of origin.

Not only P. Syringae survive the transport over several hundred kilometers, they are also able to multiply in the cloud droplets. At low temperature, bacteria initiate the freezing of the supercooled water droplet owing to the ice nucleation active (INA) protein molecules anchored on the outer shell of the cell membrane. As liquid water converts to ice, ice crystals grow fast via water diffusion and droplet coalescence, finally returning to the ground as rain or snow.

How do microorganisms survive the freezing of the cloud droplets? Under what conditions are their chances of survival highest, and which factors play the most important role? We address these questions by freezing microscopic water droplets containing P. Syringae levitated in an electrodynamic trap under realistic cloud conditions and observing the freezing events with a high-speed video camera. The droplets are then extracted from the trap and transferred to a Petri dish containing nutritious media, where the number of surviving bacteria is determined by colony counting. We find that the P. Syringae bacteria have a good chance of survival especially if the freezing of the drops takes a lot of time and the bacteria are able to adapt to the new conditions. At low ambient temperature, the bacteria counteract rapid freezing by initiating ice nucleation at low supercooling, highlighting the role of the INA proteins in the survival mechanism.  By modelling the water flow through the cell wall during freezing numerically, we demonstrate that the permeability of the bacteria cell membrane plays a decisive role in the fight for survival in a freezing environment. Thus, we suggest an explanation of the bacteria survival mechanism based on the thermodynamic model.