Let’s start off by understanding what ice nucleation is. Thermodynamically, nucleation is the process of formation of a new distinct phase containing an ensemble of nuclei/atoms that can irreversibly grow into larger sized nucleus staying as a part of the body of an existing metastable parent phase. So, ice can also grow in a similar fashion following two primary pathways:
- Homogeneous freezing of supercooled cloud droplets or solution particles at around −37 °C or below
- Heterogeneous nucleation triggered by ice-nucleating particles (INPs) like aerosols/bacterial proteins like InaZ, etc.
Bioprecipitation is a hypothesized concept of making rain/snow by bacteria proposed by David Sands from Montana State University in 1982. It is a kind of feedback cycle in which land plants generate small airborne particles, known as, aerosols, that contain plant-associated microorganisms which can influence the formation and evolution of clouds. This is a mechanism of ice nucleation as we know from the above.
Such mechanisms lead to the formation of ice in clouds which is required for snow and rainfall. Precipitation, in turn, can have a beneficial effect on plant and microbial growth. Despite the fact that dust and soot particles can also serve as ice nuclei, they cannot catalyze freezing of water at such warmer temperatures as biological ice nuclei. Recently, it has also been observed that such ice-nucleating bacteria may be present right up in the clouds as part of an evolved process of dispersal. These bacteria express ice-nucleating proteins that are used for snowmaking.
- Morris, C.E., Conen, F., Alex Huffman, J., Phillips, V., Pöschl, U. and Sands, D.C. (2014), Bioprecipitation: a feedback cycle linking Earth history, ecosystem dynamics and land use through biological ice nucleators in the atmosphere. Glob Change Biol, 20: 341-351. https://doi.org/10.1111/gcb.12447