GGR Newsletter
July 2025
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Cloudy with a chance of microbes!
Ice-forming bacteria take to the skies
Nishant Panicker
July 2025
It has been known for a while that bacteria can float around in the atmosphere, and can “fly” or be transported thousands of miles on dust particles and countless other surfaces. In a recent study on the chemistry of airborne dust particles, researchers sent a plane 10,000 feet into the sky, to discover unanticipatedly diverse microbial growth and viability, including the discovery of infectious microbes. It isn’t surprising then to learn that atmospheric climate processes are not solely governed by larger-scale atmospheric physics, but are also influenced by microbial life. There are certain specialized bacteria that have the ability to kickstart the formation of ice in clouds, one of the processes critical for precipitation. This seemingly small interaction could have profound implications for the water cycle.
Contrary to popular public perception, pure water doesn’t usually freeze at 0˚C. When temperatures reduce, water molecules that are typically moving around in solution begin to lose energy and enter into a crystalline network that we call ice (Figure 1). Pure liquid water can actually supercool to as low as -38˚C prior to a transition from liquid into solid state. For ice formation to occur at temperatures above -20˚C, a solid particle called a nucleator, is required to trigger the initial process that precedes the formation of ice crystals, a process known as nucleation. Water commonly freezes around 0˚C because of the presence of nucleators, typically impurities like mineral dusts and other salts, that raise its freezing point to 0˚C.
Figure 1. An animated simulation of the formation of crystalline ice upon lowering the temperature of water. This animation was created by Dr. Angel Herráez at the University of Alcalá, Spain, is hosted at the hyperlinked website, and is being distributed under a Creative Commons License.
Of the bacterial species that demonstrate the capacity to trigger ice formation, the first identified and most well-studied is Pseudomonas syringae. Their capacity to initiate ice formation allows them to trigger freezing even at surprisingly warm temperatures as high as -2˚C. In comparison, even mineral dust requires a temperature of around -15˚C to seed ice formation. How these ice nucleating bacteria are able to achieve this is another one of those fascinating tricks of biochemistry! It involves a family of specialized proteins that these bacteria have anchored to their outer cell surfaces, called Ice Nucleating Proteins (INPs).
The fundamental principle behind how INPs work is in their interactions with water. INPs actively impose a structural ordering on the network of water molecules, creating an arrangement that significantly lowers the barrier for ice crystal formation (Figure 2). This ordering significantly increases the efficiency of nucleation as the temperature reduces towards the melting point of water. While individual INP molecules are too small to facilitate high efficiency ice nucleation, they are able to self-assemble into larger structures. It is through their collective action that they are able to create the stable conditions for ice nucleation, allowing them to freeze at remarkably high sub-zero temperatures. Aggregation into larger structures allows for the creation of an extensive and stable surface that not only facilitates initial nucleation but also allows the ice crystal to propagate from the protein surface into the surrounding bulk water, overcoming the barrier further ice crystallization and growth to a larger size.
Figure 2. An illustration of the interaction of Ice Nucleating Proteins with water molecules to signify how INPs facilitate ice nucleation at much higher temperatures, directly reproduced from Roeters et al, Nature Communications (2021). (A) At room temperature, the ice nucleating sites are submerged in the protein film, with relatively low order between the water molecules. (B) As the temperature reduces, the INP reorients itself and the active site of the protein is exposed to the interfacial water layer, increasing the order between the INPs and the water molecules, and hence promoting ice nucleation. (C) This schematic shows a proposed model for the orientation of the INPs at the bacterial surface, with the long protein axis being parallel to the surface of the bacterial cell. This orientation increases the size of the ice nucleation site and hence maximizes the ice nucleation activity.
Once airborne, carried by wind from plants and soil, ice nucleating bacteria serve as highly efficient surfaces for crystallization and condensation. They provide an essential template around which water molecules in the clouds can aggregate and form ice crystals. This has a direct ecological impact as it influences the water cycle, affecting precipitation through snow and rain. Rainfall itself is known to stimulate the release of these bacteria from plants and soil into the air, creating a fascinating feedback loop. The process by which microbes act as condensation nuclei to trigger snow or rainfall is known as bioprecipitation. There are several studies that provide evidence for the occurrence of bioprecipitation.
In one study conducted in 2008, researchers found that the abundance and distribution of ice nucleating proteins in snow and rain samples was directly correlated with the amount of precipitation and the season of deposition. Another 2014 study found that the core of hailstones from three different storm sites near the Rocky Mountains contained bacterial ice nuclei that were capable of freezing water at warm subzero temperatures. It is furthermore interesting to know that many of the bacteria that demonstrate the capacity for ice nucleation are plant pathogens. As plant pathogens, these bacteria cause frost injury to the plant tissue by increasing the nucleation temperature of water, enabling the bacteria to access essential nutrients. Some species of bacteria like P. syringae have such a high ice nucleation efficiency, that they are even used in commercial products.
Have you ever wondered how ski resorts are able to maintain their snow cover even without considerable snow fall in a given season? A commercial product called Snomax (Figure 3) is composed of the INP derived from P. syringae, which serve as templates for high-efficiency, relatively high temperature ice nucleation. The ice nucleating proteins are first extracted from the outer cell surface of P. syringae and are then concentrated, freeze-dried and sterilized. The enhanced ice crystallization happens in proximity to the nozzle of “snow guns”, allowing the snowmaking process to use almost all of the available water molecules for ice nucleation. The utility of these INPs considerably increases the volume of snow produced at relatively high temperatures, with less energy and water.
Figure 3. A Snomax snow-making machine.
Despite their role in ice nucleation, it is unclear how significantly these bacteria contribute to global atmospheric processes. The aspects of their contribution that remain unclear are their overall abundance and their capacity to directly trigger widespread precipitation. However, the existence of these microbes and the mechanisms by which they could impact the water cycle serve as more evidence of the often unseen and largely unknown ways our microscopic world reveals its influence over larger macroscopic phenomena that shape our environment.
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