I was 12 years old when I first learned to ski. Our family was visiting relatives in Washington for Christmas, and after a good snow the previous day, my sisters, cousins, and I headed for the slopes. None of us from Oklahoma had ever been skiing before, but our cousins were gracious hosts and good teachers. We not only learned how to ski that day but also had such great fun and worked so hard at it that we devoured my aunt’s unending stew before surrendering to sleep and the next day’s soreness, stiffness, and stories. Ever since then I have loved the snow. I am captivated with the beauty of snow as it blankets everything, burying the bleak grays of asphalt and urbanization. Snow also reminds me of the Scripture in Isaiah that invites us to reason together that though our sins are like scarlet, God will make us whiter than snow.
Microbes Critically Contribute to Earth’s Water Cycle
Water doesn’t freeze at 0°C (32°F). It actually has to be much colder, -48°C, for pure water to freeze! Thankfully, it doesn’t have to be that cold for it to snow. Earth’s atmosphere is full of microcontaminants that can serve as ice nucleators, allowing water to crystalize (a necessary first step for precipitation) at warmer temperatures. As it turns out, microbes play a critical role in ice nucleation, too.
New research reported in Science Advances shows how an ice-nucleating protein on the outer surface of the bacterium Pseudomonas syringae (inaZ) imposes structural ordering of adjacent water molecules resulting in biogenic ice nucleation. The hydrophilic-hydrophobic-hydrophilic composition and structure of the amino acids in inaZ seemingly allow for the hydrophilic outer regions to drive the ordering of the water molecules for ice nucleation. In between the outer regions, the hydrophobic threonine ladder motif enables clathrate water to decouple from the bulk water, creating a water-hydrophobic interface that mimics the water structure at a water-vapor interface. Water-vapor interfaces have been shown in theoretical and experimental studies to enhance ice nucleation. In short, inaZ helps create snow and rain.1
This type of alternating water structuring by a hydrophilic-hydrophobic-hydrophilic repeat may account for the exceptional ice-nucleating ability of inaZ. The study also shows that energy (heat) transfer is particularly efficient at the P. syringae-water interface compared to controls. This data indicates that the two conditions necessary for ice nucleation (the alignment of water into an ordered structure, and the effective removal of heat due to phase transition) are both met by P. syringae. As a result, ice nucleation occurs at temperatures as warm as -2°C in the presence of P. syringae.
Microbes Are Exceptionally Good Ice Nucleators
This research highlights that we’re only beginning to understand the underlying mechanisms between P. syringae and ice nucleation. But the phenomenon of P. syringae’s effect on ice nucleation dates back to the 1970s, when scientists were first learning that these bacteria are better than other ice nucleators at achieving ice nucleation at warmer temperatures. Scientists have also known for several decades that other primary biological aerosol particles, as well as soot and dust, contribute to ice nucleation and cloud condensation nucleation.2 Numerous studies indicate that mineral dust particles are relatively efficient ice nuclei, but that at lower temperatures, soot particles can also nucleate ice. Interestingly, however, the most active at the highest subzero temperatures are those of biological origin, including pollen, fungal and bacterial spores, bacteria, and viruses.
Historically, the techniques scientists used to identify primary biological particles—techniques such as culture assays and light microscopy—failed to detect many viruses or proteins. These techniques only detected certain viable microbes and large microscopic entities (>2 µm) such as bacteria, fungal and bacterial spores, and pollen. With today’s metagenomic analyses of the environment, we know there are far more bacteria and viruses in our surrounding environment than we ever imagined (104 viruses/m3 of air). So it is extremely likely that thousands or tens of thousands more bacterial proteins like P. syringae’s inaZ exist and serve as excellent facilitators of the earth’s water cycle, sustaining life on earth and quite possibly providing repositories for human discovery, use, and flourishing. Since viruses (1031) outnumber bacteria (1030) at the estimated rate of 10:1, it’s quite likely virus particles or proteins are contributing to cloud condensation nucleation, too.
Why We Should Thank God for Microbes
P. syringae, a plant pathogen contributing to frost damage of crops and other plants, might be argued by some as an example of a harsh nature incompatible with a good, all-powerful, loving God. But if we consider its important role in ice nucleation and the precipitation cycle (creating snow, sleet, and rain), we can see a reason to believe that a good God has created a complex ecosystem for human survival, adaptation, and flourishing that includes organisms like P. syringae. Not only that, but P. syringae assists in the production of artificial snow, and its ice-nucleating proteins can be used for or mimicked in biomimetic materials for controlled interfacial freezing. In God’s providence, P. syringae is also a great resource for human flourishing.
So if you enjoy the snow as much as I do, or if you’re someone who is thrilled that artificial snow production allows you to ski more often and longer than the unaided ski season would allow, you might add bacteria like P. syringae to your prayers of thanksgiving.