The fungus growing in the soil outside your front door is already triggering rainfall above your city, and until March 2026, every scientific model calculating how rain forms had left it out entirely.
Findings published in Science Advances in March 2026 record proteins inside common soil fungi that force water droplets to freeze at minus 5.6 degrees Celsius, the same temperature at which aircraft spray silver iodide crystals to artificially seed clouds and produce rain.
Rain does not simply fall. Every raindrop needs a trigger. Pure water sitting in a cloud will not freeze on its own until it reaches minus 46 degrees Celsius, which is far colder than most clouds ever get. Something solid has to give water molecules a surface to grab onto and crystallise against, the same way ice forms faster on a cold metal railing than on open air. The warmer the temperature at which that trigger works, the higher up in the atmosphere a cloud releases its water, and the more rain actually reaches the ground below. Bacteria have been known to carry proteins that do this triggering job for decades. Fungi, which live in soil on every continent and push billions of spores into the air every single day, were not thought to work the same way. They do.
The fungi responsible belong to a family called Mortierellaceae. You have almost certainly walked over them. They grow in garden soil, agricultural fields, forest floors, and Arctic tundra on every inhabited continent. They are not rare, not exotic, and not confined to any particular climate. What nobody knew until now is that they produce a protein that drifts out of the fungus, floats freely in water and air, and forces ice to form at temperatures that directly overlap with where clouds produce rain and snow globally.
The way these proteins work is simpler than it sounds. Each protein is shaped like a tiny rigid coil, about one ten-thousandth the width of a single human hair. Along the outside of that coil sit repeating chemical patterns that grab water molecules and force them into the same locked geometric shape that ice has. When three of these coils drift close enough together in water, opposite electrical charges on their surfaces pull them into contact, the same way the north and south poles of two magnets snap together. Three coils joined side by side create a surface large enough to freeze water at minus 7.5 degrees Celsius. Four coils joined together push that up to minus 5.9 degrees Celsius. Five coils locked side by side, the largest grouping recorded, freeze water at minus 5.6 degrees Celsius, matching the performance of the silver iodide that cloud seeding companies charge governments to spray from aircraft.
What makes these fungal proteins genuinely unlike anything previously found is that they work entirely on their own. Bacteria carry similar proteins, but bacterial versions need to be anchored to a cell membrane to work at full strength. The membrane acts like a mounting board, holding hundreds of individual proteins flat and organised so their combined surface is large enough to trigger freezing near zero degrees. Pull the proteins off the membrane and their performance collapses. The fungal proteins need no mounting board. The fungus secretes them directly into its surroundings as loose dissolved molecules. To test this, researchers washed samples of two Mortierellaceae species, Mortierella alpina and Entomortierella parvispora, in pure water and pushed that water through increasingly fine filters. The finest filter had holes small enough to catch and remove whole cells, cell fragments, and membrane pieces. Ice-triggering activity came through every filter completely intact. The proteins were alone, unattached, and still working.
Their toughness goes well beyond that. These proteins stay active after being soaked in liquids as acidic as stomach acid and as alkaline as drain cleaner. Most proteins fall apart long before reaching either extreme. The fungal ice proteins held their function across that entire range. They also survive being frozen and thawed repeatedly and remain active even when diluted to concentrations so low they can only be detected with specialist laboratory equipment. The reason is structural. Each protein chain contains eight chemical bonds formed between sulphur atoms at specific points along its length. Those bonds act like internal clamps, holding the protein’s shape rigid even under conditions that would destroy most biological molecules. Bacterial ice-nucleating proteins have no equivalent clamping system, which is part of why they depend so heavily on the membrane for support.
The DNA story behind these proteins is the part that took researchers by surprise. A specific gene called InaZ sits inside bacteria of the genus Pseudomonas, and it is responsible for producing their ice-nucleating protein. Pseudomonas syringae, one of the most studied strains, uses this protein to freeze water inside crop plant cells, rupturing them so the bacteria can feed on what spills out. It is the gene responsible for frost damage on farms worldwide. When researchers sequenced the complete genetic blueprints of Mortierella alpina and Entomortierella parvispora, they found near-identical copies of that same bacterial gene sitting inside the fungal DNA. The fungal copies match roughly 58 percent of the bacterial gene’s sequence overall and nearly 100 percent of the specific section that handles ice binding. Tracing the evolutionary history of those gene sequences placed all 15 fungal variants found across the Mortierellaceae family at a single branching point shared with Pseudomonas mandelii and closely related bacteria. The chemical makeup of the fungal gene copies also sits closer to bacterial norms than to typical fungal genes, a signature that lingers for millions of years after a gene moves between unrelated organisms. At some point in ancient history, an ancestor of today’s Mortierellaceae fungi picked up a working bacterial InaZ gene directly, not through normal reproduction but through a process called horizontal gene transfer, where one organism absorbs functional DNA from a completely unrelated species. The fungi kept it, modified it over millions of years, and eventually produced a version that works without the membrane the bacteria require.
Every atmospheric model currently used to estimate how much biological material seeds ice formation in clouds treats fungi as contributors of large, relatively inefficient particles that only work at temperatures colder than minus 8 degrees Celsius. The Mortierellaceae proteins work at minus 5.6 to minus 7.5 degrees Celsius, inside the temperature band those same models already credit to bacteria, and they travel through the atmosphere as tiny dissolved molecules small enough to pass through a filter that blocks whole cells. Field measurements above agricultural land in Nottingham previously logged tens of thousands of fungal spore-forming units per cubic metre of air during peak seasons, and those counts tracked only intact spores, not the far smaller dissolved proteins. The total amount of Mortierellaceae ice-triggering protein currently reaching cloud-forming altitudes above any region on Earth has never been measured. Three species carry confirmed proteins: Mortierella alpina, Entomortierella parvispora, and Podila clonocystis. Twelve additional Mortierellaceae species beyond those three carry the same transferred bacterial gene in their sequenced genomes, placing the confirmed producers at a fraction of the likely total within this single fungal family.
Source:
A previously unrecognized class of fungal ice-nucleating proteins with bacterial ancestry, Science Advances, March 2026. https://www.science.org/doi/10.1126/sciadv.aed9652
