Some fungi can affect the weather. And now we know how they affect us.

Some fungi can produce proteins that freeze water, which can reach the atmosphere and cause rain. Now, scientists have discovered the secret of this process – an ancient bacterial gene.

Researchers knew that some bacteria have proteins in their cell membranes that allow them to freeze water at relatively high temperatures of about 23 degrees Fahrenheit (-5 degrees Celsius), a process known as ice nucleation. Certain species of fungi can do this as well, but little was known about how it functions in that kingdom of life.

“We just wanted to know how this works,” said Boris Binatzer, a microbiologist at Virginia Tech and co-author of the new study, published March 11 in the journal Science Advances.

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Vinatzer and colleagues studied the genomes of two fungal strains in the family Mortierellaceae and found their ice nucleation proteins. They had some clues. They knew that the protein was secreted into the environment rather than attached to fungal cells, and they also knew approximately how large the protein was. So they looked for genes that had these characteristics and were similar to known bacterial ice nucleation proteins.

They were surprised to find a candidate that was nearly identical to a bacterial gene called InaZ. When the fungal genes were transferred to yeast cells, the yeast also acquired the ability to make ice.

“We confirmed that that particular DNA fragment actually makes the ice nucleating protein,” he told Live Science.

This suggests that at some point in the past, perhaps millions of years ago, an ancestral fungus acquired a gene from a neighboring bacterium and made it its own through a process known as horizontal gene transfer.

But how fungi use this ice-making ability, and what evolutionary advantage it confers on them, is less clear. “Right now we have no idea,” Binatzer said.

Bacteria with ice nucleation proteins are often those that attack plants, such as Pseudomonas syringae, which infects corn. Scientists believe these bacteria can use ice-forming proteins to damage plants, allowing nutrients to leach out and bacteria to invade.

One of the fungi found in the new study came from lichens, hybrid colonies of fungi and algae that grow on rocks and trees. Vinatzer speculated that the ice nucleating proteins allow the fungus to draw water from the air, providing the lichen with a necessary but scarce resource.

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“In the morning when the humidity is high and the temperature is low, the fungal proteins can cause frost on the lichen, which then melts and provides water later in the day,” he said.

But perhaps the most interesting aspect of these ice-making bacteria and fungi is that they may be able to influence the weather, seeding clouds and causing rain.

Ice-making bacteria like P. syringae are part of the water cycle and are known to play an important role in precipitation. They are drawn into clouds by wind and evaporation, where their ice-nucleating ability produces tiny crystals that eventually become large enough to fall as rain or snow. Ice nucleating proteins secreted by fungi likely undergo a similar process, Binatzer said.

Because a single fungus secretes many proteins, each acting as a separate ice nucleus, there may be more proteins in a cloud than there are bacteria that produce rain. “This suggests that fungi may actually be more important than bacteria in influencing climate,” he said, noting that it could benefit the entire ecosystem, not just fungi on land.

These newly discovered fungal proteins may also be useful to humans, Binatzer suggested. Currently, cloud seeding operations use a toxic chemical called silver iodide to generate ice crystals, but perhaps it could be replaced with a benign organic protein.

“These proteins have the potential to replace toxic silver iodide,” Binatzer said. “If you know how to generate them, why not use that instead?”