Mushrooms Can Make It Rain: Study

They taste great in soup and can get you high if you eat the wrong ones. Now, scientists have discovered an unexpected function the humble mushroom—their spores can play a role in stimulating rainfall.

A study published in the journal PLOS One on Wednesday detailed how scientists revealed a previously unknown system, wherein rain promotes mushroom growth and the release of mushroom spores—the fungal equivalent of sperm cells that allow mushrooms to reproduce—that can promote rainfall in humid environments.

After it is released into the air, a mushroom spore acts as a nucleus around which water vapor in the air coalesces and condenses into droplets. When enough of these droplets are formed, they stick together to form clouds and eventually result in rain.

Professor Nicholas Money, an expert in fungal biology at at Miami University in Ohio and a co-author of the paper, says that this mechanism could play a role in regulating precipitation in rainforest environments. "If you look at satellite photographs of rainforest canopies, you see that lots of small clouds form in the afternoon above the tree canopy, and these are locations where you've got very high densities of mushrooms and they're releasing lots of spores," says Money. "We're not saying this is the only way in which clouds form but this could be a stimulus for cloud formation, which is a rather amazing link."

According to the study, fungi including mushrooms release millions of tons of tiny spores into the atmosphere each year. Basidiomycete fungi—a group of around 30,000 described species that includes mushrooms—can release up to 30,000 spores per second.

In this type of mushroom, spores are catapulted from under the cap of the mushroom into the air via a mechanism known as Buller's drop. The mushrooms secrete a small amount of sugar, which attracts surface water and forms a large droplet at the base of the spore. Eventually, this droplet is released, and the force results in the spore being ejected into the air at a speed of 1.8 m/s. Spore dispersal is a similar mechanism to pollination in flowers and aims at increasing the mushroom's chances of reproducing.

By observing the spores using an electron microscope, the researchers found that a single spore functioned as a nucleus around which water vapor coalesced and formed droplets. While the same effect is observable in other kinds of airborne particulate matter—such as pollen released by flowers—the effect was particularly pronounced in the mushroom spores.

The system is an example of a positive feedback loop, whereby rain promotes mushroom growth, which in turn promotes the release of spores and that produces more rain. The researchers said that the rainmaking properties would be most noticeable in tropical habitats that support large populations of mushrooms, and that the system could function in as many as 16,000 species of mushrooms.

The mechanism works only in very humid environments—the equivalent of a bathroom after taking a shower, Money says. However, he points out that mushrooms could be stimulating rainfall in temperate regions such as the U.K., where humidity can be high in cloudy areas. "If you look... above the British countryside, there's very, very high concentrations of water vapor," says Money. "There's no reason in principle that this [mechanism] couldn't be operating in other areas."