Fungal spores: the root of rain?

Posted on September 9, 2015   by Benjamin Thompson

There's a branch of chaos theory suggesting that the minute air movements caused by a butterfly flapping its wings could ultimately cause a distant hurricane. Yesterday, at the International Meeting on The Invasive Fungus, Professor Nik Money, from Miami University, USA, described another tiny force that might have a huge effect.

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Rather than being a mathematical metaphor, Nik is interested in a biological process that could be driving real weather patterns right now. He’s investigating how fungal spores – all fractions of a millimetre in diameter – might be responsible for rainfall hundreds of metres up in the air.

If asked to describe a fungus, many of us would first think of the species Agaricus bisporus, the edible mushroom that we’re used to cooking with. This species and others like it are members of the basidiomycete group of fungi, of which the mushroom portion is just a factory that grows out of the ground to release the spores into the air.

On the underside of the mushroom’s cap are comb-like structures known as ‘gills’, within which fungal spores are produced. Molecules on the surface of the spores encourage water droplets to condense upon them; when these droplets reach a certain size both they, and the spores they are in contact with, are fired out of the gill at an incredible speed, estimated in some cases to be as high as 1.2 metres per second. Tens of thousands of spores can be released by each mushroom every second.

This ordnance-based process of spore dispersal is known as ‘ballistospore discharge’ and the water droplet on the spore is named a ‘Buller’s drop’, after the eccentric discoverer of the mechanism, A. H. Reginald Buller. Once airborne, the Buller’s drop evaporates, and the spores disperse.

Despite being released only a few centimetres from the ground, there is a body of evidence to suggest the presence of fungal spores at high altitudes. Much of this work has come from researchers in the Biogeography Department at the Max Planck Institute for Chemistry, Germany, who measured high levels of the chemical mannitol in the air above the Amazon Rainforest.

Mannitol is a small molecule found on the surface of fungal spores and was used as a marker to indicate their presence in the atmosphere. Given the amount of mannitol they detected, the researchers estimated that each year, millions of tonnes of fungal spores are released into the skies. Given the size and relative mass of an individual spore, the numbers involved must be vast.

If this number of spores can be found swirling around in the atmosphere, why is the world not covered in Amazonian mushrooms? As Nik explained, “While some spores are capable of travelling long distances, most of them don’t survive very long. The spores that get up into the atmosphere are not necessarily going to be alive when they come down in rain.”

“A single bracket fungus releases trillions of spores; only two need to find a new home to start a fresh colony. The chances of any one spore making it are lower than the chances of you or me winning the lottery.”

Nik and his colleagues, Maribeth Hassett and Mark Fischer, are currently looking to see whether these atmospheric spores are capable of acting as ‘cloud seeds’ – particles upon which water in the air can condensate into clouds, and, ultimately, rain. The group has been studying spores using an Environmental Electron Microscope, which lets them observe in fine detail how they behave in different humidities.

Preliminary results suggest that spores that have been released are capable of reforming Buller’s drops, even after they’ve been released from a mushroom, likely due to the mannitol on the surface of the spores. This effect was seen regardless of whether the spore was alive or dead.

While the group’s research is still in its early stages, Nik thinks that this phenomenon could be an important part of local weather patterns. “We can’t say this is definitely happening,” he explains, “because we’ve not been up there and seen this taking place in the clouds. But we’ve shown, at least in principle, that spores can condense water even after they’ve been released from mushrooms.”

Image: Anita on Flickr under CC BY-NC-ND 2.0