From Cropland to Cities: Aerial Transport of Fungi

Fungus. When you first hear the word you may think of mushrooms living on a rotting log or in the red sauce on top of your pasta. But most fungi don’t live above ground. They live in the soil and some fungi even “make friends” with plant roots. We call this type of fungi, mycorrhizae. Within this group, there is a particular type of fungus called Arbuscular mycorrhizae which is a fungus that penetrates plant roots to form an intimate association. The fungus spreads its “arms and legs” (called hyphae) and moves through the soil in search of hard-to-reach nutrients. The mycorrhizae then shares some of those nutrients with the plant it attaches to and in return the plant gives the fungus sugar produced through photosynthesis. It’s mutual trade that makes both organisms happy!


This is what we typically think of when we think of mushrooms but there is so much more going on below the soil surface.
https://commons.wikimedia.org/wiki/File:Armillaria_mellea,_Honey_Fungus,_UK_1.jpg

Many plants rely on intimate associations with fungi, yet we don’t fully understand how fungi colonizes the soil in the first place! This leaves a huge knowledge gap that, if filled, could prove extremely useful. For example, imagine you have a rooftop garden. You have a box full of soil you bought from the hardware store with no fungus added. The box is full of vegetables that one day you hope to feast upon! Those vegetables may benefit from mycorrhizae but if they weren’t present in the soil to begin with.

So, how do the fungi get to the rooftop?

To figure that out, researchers from DePaul University set up a year long experiment on top of a building in Chicago. They wanted to know if there were spores, which are like fungal seeds, floating around in the air and landing on the soil to form associations with plants. Just like plant seeds, the spores of fungi are sometimes blown in the wind and eventually land in a new area that they can colonize. Using a special filter, the researchers were able to capture microscopic particles that blew in on the wind. Once a month for a year, the researchers would collect the filter and determine what fungal spores they had captured.


This is an image of the rooftop garden on top of Chicago City Hall. How could fungi get all the way up there?
https://commons.wikimedia.org/wiki/File:Chicago_City_Hall_rooftop_garden.jpg

In one year the researchers captured nearly 50,000 fungal spores!

Most of the spores were small, about as thick as a piece of paper. These fungi were detected in the filters during every month of the year with the most fungi captured in August and the least in March. The researchers also found many different species of fungi based on microscopic identification and DNA sequencing. Interestingly, the abundance of spores was not correlated with wind speed, temperature, or soil moisture, which were all predicted to impact how microbes move through the air. In fact, spore abundance decreased with higher wind speeds which seems counter-intuitive. This could be due to the seasonal timing of spore production. If spores aren’t produced during the time of the year with the highest winds, it could explain why spore abundance was low when winds were high. Local soil disturbance could have also affected the abundance of spores. When nearby farms till the fields, they kick up a lot of soil which could allow the spores to move from the soil to the air. Corn and soybean harvest in the region corresponds with the high August to November spore count.


Harvesting a corn field in the fall. This may be one of the sources of liberation of fungal spores. https://commons.wikimedia.org/wiki/File:Corn_Harvest_(9625542844).jpg

This was the first study that documented the types of fungi with aerial dispersal and described what they looked like. Fungi that travel through the air have unique characteristics.  In general, small and smooth spores are more likely to travel to new sites via air than large spores with lots of decoration on outside. Based on this study, the author’s developed a conceptual model describing how fungi move from the soil to the air and to a new patch of soil. The first step is liberation. The spores must somehow be released from the deeper layers of the soil. This may be through human soil disturbance such as farming, recreation, or mining or natural disturbances such as storms and rainfall. Second, the spores must have some means of transport either by wind or animal movement. And finally, the spores must find a good spot to land and form and make friends with the local plants.

In summary, fungi may be passively reestablishing urban areas due to local soil disturbance. The fungi that are able to passively move through the air have a unique set of traits and are primarily small and un-decorated. Understanding the passive reestablishment of fungi, particularly in urban areas could be beneficial for urban ecological restoration efforts. Plants that live with the beneficial fungi grow better. Knowing that these beneficial fungi can move from the cropland to the cities may help the urban jungle grow. As cities move towards green infrastructure, like green roofs and urban farms, promoting the passive transport of these fungi and other beneficial microbes will help these projects succeed and bring a bit more green to the concrete metropolis.

Source:

Chaudhary, V.B., Nolimal, S., Sosa‐Hernández, M.A., Egan, C. and Kastens, J. (2020), Trait‐based aerial dispersal of arbuscular mycorrhizal fungi. New Phytol. doi:10.1111/nph.16667

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