All About Actinomycetes (Excerpt: “Microbe Science For Gardeners”)

In this excerpt from "Microbe Science for Gardeners," author Robert Pavlis digs into why soil smells good and if dirt really helps depression.

by Hobby Farms HQ

The following excerpt is from Microbe Science for Gardeners (New Society Publishers, September 2023) by Robert Pavlis.

Microbe Science for Gardeners book cover
courtesy of New Society Publishers

Actinomycetes are called filamentous bacteria, or mold bacteria. They look and grow like fungi but are biologically similar to bacteria. They grow hyphae-like threads that consume resistant organic matter, and they are tolerant of dry soil, alkaline soil, and high-temperature conditions. They produce antibiotics such as streptomycin and actinomycin that stop the growth of other microbes. Some of these are available as commercial drugs.

Actinomycetes tend to be found in decaying organic matter.
They can protect plant roots from disease, and in a few cases they cause diseases such as potato scab. Their affinity for higher temperatures and their ability to decompose tough organic matter makes them an important contributor in hot composting. These organisms are responsible for the earthy smell of damp, well-aerated soil.

Frankia are special actinomycetes that form symbiotic nitrogen-
fixing nodules on over two hundred species of nonleguminous plants. Most are trees and shrubs such as alders, sea buckthorn and Casuarina species, many of which are pioneer species that grow in very poor soil.

Why Does Soil Smell So Good?

The earthy smell that rises after a summer rain is so pleasant that we use it to scent perfumes. One of the compounds responsible for this fragrance is geosmin, and its biology in soil is very interesting.

Geosmin is made mostly by an actinomycetes called Streptomyces,
which makes a host of other compounds including the well known
antibiotic streptomycin. Scientists have been wondering for quite some time why this organism makes geosmin, and they now have
some insight into this.

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Streptomyces grow mycelium threads through the pores in soil.
When they run out of nutrients, it is time to move on, and they
form spores. These spores spread by wind and water. As part of the process of producing spores they also produce geosmin, which attracts springtails.

Springtails are tiny critters (hexapods) that get their name from their ability to hop around like fleas, and they have a particular attraction for geosmin. As the springtails rummage through soil looking for geosmin, the Streptomyces spores get attached to the springtails who then spread them throughout the soil. Springtails are jumping Ubers for actinomycetes.

Microbe Science for Gardeners
courtesy of New Society Publishers

Microbe Myth: Soil Is an Antidepressant That Makes You Feel Good

You probably saw the error-riddled meme that says, “Soil is an antidepressant. The smell of mycobacterium vacii, a microorganism found in soil, compost and leaf mold, lights up neurotransmitters that release serotonin, a mood-lifting hormone.”

Gardeners were quick to accept these facts, but I had a close look at the science behind this. First of all, there is no such organism; however, Mycobacterium vaccae does exist.

Serotonin has been well studied and is known as the “feel-good hormone.” It plays a key role in staving off anxiety and depression. So, if the smell from microbes in soil causes higher levels of serotonin, it is quite likely that these smells make us feel good. However, the pleasant smell of soil is due to geosmin which is made by Streptomyces bacteria, not Mycobacterium vaccae. It is also not linked to serotonin levels.

There is no scientific evidence that exposure to Mycobacterium vaccae or geosmin changes our serotonin levels. As with most gardening memes, there is a smattering of truth, but most of the message is wrong. Don’t believe gardening memes.

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