Actinomycetes: The White Stuff in Your Hot Compost

When you dig through your blazing hot compost pile and find dense pockets of white filaments, you might either be alarmed or excited.

I love to propagate fungi in my vermicomposting efforts. So I get a little excited when I see what appears to be a healthy fungal population.

These white stringy elements appear in hot compost, especially when that compost has a woody component. These are called actinomycetes, and should be considered a normal part of the hot composting process.

In this article, we’ll go into a little greater depth on actinomycetes, what they do, and if they are harmful in a worm composting environment.

What Are Actinomycetes? 

Actinomycetes and fungi share similar light grey coloring, both propagate via spores, and even share common purpose as effective lignin and cellulose decomposers.

They were even once regarded as fungi due to their fungi-like filamental structure. Even the “-mycete” suffix is derived from “mycelia,” indicating early categorization as a fungus.

But they are actually bacteria.

There are several strains of actinobacteria, but the vast majority found in soil and compost are of the streptomyces variety which, according to the National Institute of Health, is actually the source for over two thirds of all antibiotics.

How Do Actinomycetes Work?

Unlike in medicine, actinomycetes don’t function as a biology killer. In compost and soil, they serve as compost facilitators, nutrient cyclers, and pest suppressors.

Woodier compost ingredients like stems, twigs, and tree bark are attractive targets for decomposition by actinomycetes, which tend to proliferate only in aerobic conditions in the thermophilic and cooling phases of the compost cycle.

Actinomycetes are also more active in substrates with a higher pH.

Functions of Actinobacteria

Composting Woody Material

Actinobacteria thrive in woody conditions. They produce a lignocellulolytic (there’s a 2-dollar word for you) enzyme that effectively breaks down complex compounds like cellulose, chitin, lignin, and protein.

Nutrient Cycling

Actinomycetes are a “nitrogen-fixing” bacteria, capable of breaking the bonds of organic nitrogen atoms. Once those bonds are broken, that nitrogen takes a more plant-usable ammonium or nitrate form.

Aside from converting carbon and nitrogen, they can also convert other organically-bound nutrients like phosphorous.

Pathogen & Pest Protection

Actinomycetes exhibit certain antibiotic properties that can interfere with biochemical processes in targeted microorganisms. They can also produce enzymes and toxins which can prevent pests from flourishing.

Actinobacteria in soil can also help harden a plant’s own defenses. For instance, they can stimulate the plant’s production of protective phytoalexins (there’s another 2-dollar word) to prevent further damage by pathogens.

Are Actinomycetes Harmful in a Worm Composting Bin?

Absolutely not.

Here at the Urban Worm Company, we regularly encountered the stringy white stuff in our aerated static bin hot composting system. Our wood chip-heavy compost mix included a healthy dose of cellulose which is an attractive breeding ground for actinobacteria.

And we had no issues with worms avoiding it once we feed it to our continuous flow through (CFT) vermicomposting system.

While there is no harm in introducing organic material with actinomycetes to your bin, do not expect this complex bacteria – especially in its visible form – to proliferate. A “normal” home worm bin fed a diet of kitchen scraps and paper waste will not feature the higher temperatures and woodier material where actinomycetes would grow.

Summary: Actinomycetes Are Friends Not Foes

While not fungi, that white stuff in your compost is behaving in a fungi-like manner by breaking down woody material and cycling nutrients. It also helps plants ward off nasty pests and pathogens.

We welcome these wood-loving microbes!

Want to Learn More About the Soil Food Web?

Actinomycetes are “characters” in what we know as the soil food web.

Check out what everyone else does in our comprehensive post about what else is under your feet.

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