Comb water balance and fungal symbiosis

A hydrophobic fungus comb

The fungal symbiosis of Termitomyces and Macrotermes is thought to be largely digestive in character: the temites provide the fungi with a rich source of pre-processed wood and the fungi provide the termites with a pre-digested and enriched diet.

Still unresolved, however, is how the symbiosis came to be in the first place. There are two wrinkles that factor into any thinking on this question.

First, there is the problem of the so-called allofungi. A fungus comb contains viable spores of more than 20 species of wood-digesting fungi. Yet, only theTermitomyces spores germinate. The other spores remain viable and will germinate under the right conditions, and when they do so, quickly overwhelm the Termitomyces culture. How do the termites (and the fungi) pull off this trick?

Second, there is the phenomenon of nest humidity regulation. Martin Luscher was thought to have solved this problem through the "air-conditioned termite nest. We now know this model is unsupportable. What, then, is to take its place?

The fungal culture actually plays a surprising role in smoothing both wrinkles, as I show on this page, and here.

The hydrophobic fungus comb

Wetting of the fungus comb

If a drop of water is placed on an inside surface of the fungus comb, water does not wet the comb immediately, but sits as a drop on the fungus comb surface. After about twenty minutes, the drop begins to wet the comb, as seen by the darkening of the comb surface beneath the drop. Even after four hours, though, there is still a recognizable drop present on the surface.

The hydrophobic surface is probably the result of the dense mat of fungi that coat the surface. These are the hyphae of Termitomyces. You can see how dense is the mat from these scanning electron micrographs, kindly provided by Jason Nadler of the Georgia Tech Center for Biologically Inspired Design.

fungal hyphae

The comb is not uniformly hydrophobic. The top margins of the combs, where the termites add new pellets of inoculated slurry, takes up water readily, whereas the flat surface of the comb, where the hyphal growth is most dense, is also the most hydrophobic. In all likelihood, the margin wets readily because fungal growth is not very dense, and water uptake is dominated by the strong matric potential of the wood fiber slurry (the same reason paper towel imbibes water readily. Where the fungal growth is the most dense, the hydrophobic surface of the fungus dominates, and water cannot penetrate to the mesh of wood fibers below. Whatever is causing the hyphae to be hydrophobic appears to break down under a water drop because the comb wets eventually.

Here is a time lapse video of the comb imbibing water either at the margins or the face. The video spans about 45 minutes of time.

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Evaporation and uptake of water vapor by the fungus comb

The comb itself is quite moist: on average, water constitutes 35-40% of the comb's fresh mass.

The comb also has some unusual hydric properties. If a comb is exposed to dry air, it loses water by evaporation. This is very slow, with a time constant of about 45 hours. However, if a comb is exposed to air that is saturated in water vapor, it absorbs water from the air. This is a longer process, with a time constant of about 80 hours.

Fungus comb water vapor fluxes

Thus, the comb is both hydrophobic to liquid water and hygroscopic to water vapor, as long as the local humidity is above about 80%. If local humidity is less than about 80%, the comb evaporates water.

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Comb water balance and fungal ecology

mold growth in wetted comb

Normally, only Termitomyces spores germinate on the fungus comb. There are viable spores from more than 20 different fungal species in the comb, however. Why do they not germinate?

Chemical suppression is unlikely, because these allofungal spores germinate readily when the comb is removed from the colony. This occurs if the comb is wetted. In the figure to the right, we see two pieces of fungus comb. The one on the right has had its structure disrupted so that it readily takes up liquid water from a wet filter paper. The undisrupted comb also wets but much less so than the disrupted comb. Wherever the comb is wetted, there is a rich growth of other types of mold.

Here are some closeup images of some of these allofungi.

allofungal growth

The allofungi in general are wood-digesting fungi related to the damp-rot fungi. These are common on rotting logs in woods. They commonly grow very fast. If they grow in competition with the more slowly growing Termitomyces, they quickly outcompete the Termitomyces and drive them to extinction in the comb. This is a major threat to the termites, because they depend upon the slow growth of the fungal symbiont, which leaves enough partially digested wood in the comb for the termites to eat. Fast growing fungi take up their digestate quickly, depriving the termites of it.

comb water balance

We believe that the growth of the allofungi are suppressed by maintaining a dry environment inside the comb. The hydrophobic nature of the comb material ensures that metabolic water from Termitomyces metabolism can be effectively pushed out of the comb into the air, keeping the comb interior dry. When this fails and the comb accumulates liquid water, this promotes the germination of the allofungi. Thus, the Termitomyces rely on the peculiar structure of the comb, built by termites, to help them keep the local environment dry enough to suppress the growth of its fungal competitors. Win win!

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Termite pages

Termite home

Structure

Endocasting

Social homeostasis

Nest temperature

Water homeostasis 1

Water homeostasis 2

Water homeostasis 3

Fungal symbiosis

Fungal symbiosis and water 1

Fungi and water homeostasis 2

Gas exchange 1

DC vs AC Gas Exchange

Gas exchange 2

Gas exchange 3

Gas exchange 4

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