Fungal symbiosis and water homeostasis

water imbibing fungus comb

The fungal culture of Macrotermes may be tied up with one aspect of the "air conditioned termite nest" idea, namely regulation of nest humidity. In Martin Luscher's conception, excess humidity in the nest is vented away by the metabolism driven circulation of air between the nest and porous mound surface.

It truly is essential that nest humidity be regulated (see the 'Fungi and Water 1' link to the left). And compared to temperature, which is not regulated at all, nest humidity is tightly regulated. As it turns out, the mound structure or mechanisms of gas exchange have nothing to do with it.

The comb as a humidity "sponge"

a humidity sponge

How a fungus comb can act as a humidity sponge.

A sponge is a reversible storage device for liquid water. Depending upon the hydration state of the sponge, water can move across the boundary between the sponge and environment. If the sponge is dryer than the outside, water is drawn across the boundary into the sponge, which holds it. If the environment is dryer than the sponge, water will cross the boundary in the other direction.

A typical sponge moves liquid water. Sponges can move water in other forms as well. What matters is a difference in the so-called water potential. Water potential is a measure of the capacity of water to do work (it has units of joules per cubic meter). If there is a difference in water potential between two points, water will move down the wate potential gradient. Water potential often involves liquid water but it can also involve vapor phase water: humidity can be converted to a water potential with a fairly simply calculation.

The tendency of the fungus comb to maintain an internal humidity of around 80% makes it ideally suited to be a humidity sponge. When humidity of the air exceeds 80%, vapor phase water moves into the fungus comb, where the water potential is less. When local humiidity falls below 80%, water will now flow from the liquid phase in the fungus comb to the vapor phase in the air.

The consequence will be a damping of local humidity. The fungus comb is acting as a humidity sponge.

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Capacity of humidity damping in the colony

The comb's hydric properties may enable it to act as a humidity sponde, but will this make any difference in the umidity balance of the colony? Whether it does so depends upon the capacity of the fungus combs in aggregate within the colony

A typical Macroterem colony houses from 25-40 kg dry mass of fungus combs. Because the fresh comb is about 40% water, this means the colony's fungus garden holds in it 20-30 kg of liquid water. So there is ample capacity for the fungus garden to be a water store.

fungus comb in gallery

To be an effective humidity damper, the fungus garden should be able to exchange water with the air at sufficiently high rates. There are three factors that are significant:

  • The comb should have significant capacity for water storage. That seems established.
  • The fact that the fungus comb is confined in a gallery. This means that each fungus comb has to damp humidity in only a small volume.
  • The surface area for exchange. Flux of vapor across the boundary depends upon the difference of water potential, and how large a surface area there is for exchange.

The first two points seem reasonable. THe hard point is the last: just how large is the surface area for exchange?

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How to measure the exchange capacity of the fungus garden

The fungus combfluorescein comb

The fungus comb has an extraordinarily complex shape. Its basic morphology consists of vertical sheets, roughly 2 mm in thickness, folded into the compact comb. The comb is permeated with vertical air spaces. You can see this by taking a cross section right through the comb (bottom).

To quantify these properties, we need to measure the comb's volume and surface area. Volume is no problem, that can be estimated by a simple water displacement technique.Measuring the comb's surface area presents a challenge. You can see how we solved it in this video.

It seems these will readily support air flow through them. We can visualize this by dipping the fungus comb into fluorescein dye, and then putting the fungus comb back into water. The dye streams represent the kinds of natural convection flows that can be expected in the gallery.

Here is the cross section through a comb. Note the thin walls separated by air space.

 

cross section

Below is an animation of a complete slice through of the comb on the left (done with Kate Fuller). Use the slider to scroll through the comb.

 

 

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The fungus garden has substantial humidity damping capacity

The measurements of fungus comb surface area reveals a quite remarkable characteristic: the fungus comb is structured for capacity.

The basic problem in any physiological system is how to match exchange capacity, which usually depends upon surface area, which exchange demand, which is usually dependent upon volume. In most solids, surface area scales to the 2/3 power of volume. This means that as size increases, demand invariably outstrips capacity to meet that demand. In 3D animals, this is circumvented by having highly folded exchange surfaces, like lungs.

In the fungus comb, the highly folded sheet morphology means that scaling problem does not exist: capacity for exchange tracks demand for exchange one-to-one.

scaling of sa to v

This makes the estimation of aggregate comb surface area much easier. From some simple calculations (outlined in the video above), we estimate that the fungus garden has, in aggregate, about 40 square meters of exchange surface. The human lung has about 160 square meters.

It seems therefore that the fungus garden does have significant capacity to damp nest humidity.

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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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