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Who digests the wood?

The voracious and taxonomically isolated termite Mas-totermes darwiniensis lives in underground nests that may house more than a million individuals. They have been called "white ants," although they are not true ants. Termites eat anything that contains carbon polymers, thus destroying trees and wood, but they also consume keratinous materials, tires, and even the coverings of electrical cables. In the geological past they have been present throughout the planet, but now they are restricted to the Darwin region of northern Australia, outside the savannah area. These termites and their ancestors are thought to have been eating old dead wood for at least 400 million years, since the Devonian, when Earth's forests consisted of tree lycopods and giant seed ferns. But neither this species nor any of the termites with more restricted diets is in fact able to digest ingested wood. So who digests it?

If it were not for the termites, dead wood, which resists degradation by weathering and water, would accumulate in savannahs and forests in large quantities. It is very probable that the first termites to evolve were wood-eaters. They were arthropods (order Isoptera) and appear to have been related to some social insects like the wood-eating cockroaches (order Blattaria). There are now only a few species of wood-eating cockroach (for example, Cryptocercus punctatus, in the Allegheny area of the southern Appalachians), but about 2,000 species of about 210 genera of wood-eating termites occur in savannahs, tropical rainforests, grasslands, and deserts. These species, except for Mastotermes, are known as "lower termites" because they were the first termites to evolve from cockroaches. They may be subterranean termites (family Rhynotermitidae) or aboveground forms that feed on damp wood (family Hodotermitidae) or dry wood (family Kalotermitidae).

Termites eat wood and excrete sandy fecal pellets. But no animal can digest wood. All wood-eating termites and roaches have large paunches harboring incredibly complex microbial communities that digest the wood. Highly motile protists and bacteria fill expansions of the hind end of the termite digestive tract (the section called stomachs, intestines, or hindguts). These are enormously swollen structures, and their characteristics influence the body size, shape, and color of the termite.

On the inner surfaces of the termite's muscular intestinal walls are epithelial cells connected to each other. The swollen intestine, lined with chitin, forms an active and thriving tube housing hundreds of different species of anaerobic motile zoomastigiinids. Each species of termite has its own range of protists, and they in fact digest the wood. They advance by flexing their sensory undulipodia, but, like garbage trucks, they take in the suitable wood fragments through the hind end. Some of these protists produce enzymes that break down the cellulose in wood into sugars such as cellobiose or even smaller carbon compounds such as acetate. It is known that at least four genera of termite symbionts--a Trychonympha hypermastigote, a Coronympha calonymphid, a Tricercomitus trichomonad, and a Tricercomitopsis trichomonad--can live in isolated cultures on purified cellulose fibers.

Bacteria are also very abundant in the rear digestive system of wood-eating termites, but no cellulose-digesting bacteria have been isolated from them. They surely fulfill other purposes; methane-producing and nitrogen-fixing bacteria have been isolated from termite intestines. Their tasks might include removing the hydrogen and carbon dioxide, subsequently released as methane, and supplying abundant fixed usable nitrogen.

Soon after hatching, each healthy individual from a termite colony, including alates, soldiers, and workers, already has in its intestines the typical components of the bacterial and protist gut community. Microbial symbionts ingest, digest, reproduce, and die, and some even mate in the dank anoxic space of the food-filled intestine. They are easy to observe. It is only necessary to remove the hindgut through the anus into a solution isotonic for insect hemolymph (or saliva, if you are in a hurry), and they can be seen with a simple optical microscope. A fantastic bustling world appears before the viewer's eyes.

The relative proportions of the different protist groups (parabasalians, pyrsonymphids, calonymphids, etc.) and bacteria (arthromitid spore-formers, spirochaetes, etc.) change with the seasons, the health of the individual, geographical location, and other factors. Basically, however, it is clear that without their symbionts termites couldn't digest the wood they eat.
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Publication:Encyclopedia of the Biosphere
Date:Mar 1, 2000
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