Life in the Soil - Soil: Reflections on the Basis of our Existence - page 72

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Concentration to resources. This photograph, taken from space, shows Earth's night-time illumi-
nation. The River Nile, with its fertile basin, appears as a root in North Africa. Cities dot the map
as bright spots. The black areas—oceans and deserts—are more or less devoid of human activity.
The photo could also represent biological activity in the soil. Life in the ground, like human life,
is concentrated in areas where resources are plentiful. A fallen leaf provides easily accessible
nutrition for fast-growing bacteria and fungi. Plant roots leach sugar, and here the activity level
is hundreds of times higher than a few millimetres away from the root surface. The dark areas
could represent mineral grains that are resistant to weathering or areas with old organic matter
depleted of accessible nutrients

can survive on a resource composed of carbon, oxygen, hydrogen and little else.
Predators eat prey rich in protein and with a similar chemical makeup to them-
selves, but how do termites ingest enough nitrogen, and where does the surplus
carbon go when they convert their food into biomass? The answer is that termites,
like all other herbivores, exist in symbiosis with microorganisms in their gut.
I learnt how to study these microorganisms at a conference in Halifax,
Canada. You can dissect a termite by removing the head with tweezers and
removing the alimentary canal with a needle. You then squeeze out the contents
of the gut in a drop of water, put them under a cover glass and examine them
with a microscope. You will then observe a myriad of small organisms of all
shapes and sizes swimming around in the water—long, corkscrew-shaped crea-
tures wriggling alongside oval-shaped microorganisms that use small rotating
hairs for forward motion.
The former are spirochetes—giants of the bacterial world that can grow to a
tenth of a millimetre in length (almost one hundred times longer than most other
bacteria). They ferment carbohydrates decomposed from wood fibres in the anaer-
obic conditions prevailing in a termite's stomach. Some spirochetes are known for
causing illness in humans, including the tick-borne Lyme Disease.
The oval organisms are unicellular ciliates. They also feed on carbohydrates
and contain archaea that convert the carbon dioxide and hydrogen produced dur-
ing fermentation into methane. Thus, excess carbon, oxygen and hydrogen in
termites' plant food is released into the atmosphere as carbon dioxide and meth-
ane. By eating large quantities of wood, termites ingest sufficient nitrogen for
their needs.
Bacteria and ciliates gain substantially from this symbiosis. Theirs is a shel-
tered world offering protection from predators and unfettered access to food. For
their part, the termites do not have to compete with other animals for food as they
alone are capable of eating wood.
Termites are widespread in tropical and subtropical areas. Anyone who has
been on safari in Tanzania's Serengeti National Park will recall the ubiquitous
sight of tall termite mounds. The insects build labyrinths of underground tunnels
and drag in bits of wood and other organic matter when collecting food for the col-
ony. The mound is literally just the tip of the iceberg. In some savannah areas, ter-
mites dispose of a quarter of all fallen leaves, and play the role of composters just