Geology Reference
In-Depth Information
needed to digest the recalcitrant lignin and cellulose. One of their strategies involves a
chemical that we use to bleach our hair and as a component of rocket fuel—hydrogen
peroxide. The fungi secrete this into the wood along with ferrous ions to create a host of
wood-rotting free radicals. This is the dreaded brown rot, of which dry rot in buildings
is an example. In this particular strategy, enzymes inside fungal cells make the hydrogen
peroxide—other strategies involve exuding digestive enzymes directly into the wood. It
is even possible that the bacteria were the first to develop wood-rotting biotechnologies,
and that the fungi picked up these decomposer's arts via fragments of bacterial DNA in
the soil, or even in the wood itself.
Now, at long last, the decomposer fungi could infiltrate their tubular mycelia into the
woody carcases of dead trees. Mycelia flowed like water along lines of least resistance
in the wood and gained access to the bountiful supply of nutrients and minerals so long
denied them. Feasting on wood, fungi saved the earth as a serendipitous by-product of
their appetite. They consumed oxygen and released carbon dioxide, thereby helping to
regulate the flux of heat out of our planet's membranous and highly volatile atmosphere.
Decomposer fungi also help to keep the planetary carbon cycle turning by consuming
leaves that are not particularly palatable to those other prominent decomposers—the
bacteria. In the vast boreal forests of the north the needle-like leaves of pine trees defy
bacterial decomposition, as do the leaves of beech and other trees in the temperate
forests of Europe. Both types of leaf contain large amounts of lignocellulose, a problem
for most bacteria, but not, as we have seen, for fungi. As hyphae digest fallen,
lignocellulose-rich leaves, decomposer fungi consume oxygen and release carbon diox-
ide gas that swirls back into the air, helping to adjust the earth's temperature.
Some fungi live harmlessly inside the tissues of plants, including trees— these are the
endophytic fungi. The diversity of endophytic fungi in a single tree can be immense—up
to 100 species in some cases. Many spend their lives as benign presences within their
host; others become pathogenic if it becomes stressed. Some endophytes wait patiently
for their tree to die, whereupon they feed on the dead wood and produce spores. Some
kill and therefore prune tree branches when the tree cuts off the water supply to its own
limb. Others colonise conifer needles in greater density as the needles age, killing the
needles when they become so old or so shaded that they no longer produce sufficient
sugar. Many endophytic fungi protect their hosts by producing substances that deter
predators and kill pathogens—this effect is particularly important in grasses. Endophyt-
ic fungi could soon have a great influence on the earth's temperature if plants experience
greater stresses because of climate change. Many endophytes will then kill and digest
their hosts, releasing carbon dioxide to the atmosphere.
Decomposer, endophytic and mycorrhizal fungi thus run much of the short-term car-
bon cycle in partnership with the land plants. Many billions of tonnes of carbon are re-
