Geology Reference
In-Depth Information
The mycorrhizas confer other precious gifts: they give trees and other plants resist-
ance to pathogenic fungi and bacteria, and also the ability to filter out toxic metal ions
that would otherwise accumulate in plants growing in the acidic soils of heathlands and
moorlands. As we have seen, in exchange, the plant gives the fungus its own hard-won
gift: sugars made by photosynthesis in its sunlit leaves. In nutrient-poor soils, up to 25%
of the plant's photosythetically produced sugars can be given over to the fungus, which
ably converts them into more of its body tissue, greatly expanding its mycelial domain
to the benefit of both partners. In nutrientrich soils, the benefit to plants is less, and so
they often actively discourage partnerships with mycorrhizas. Adding chemical fertil-
izers to soils inhibits mycorrhizas—a good argument in favour of ecologically sensitive
agriculture.
Fungi and the carbon cycle
Trees today grow to immense sizes thanks to this massive mycorrhizal extension of their
absorptive capacities. But this remarkable partnership nearly turned into a global dis-
aster when the first trees appeared some 385 million years ago, held skywards in great
leafy spires by a new fibrous material called wood. This novel substance is now a ma-
jor component of the biosphere: 90% of contemporary forest biomass is in wood—40%
of net annual productivity in forests is woody. The main component that makes wood
strong is a carbon-ringed macromolecule called lignin that links itself to molecules of
cellulose, creating an association known as lignocellulose. In woody tissue, lignocellu-
lose is arranged as a series of closely packed tubes along which nutrients flow in both
directions between leaves and roots. Wood is of such great structural complexity that
for about 80 million years it defeated the best digestive efforts of the decomposer fungi
(and some bacteria too, it must be said) to access the ample nutrients and minerals it
contained. During this long interval, a huge amount of carbon extracted from the air
accumulated in the woody bodies of dead trees with two rather serious, interconnected
consequences for the earth: global cooling and a potentially dangerous increase in the
oxygen content of the atmosphere. Much of the dead wood was converted into the coal
that we so foolishly burn today (If you have gone hazy about how carbon, oxygen and
temperature are connected, do please look back to page 171) .
Had the dead wood continued to pile up, Gaia might have frozen to death or been
consumed in massive global wildfires by the extra cohorts of oxygen molecules in the
air. And pile up it did, despite the best efforts of various organisms, fungi included, to
break it down for food. Finally, after what must have seemed like a highly frustrating 80
million years of grinding evolutionary toil, fungi invented the cunning chemical toolkit
Search WWH ::




Custom Search