Geology Reference
In-Depth Information
The fungi needed their 75 million year head start to discover how to extract scarce
minerals from the rocks, how to gather nutrients and how to resist droughts and other en-
vironmental extremes. One of their strategies was to form partnerships. Early on during
their tenure on land (and perhaps earlier, when they lived in the sea) fungi discovered
the benefits of intimate relations with photosynthetic algae. The fungi provided minerals
and a tough, secure environment for their more vulnerable photosynthetic partners, who,
in turn provisioned their hosts with reliable supplies of much-needed sugars. As a result,
both partners grew better together than alone, and the association flourished. Sometimes
photosynthetic bacteria were involved rather than algae, but both kinds of intimate asso-
ciation gave rise to a new emergent life form that has been of great importance to Gaia
to this day: the lichen tribe (of which more later).
The first land plants faced great difficulties, for their root systems were poorly deve-
loped and soils were thin and of poor quality. The fungi had proven their aptitude for
running successful relationships with partners that photosynthesised. They had, after all,
helped to create the lichens that had covered rocks all over the planet for millions of
years. Perhaps it was easy for the partnership-prone fungi (but probably not the lichen-
forming ones) to discover that the enfeebled plants were more than willing to give them
precious sugars in exchange for phosphorus and other minerals. If so, the mycorrhizal
fungi came easily into existence.
In this view, fungi and plants evolved separately. A highly controversial alternative
proposed by biologist Peter Astatt is that plants are, in fact, 'inside-out lichens'. In
Astatt's view, plants, like lichens, arose out of intimate associations between fungi and
algae, but this time the alga provided structure and the photosynthetic clout, whilst the
fungus morphed into the tubes (xylem and phloem) that shunted nutrients around the
newly emerged organism. Free-living mycorrhizal fungi must soon have connected with
the roots of these inverted lichens, provisioning them with minerals and nutrients.
In whatever way it came into existence, the new association between plants and my-
corrhizal fungi was a great success. It allowed plants to colonise much of the earth's
surface. It turned the earth green and made it possible to fix huge amounts of carbon
dioxide from the air in the bulky bodies of trees. To this day, mycorrhizas use their soph-
isticated digestive and distributive powers to provision their plant partners with phos-
phorus, nitrogen, copper, zinc and large amounts of water from sources far beyond the
spatial and biochemical reach of plant root systems. Mycorrhizas save plants the consid-
erable metabolic expense of constructing extensive root systems by gathering nutrients
for them from widely scattered and evershifting patches of food that plant roots would
find difficult if not impossible to reach. The extent of mycorrhizal support for trees is
staggering. Recent estimates suggest that an individual tree can be associated with hun-
dreds of thousands of kilometres of fungal hyphae.
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