Geology Reference
In-Depth Information
fruticose ones that look like beards or strands of hair. Mycologists quite rightly claim
them all as fungi because most lichen body tissue consists of fungal hyphae surrounding
photosynthetic algae or bacteria in a kind mycelial sandwich.
Lichens are some of the hardiest organisms on the planet. Over hundreds of millions
of years they have honed the subtle art of colonising one of the harshest habitats on
earth: barren rock. Some are happy enough living on the outside surfaces of rocks. Oth-
ers go further, loving nothing more than to bore physically into their rocky hosts. Yet
others ooze and settle into natural cracks and fissures. Lichens mineralize their stony
habitats by weathering them using various subtle physical and chemical techniques. For
creatures so seemingly insignificant, lichens do this surprisingly well despite their often
slow rates of growth (around one millimetre per year). Thanks to their gentle ministra-
tions, silicate rocks vanish from ten to one hundred times faster than do rocks weathered
only by rain water and carbon dioxide.
Lichens physically weather rocks by pulverising them. Surface-living lichens anchor
themselves with small hair-like extensions called rhizines which physically fracture the
rock when they grow. Dark lichens of this sort break up the rock underneath them when
their rhizines swell up with warmth on sunny days. Those lichens that live inside rocks
swell up when wet, splitting the rock into fragments that provide greater surface area for
chemical weathering.
Lichens chemically weather rocks by dissolving them. The fungal partner releases
a wide spectrum of specially crafted organic molecules collectively known as lichen
acids. Some of these are chelating agents that capture metal ions from the rock in com-
plex molecular cages. Simpler organic compounds are also exuded, such as oxalic acid,
which combines with calcium in the rock. Many lichens host diatoms that absorb silica
released by chemical weathering, further enhancing the demise of the rocks. The fungus
absorbs some of these and other minerals through its cell walls. The rest are left behind
to make the beginnings of soil, supplemented, in due time, by lichen corpses. Lichens
begin the process of plant succession, for eventually lichen-derived soils are thick and
nutritious enough to support mosses, and so the soil builds up further until more com-
plex plants arrive with their own fungal partners—the mycorrhizas. In these various
ways, over hundreds of millions of years, lichens have played a major role in cooling the
earth by weathering calcium ions out of silicate rocks and marrying them with carbon
atoms from the atmosphere.
Fungal philosophy
