Geology Reference
In-Depth Information
the planet plummeted down into an ice age as oxygen hunted down methane, converting
it to water and carbon dioxide— which, molecule for molecule, is about 20 times less
powerful a greenhouse gas than methane, its hydrogen-rich progenitor.
Bacterial Mergers
It was Lynn Margulis who championed the idea that the rise of oxygen may have fed
back to trigger changes of massive significance amongst the denizens of the bacterial
world. Like the sun and volcanoes, oxygen is both a giver of life and a dealer of death,
for the gas is so highly reactive, so passionate in its quest to complete its outer orbit of
electrons, that it happily attacks the complex molecules inside living bacterial cells. It
could be that this new danger from oxygen prompted cells to develop a protective mem-
brane to shield the genetic material in the nucleus, but in any case the new oxygen-rich
environment quickened the global bacterial metabolism, allowing it to weather rocks on
the land surface ever more effectively. As a result more nutrients were made available
to life as a whole, strengthening Gaia's coupling between life and her realms of rock, air
and water.
This newly enriched environment opened up possibilities for cooperative relation-
ships amongst the bacteria that could only have been dreamt of by the inhabitants of
the ancient methane-dominated world. Perhaps the first of these new associations took
place between the highly mobile corkscrew spirochete bacteria and more sedentary bac-
teria such as Thermoplasma who lived out their lives in sulphur springs. Spirochetes are
voracious predators, and at first may have remorselessly devoured beings such as Ther-
moplasma, but they soon discovered that a much more effective strategy was to attach
themselves to the outside of their prey to suck up the nutrients leaking out of their cell
membranes. Spirochetes are forever on the move, and so this arrangement meant that
both partners were constantly propelled into new environments, some of which must
have contained fresh supplies of food. The association worked as a stable configuration
which enhanced the wellbeing of both partners. A similar association between free-liv-
ing spirochetes and a host organism exists today in a most unlikely place—in the intest-
ines of termites in the outback of northern Australia. These termites ( Mastotermes dar-
winiensis ) eat wood—a major challenge, because the lignin in the wood cannot be di-
gested by the unaided insect metabolism. Luckily for the termites, help is at hand in the
form of wood-digesting microbes living inside their hindguts. One of the most spectacu-
lar of these beings, a large single-celled protist called Mixotricha paradoxa ( Figure 37 ),
provides the termites with a highly efficient wood digesting service. To do this, Mixo-
tricha needs to move around in the termite hindgut, and this it does with the help of three
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