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Oxygen—polluter or eutrophicator?
In the early stages of the Gaia hypothesis, Lynn Margulis and I
often talked of the rise of free oxygen
in the atmosphere as if it were a disaster. We called it the first
and greatest air pollution incident, where vast ranges of species
of organisms were killed off by oxygen poisoning.
I now think that the opposite is true. Oxygen
is poisonous, it is mutagenic and probably carcinogenic, and it
thus sets a limit to life spans. But its presence also opens abundant
new opportunities for organisms. At the end of the Archean, the
appearance of a little free oxygen would have worked wonders for
those early ecosystems. Oxygen was good for two reasons: first,
its presence enabled consumers to recycle the organic matter of
the photosynthesizers much more rapidly than could be done by the
methanogens: and second, oxygen would have changed the environmental
chemistry. The oxidation of atmospheric nitrogen to nitrates would
have increased, as would the weathering of many rocks, particularly
on the land surfaces. This would have made available nutrients that
were previously scarce, and so allowed an increase in the abundance
of life.
Geophysiological models of the change suggest
that even the denizens of the anoxic regions would have benefited.
This is simply because oxygen is, in a sense, a eutrophicator -
that is, by increasing nutrient levels it encourages surface "blooms"
of algae, and these deny light and oxygen to life below, which soon
dies. The increased supply of dead plants in its presence would
have enriched the methanogens until consumers took over this role.
(see
variation 19 and note early form of the theory)
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