Geoscience Reference
In-Depth Information
oxygen-producing organisms. These ancient Rubiscos had an inherent
oxygenase activity;
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this was an unintentional property of the enzyme
but of no consequence since no oxygen was present. The ancient Ru-
bisco, possibly first present in anoxygenic photosynthetic purple bacte-
ria, was adopted by the earliest oxygenic phototroph at a time when
atmospheric oxygen concentration was very low.
14
As oxygen concen-
tration rose, both in local environments and in the atmosphere,
15
organ-
isms developed a number of strategies to deal with the shortcomings of
Rubisco's oxygenase activity. These remedies include the evolution of
less oxygen-sensitive Rubiscos and the development of various strate-
gies to concentrate CO
2
within the cell, raising the ratio of CO
2
to O
2
.
Such strategies are widely used by plants and cyanobacteria today.
Thus, Rubisco provides a beautiful example of how evolution does
not lead to higher-order perfection. In this case we see that once Rubisco
was adopted as the carbon-ixing enzyme for oxygenic phototrophs,
this path was taken, and subsequent evolution through natural selec-
tion has acted to minimize the expression of Rubiscos imperfections,
rather than to find a replacement. But its properties are still not ideally
suited to the task.
We also see that it makes sense to view the evolution of oxygenic pho-
tosynthesis in the broader context of the evolving ancient biosphere.
We can see, for example, where many of the parts comprising the oxy-
genic photosynthetic process were derived. Viewed in this context, it
is not surprising that oxygen evolution took some time to develop and
why it was not part of the original biosphere as we explored in the last
chapter. Oxygenic photosynthesis was not, however, assembled as a
puzzle from a number of preexisting pieces. Many unique biological
innovations, like the manganese cluster and chlorophyll biosynthesis,
for example, were integral parts of its evolution. Could other pathways
have led to oxygenic photosynthesis? Perhaps. This, I think, is a fasci-
nating question, and imagining alternative pathways to phototrophic
oxygen production may help us visualize how this process could pos-
sibly have developed elsewhere, beyond Earth.
