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reducing (electron yielding) to easily combine with NADP
+
to form
NADP(H).
We are not quite finished. If we just stop here, the electrons packed in
NADP(H) would simply accumulate. This cannot and does not hap-
pen. Indeed, the electrons in NADP(H), which, if you remember, were
ultimately sourced from water, are transferred to CO
2
, producing the
organic compounds used to build cells. This process is known as carbon
fixation, and the incorporation of CO
2
is promoted by an enzyme known
as Rubisco.
9
Indeed, Rubisco-promoted carbon fixation is the basis for
virtually all the food we eat and nearly all of the fossil energy we use. It's
been a long ride indeed, but in the end, the cell gets just what it needs
to grow, and it spits out O
2
as a waste product in the process.
With this brief description as a backdrop, we will approach the evo-
lution of oxygen-producing organisms by considering the evolution,
and assembly, of its basic constituent parts. We will focus on the follow-
ing key questions: (1) hat is the evolutionary history of chlorophyll?
(2) hat are the evolutionary histories of PSI and PSII? (3) hat is
the origin of the oxygen-evolving complex in PSII? And finally, (4) what
is the evolutionary history of Rubisco? In addressing these, we will seek
to understand the complex path leading to the evolution of oxygenic
photosynthesis on Earth.
As hinted at in the last chapter, oxygenic photosynthesis was probably
not the first type of photosynthesis; that honor goes to anoxygenic pho-
tosynthetic bacteria. I will not use much time exploring the evolution of
those earliest photosynthetic organisms, and indeed, not much is known.
However, as we shall see below, the evolution of oxygenic photosyn-
thesis makes most sense when viewed in the light of anoxygenic photo-
trophs as precursors. Thus, we will look to anoxygenic phototrophs
more than once to find clues to the origin of oxygenic photosynthesis.
We begin with chlorophyll. I remember well my mom's frustration as
I tried to sneak through the back door with bright green stains on my
new jeans or, even worse, on my white Sunday shirt. Other than chal-
lenging mothers (and fathers) to keep their kid's clothes in a reasonable
state of shine, green-staining chlorophyll, as we saw above, also serves
several different functions in an oxygen-producing organism; it is an
integral part of the antenna complex and it is a critical component of
both photosystems. The main importance of chlorophyll for oxygenic
