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are not at all clear, these two pre-existing reaction centers were con-
nected to form the coupled photosystems in oxygenic photosynthetic
bacteria. The connection may have occurred as the reaction center of
one type was transferred into an organism containing the reaction cen-
ter of the other type; alternatively, the two types of reaction centers were
coupled already in an original anoxygenic photosynthetic organism,
which later developed phototrophic oxygen production. In either of
these scenarios, anoxygenic phototrophs once contained both types
of photosystems. If the first scenario is true, then the precursor anoxy-
genic phototrophic organism, with the two photosystems, has appar-
ently been lost from nature (but, see below). If the second scenario is
true, then modern GSBs and purple bacteria have each lost one of the
two reaction centers. Bob Blankenship argues that it's impossible at
present to distinguish between the two possibilities, and perhaps it is.
However, John Allen from the University of London likes the idea that
the two reaction centers once existed together in the same anoxygenic
phototroph, perhaps derived from a single precursor through a gene
duplication event. 12 In his view, each of the photosystems was of service
to the organism, and either one or the other was chosen for use by the
organism depending on environmental conditions. At some point these
two photosystems combined. In Allen's view, anoxygenic phototrophs
may well exist today with both reaction centers intact. It would cer-
tainly be exciting to find such an organism!
hile we have come closer to understanding the evolution of oxy-
genic photosynthetic organisms, we have yet to explain how oxygen
came to be produced. Here, we need to focus on the oxygen-evolving
complex (OEC), containing in its core a cluster of 4 Mn atoms and a Ca
(calcium) atom. Just how the whole thing works is still under active
debate, but it is well known that Mn atoms are the workhorses here.
Basically, these atoms act as a biological capacitor. In order to form O 2 ,
the oxidized form of P680 removes 4 electrons from the Mn-complex
(not directly from water). These 4 electrons are replaced as two water
molecules liberate electrons, and thereby form O 2 . Curiously, the oxygen-
evolving complex, and in particular the manganese cluster, is unique to
oxygenic phototrophs. Nothing like it is known anywhere else in biol-
ogy. Jason Raymond and Bob Blankenship, however, have some ideas
as to where it might have come from. They have developed new methods,
 
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