Geoscience Reference
In-Depth Information
phototrophic organisms that conduct exactly the same process in the
synthesis of both chlorophyll and bacteriochlorophyll molecules. Xiong
and Bauer determined the DNA sequences of several of these genes, and
when comparing sequences from among the same genes, those from
anoxygenic phototrophic bacteria always appeared more ancient than
those from oxygenic phototrophs. This is good evidence that chloro-
phyll biosynthesis is more modern than bacteriochlorophyll biosynthe-
sis, suggesting that bacteriochlorophyll came first. Therefore, the fact that
chlorophyll has a somewhat simpler final biosynthetic pathway must be
balanced against other evidence that bacteriochlorophyll, in fact, origi-
nated first.
Thus, our idea that anoxygenic photosynthesis predates oxygenic
photosynthesis seems pretty well supported, as does the idea that chlo-
rophyll biosynthesis emerged later, perhaps together with the evolution
of oxygenic photosynthesis. But why? hy did organisms irst use bac-
teriochlorophylls, the more complex biosynthetic product, rather than
starting off immediately with chlorophyll? It could be by chance that
the bacteriochlorophyll pathway evolved first, and since it worked, there
was no evolutionary pressure to incorporate a new pigment system. Or
it could be, as Martin Hohmann-Marriott from the University of Otago
and Bob Blankenship from Washington University (more on Bob below)
pointed out, that the early enzymes forming bacteriochlorophyll could
have conducted multiple steps in the synthesis process, essentially by-
passing the chlorophyll step, or producing chlorophyll as only a minor
product. Only later, as the genes evolved, did chlorophyll synthesis be-
come a major formation pathway in oxygen-producing organisms. This
seems possible, but the reality is that we still have much to learn.
We now turn our attention to the reaction centers, PSI and PSII,
the heart of the oxygenic photosynthetic process. Bob Blankenship of
Washington University has studied photosynthesis for many years and
he, together with his former student Jason Raymond, have probably
thought about the evolution of photosynthesis as much as anyone. As
they say, pictures tell a thousand words, and long ago Bob found simi-
larities in function and structure between PSI and PSII, and the reac-
tion centers used by anoxygenic phototrophs. Let's try to see what Bob
saw, by reference to
igure 3.3.
Bob compared the biochemical pathways
of PSI and PSII to those of anoxygenic phototrophs. Through this com-
