more bad design and the origin of chloroplasts
Organization: animal -- coelomate -- deuterostome
From: firstname.lastname@example.org (Chris Colby)
I got an email message awhile back correcting a mistake in a
post of mine concerning bad design. In the post it mentions
endosymbiosis as the origin of chloroplasts and says that
chloroplasts came from cyanobacterial ancestors. This isn't
the whole story. Chloroplasts are, incidentally, the organelles
(found in plants and some lineages of protists) where photosynthesis
The chloroplasts of red algae (a multicelluar protist)
came from cyanobacterial ancestors. The chloroplasts of green
algae (and plants, since they evolved from green algae) came from
a group of organisms closely related to cyanobacteria. The bacteria
_Prochloron_ is a living member of this group called the Prochloro-
phytes. The chloroplasts of brown algae came from a group of bacteria
as yet undiscovered or possibly extinct.
Red algae chloroplasts and cyanobacteria have chlorophyll a and
phycobilins as light harvesting molecules. Green algae (and
plant) chloroplasts and Prochlorophytes have chlorophyll a and b
and lack phycobilin. Brown algae chloroplasts have chlorophyll a
and c. Hopefully someone will stick some of these in blender some
day and sequence their rbcl gene and find out what they are related
(Prochlorophytes may have arisen multiple times from cyanobacteria.
At least, thats what two different Nature papers from 1992 claim.)
Anyway, that's the scoop on chloroplasts. I just gave a lecture on
the evolution of metabolism to the bio lab I teach. I was really
surprised so much is known (not my field, I just did some reading
on it for the hell of it.) I found out a nice little tidbit about
photosynthesis. Let me give a little background first.
Photosynthesis is (loosely) the conversion of carbon dioxide into
sugar (oxygen is a waste product in some forms of photosynthesis).
It is a process driven by light energy. There are two steps in
photosynthesis -- the light reactions and the dark reactions. The
light reactions harness light energy and the the dark reactions use
that energy to make sugar. Some organisms use photosystem I to
harness light energy, some use photosystem I and photosystem II.
(Photosystem II is the one that produces oxygen).
[Net photosynthetic rxn:
6 CO2 + 12 H2X -> C6H12O6 (glucose) + 6 X2 (or 12 X) + 6 H20
where X can be sulfur or oxygen
In organisms where X is oxygen, the net photosynthetic reaction
is the exact reverse of aerobic respiration. If X is sulfur, the
reaction is the opposite of some forms of anaerobic respiration.]
Photosystem I (PS I) evolved first and is used by green sulfur bacteria
and purple bacteria. Cyanobacteria (and thus algae and plants) use both
photosystem I and II. The Calvin cycle, which fixes carbon in purple
bacteria and cyanobacteria, evolved before photosystem II and hence
in the absense of oxygen. Today, organisms that use the Calvin
cycle to fix carbon lose 1/4 to 1/2 of the carbon they initially
fix because oxygen competes with the forward reaction of one of
the steps in the cycle (the step catalysed by RuBP carboxlyase)!
This is a great little tidbit of bio-trivia and something that
makes perfect sense in light of evolution; but something that
would seem to be hard to explain if plants were designed by an
This obstacle has been overcome by one group of organisms
that initially fix carbon by a newer process, then pass it to
the Calvin cycle in cells in the interior of the organism away
from high oxygen levels. These are called C4 plants -- they are
a group of grasses. They are in the angiosperm division and the
monocot subdivision of plants.
I leave you with a cladogram summarizing one hypothesis about
the evolution of metabolism:
bacteria purple bacteria cyanobacteria
light rxn(s)= (PSI) (PSI) (PSI and PSII)
dark rxns= (reverse TCA) (Calvin) (Calvin)
| \ /
| \ /
other eubacteria | |
| \ /
The above cladogram gives three lineages (I left out a bunch) of
bacteria and the light and dark rxns taking place in them. The
reverse TCA cycle fixes carbon by reversing the order of the
TCA or Krebs cycle from aerobic respiration. Instead of giving
off energy and CO2, energy is added and CO2 is taken in.
Aerobic respiration did not evolve until after PSII arrived so
it looks like ancestral aerobes just modified an already
present metabolic cycle (the reverse TCA cycle) to fit their
needs (not consciously, of course -- I'm using bio-slang here.)
PS I evolved after the "other"/green-purple-cyano split. The
Calvin cycle evolved after the green/purple-cyano split and
PS II evolved along the cyano line (probably as a duplication
and differentiation of PS I).
I should point out that new evidence may change the above
inference. I read in a biology of microorganisms book that
the Calvin cycle may be present in some archaebacteria --
that would move it's origination down to before the root of
the above tree.
Not everything is known about the evolution of metabolism, but
at least as biologists we can construct hypotheses and test them
by surveying the distribution of traits across taxa. I would hate
to be a creationist and have to look at all the diversity out there
(and how it is distributed) and lamely conclude that we can't tell
anything about how it got here.
Chris Colby --- email: email@example.com ---