Alternative atmosphere plantlife

Yes, there's an alternative.

Plants using chlorophyll must, to the best of my knowledge, take in CO₂ and put out oxygen. So chlorophyll is a no-no. Fortunately, not all photoautotrophs use chlorophyll. A select few use a substance called bacteriochlorophyll (see also here). Here's its structure:

^{Atoms (vertices) not labeled are carbon atoms.}

Bacteriochlorophyll is found in, not surprisingly, bacteria! Certain "green bacteria" contain organelles called chlorosomes. The process looks like this:

Some of that can be understood from the image; more detail sheds some light on it (pun very much intended).

Much better information can be found here. The photosynthetic process involving bacteriochloropyll can be summarized as: $$\text{carotenoid} \to \text{Bchl c}_{\text{chlorosome}} \to \text{Bchl a}_{\text{baseplate}} \to \text{Bchl a}_{\text{antenna}} \to \text{Bchl}_{\text{reaction center}}$$ What does this mean? Check out this image:

Energy travels first to "cylindrical aggregates of Bchl c and carotenoids" (1), from the carotenoid to the Bchl chlorosome. Then it goes to the baseplate (2) of Bchl. From there, it goes to the antenna proteins (5) and finally the reaction center (6). There is Bchl in both the antenna proteins and the reaction center. Does this make the second image from the top a bit clearer?

Processes using bacteriochlorophyll does not use carbon dioxide, as normal photosynthesis does.

Processes using bacteriochlorophyll are actually a subset of processes of anoxygenic photosynthesis (see also here). Anoyxgenic phoyosynthesis does not produce oxygen (hence "anoxygenic"). Water is not used as an electron donor (an alternative is H₂S). This pdf has some very informative diagrams of the process (pages 6 and 7). Here's one (with someone else's notes on it!):

The big issue with using bacteriochlorophyll is that, to the best of scientists' knowledge, only some bacteria use it, not plants. You can probably circumvent this somehow by creating conditions in which normal photosynthesis (i.e. using chlorophyll) is not feasible. Perhaps H₂O isn't too plentiful, while H₂S is.

posted almost 10 years ago

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