# Alternative atmosphere plantlife

Are there any reasonably plausible alternatives to the process used by our plants on earth (Carbon Dioxide, Chlorophyll, Oxygen) around which an ecosystem could be built in a planet with a completely different atmosphere? Would every planet with life on it need to have a carbon+oxygen based life-cycle or are there any viable alternatives?

To be clear, I'm talking about a Sunlight-powered ecosystem but one where the plants do not use or produce CO2, O2, or both.

If there are any alternatives then are we able to make any guesses as to the appearance or other properties of the plants that might live in that atmosphere?

• There are pigments other than chlorophyll....carotenoids come to mind (I think I spelled that right:) ) – JDSweetBeat Mar 6 '15 at 16:32
• To clarify, you mean specifically without those three components - CO2, chlorophyll, and sun? Or just one of those at a time? Because you can get slime molds and fungi in the dark. – Isaac Kotlicky Mar 6 '15 at 16:38
• I meant changing the atmosphere, not removing sunlight. I've edited the question for clarity. – Tim B Mar 6 '15 at 17:57
• Related to this question, but not quite worth its own question, if anyone feels like having a crack - I read a science-fiction novel a while back in which an alien ship creating a new planet for humans to live on chose to use melanin as the basis for photosynthesis in the new biosphere. Is that a feasible choice? – Werrf Nov 23 '16 at 21:30
• @Werrf Hmm, my first instinct is that that would be fine as a question in its own right, but it most likely would be a duplicate of this one so maybe doing it as a comment is right. Having said that this question may cover it but the existing answers don't...so I think you would be fine to ask it. – Tim B Nov 23 '16 at 21:33

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.

All life in the universe is probably carbon-based, so every planet capable of supporting life would need sufficient carbon in the atmosphere and/or soil to form organic molecules. I wrote a short work on this a few years ago: https://www.dropbox.com/s/aai21mjdlyyw9fv/SiBiochem.pdf?dl=0

To summarize:

You need to base your biochemistry on an element with chemical properties similar to those of C (tetravalency, sp3 orbital hybridisation, etc). The only elements with the same properties as C are those in the C group, of which Si is the next lightest. The problem with this is that silanes (Si-O compounds) are all solid, inflexible and less stable than their equivalent organic molecules, which makes them difficult to use as building blocks (proteins, lipids, DNA, etc) and difficult to metabolise. Heavier elements in the C group suffer even more from this problem (and others)

As far as oxygen goes, I don't know whether another element could replace it; it's worth having a look on the internet or studying the properties of other elements in the oxygen group (or perhaps in the boron and nitrogen groups?).

If you stick to oxygen, there are plenty of molecules which could replace chlorophyll. Haemoglobin has esentially the same molecular structure (the differences are that its core is an iron atom instead of a magnesium atom and that some of the nitrogens are located in different places), so I believe haemoglobin could be usable for photosynthesis (and chlorophyll for oxygen transport in some sort of modified bloodstream) under certain conditions (I wanted to study this problem scientifically some time ago but never got around to it). You can also probably replace the magnesium with some other metallic atom and have something similar.

It's unlikely that you'll be able to get rid of nitrogen dependence if you stick to something chlorophyll-like, though.

The optical properties of chlorophyll-like molecules can be wildly different from those of chlorophyll itself (for example, haemoglobin scatters red light and absorbs green light, whereas chlorophyll scatters green light and absorbs blue and red light). Aside from any possible structural changes your non-chlorophyll-based plants might have, they would probably be primarily some colour other than green.