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I'm taking a microbial physiology course and we noted that, while some archaea are phototrophic, there are no known photosynethetic archaea. Are there any physiological characteristics that make archaea less suited for photosynthesis? Or is it simply a matter of there likely being photosynthetic archaea, but they have not yet been found (which seems like it could easily be the case, given that there are many extremophilic archaea)?

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There are photosynthetic archaea (such as Halobacterium) but the mechanism is different. They use rhodopsin-like ion pumps (bacteriorhodopsin and halorhodopsin) to move ions against the gradient and produce ATP via chemiosmosis (like mitochondria).

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  • $\begingroup$ I was under the impression that this was distinct from photosynthesis. At the very least, my professor emphasized that this was not the case, as rhodopsins do not utilize an electron transport chain in order to make a proton gradient, but photosynthesis does. $\endgroup$
    – Squirrel
    Mar 12, 2015 at 0:14
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    $\begingroup$ @Squirrel: Isn't this just a matter of definition? That is, one could define photosynthesis in very general terms as using light to synthesize energy-bearing chemicals, or specifically as the clorophyll-based synthesis used by plants. As for why no archaea use clorophyll, I don't know for sure, but would guess it's because plants evolved it after they split from archaea, and the identical process is unlikely to evolve twice. But I'd certainly be interested in a more authoritative answer. $\endgroup$
    – jamesqf
    Mar 12, 2015 at 4:40
  • $\begingroup$ @Squirrel rhodopsins work just like electron transport chains and produce ATP with the help of F₀F₁ ATP-synthases. $\endgroup$
    – WYSIWYG
    Mar 12, 2015 at 5:01
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    $\begingroup$ @Squirrel If your professor said haloarchaea don't count, then (s)he probably wanted to ask for photoautotrophs instead. Both plants and haloarchaea are phototrophs, but haloarchaea are photoheterotrophs since they cannot fix carbon. Knocking out PSII in plants will also convert them into photoheterotrophs. $\endgroup$
    – March Ho
    Mar 13, 2015 at 6:28

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