Re: Life: Turn it upside down!

On 11/04/2024 14:41, Arkalen wrote:

 As an aside I think it's really interesting that those are both eukaryotes; it actually tracks with what I said earlier about how "having their own cytoplasm" implied "having their own membranes" which implied "respiring across those membranes" because I realized later that this applied to bacteria but not necessarily eukaryotes! Eukaryotes respire using mitochondria, not their outer membrane. I still think it's interesting that the examples we'd find of endocellular parasites that (maybe, partially) gave up on metabolism would be eukaryotes. Does it means bacteria don't do this? And if so why is it eukaryotes can give up on metabolism more easily than bacteria can? It seems to intuitively make sense that getting rid of organelles would be easier than getting rid of a function that's fundamental to your cellular structure, but seeing eukaryotic modularity potentially confirmed this way is still pretty interesting.

Your original mention of ATP production associated with membranes pointed me in the direction of amitochrondriate eukaryotes for counterexamples (and I had a recollection of microsporidia as "energy parasites"). I don't see a reason to conclude that loss of ATP can't also occur among parasitic prokaryotes.
My first candidate - Mycoplasma - came out negative; they do generate ATP, but by an atypical path. Further searching however found putative energy parasites among the acutalibacteraceous clostridia.
https://www.nature.com/articles/s41396-023-01502-0
On the other, a paywalled paper reports that Chlamydia trachomatis was erroneously considered an obligate energy parasite for nearly 40 years. Google Scholar then finds what it perhaps the debunking paper.
https://onlinelibrary.wiley.com/doi/full/10.1046/j.1365-2958.1999.01464.x
Quite a number of prokaryotes are energy parasites. But we're looking for obligate energy parasites (and even that may not be enough - one could imagine a parasite which has some ATP-production capability, but not enough to meet its needs).

  It also means I still have doubts about the notion that intracellular parasites can be as simple as even giant viruses. It seems entirely possible in principle don't get me wrong; my argument is that metabolism is what separates cellular life's complexity from that of viruses so it would perfectly track that cellular life that got rid of metabolism could simplify to virus level. But the idea of *eukaryotes* - not just cellular life but *eukaryotic* cellular life containing organelles and all that jazz even if mitochondria are no longer in their number - could be as simple as viruses, giant as they might be, still begs disbelief for me. Possible in principle but pretty remarkable to witness in reality, and I'm not sure Microsporidia or Giardia reach that level.

There's even a amitochondriate animal - Henneguya zschokkei. (Looking at the Wikispedia article for this I discover that there is a hypothesis that myxozoa evolved from transmissible cancers.)
However microsporidia and Giardia were brought up in response to your specific claim about ATP; not as a claim of comparable complexity to mimiviruses. Personally I doubt (pace the problem of generating an operational definition of complexity giving an ordering), even with the overlap in genome sizes and gene counts, that any cellular organisms are simpler than mimivirus, but it strikes me that this is some that needs to be demonstrated rather than assumed. I would look among parasitic prokaryotes for candidates, with Wolbachia pipientis and Buchnera aphidicola as first ports of call. (I suspect that treating these two clades as single species in incorrect, and each of these is comprised of many species.)
Based on the overlap in genome sizes and gene counts my provisional position is that the gap between viruses and cellular organisms is narrower than generally expected.

  Thank you for drawing my attention to the possibility, it's definitely something I'll look into more.

--
alias Ernest Major