Re: Life: Turn it upside down!

On 10/04/2024 14:52, Ernest Major wrote:

On 10/04/2024 11:00, Arkalen wrote:

On 10/04/2024 11:25, Ernest Major wrote:

On 10/04/2024 07:58, Arkalen wrote:

snip

Parasites, especially intracellular parasites (including parasitic plants, which invade their hosts at the intracellular level) lack even more of the metabolism, scavenging chemicals from their hosts. With 1,000 or so genes, mimivirus also has a truncated metabolism (I don't know how it compares to say Wolbachia, but with comparable numbers of genes a comparison seems an obvious thing to investigate.) A difference between mimiviruses and intracellular parasites is that the latter have their own cytoplasm, while the former utilises the host cytoplasm as a substrate for its metabolism. That's still a big difference - but is it the only difference in kind between mimiviruses and the simplest intracellular parasitic organisms? (According to the above paper mimivirus has an immune system, which is something one could imagine a cellular organism lacking.)

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I agree those are much more similar than I'd been thinking; I was thinking of viruses as they are outside of the cell but you're right that when you consider their activity inside of the cell then there's much less reason to say that activity isn't "metabolism". Except for that whole "meta" part of "metabolism" : does mimivirus do catabolism? Do intracellular parasites?
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I'll look it up after posting but I notice you point out the difference that intracellular parasites have their own cytoplasm. I will hazard the guess that this means they have their own *membranes*, and further hazard the guess that they use respiration to generate a proton motive force across that membrane to regenerate ATP. I could see it if they didn't, after all they can get ATP from the host cell can't they. But if they do, that would be metabolism with the "meta".

 Microsporidia have lost the ability to generate their own ATP. The same is said of Giardia.

Do you have a cite on that? This paper suggests that Giardia does have metabolism, using fermentation (but then maybe it varies by Giardia species, this paper seems to be looking at one specific one):
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC88984/
It explicitly describes it as generating its own ATP unless I'm seriously missing something:
"However, certain eukaryotes, including Trichomonas spp., Entamoeba spp., and Giardia spp., are characterized by their lack of mitochondria and cytochrome-mediated oxidative phosphorylation. They rely on fermentative metabolism (even when oxygen is present) for energy conservation. Glycolysis and its brief extensions generate ATP, with generation dependent only on substrate level phosphorylation."
I'm not sure you're even completely right on Microsporidia, cf this paper:
https://link.springer.com/article/10.1007/s00436-020-06657-9
It does describe microsporidia as using the host's ATP, but also of using glycolysis to generate ATP:
"These parasites have lost canonical mitochondria and the oxidative
phosphorylation pathway, so that glycolysis is the only way to
generate ATP (Heinz et al. 2012; Corradi 2015). During the
intracellular development stage, microsporidia apparently do
not use their energy metabolism (Dolgikh et al. 2011) and
instead satisfy their energy demands by “stealing” ATP from
the host cell using unique nucleotide carriers acquired via
horizontal transfer from bacteria (Tsaousis et al. 2008;
Alexander et al. 2016)."
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.
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.
Thank you for drawing my attention to the possibility, it's definitely something I'll look into more.