1. Pyruvate.
Many protozoans are obligate anaerobes. We can do anaerobic resipration in a pinch and use pyruvate as an electron acceptor (producing lactate), later regenrating the pyruvate using oxygen. Some anaerobic protists only do anaerobic respiration. An example is blastocystis.
The Human Gut Colonizer Blastocystis Respires Using Complex II and Alternative Oxidase to Buffer Transient Oxygen Fluctuations in the Gut
Blastocystis received additional attention due to its unusual
mitochondrion . As an anaerobe, and similar to other anaerobic
microbial eukaryotes, it has lost many classic features of
mitochondria and performs no oxidative phosphorylation and lacks a
standard mitochondrial electron transport chain... Although
Blastocystis can produce some ATP via substrate level phosphorylation
using the TCA cycle enzyme succinyl-CoA synthetase = it is mainly
reliant on fermentation producing lactate, acetate and probably
propionate...
- Bare naked protons.
Some anaerobic protists oxidize carbon and use protons as electron receptors generatng hydrogen. You read it here first: eukaryotes that make hydrogen. Giardia is one.
https://www.nature.com/scitable/topicpage/the-origin-of-mitochondria-14232356/
Mitochondria of yet another kind yield even less ATP per molecule of
glucose. These are mitochondria of several distantly related
unicellular eukaryotes (protists) that lack an electron transport
chain altogether. They synthesize ATP from pyruvate breakdown via
simple fermentations that typically involve the production of
molecular hydrogen as a major metabolic end product. These
mitochondria are called hydrogenosomes and allow the cell to gain
about 4 mol of ATP per mole of glucose. Hydrogenosomes were discovered
in 1973 in trichomonads, a group of unicellular eukaryotes. They were
later found in chytridiomycete fungi that inhabit the rumen of cattle,
as well as some ciliates, and they continue to be found in other
groups. The enzymes of hydrogenosomes are not unique to these
anaerobes. They are found also in the mitochondria, the cytosol, or
even the plastids of other eukaryotes (Figure 1)
And a funky 4th kind that I do not understand well at all; see cite on degenrate mitochondria called "mitosomes".
Both of these respiration methods use glucose less efficiently than oxygen based metabolism does, but neither requires oxygen. Maybe you could get away without oxygen. In the linked question I proposed a big deep ocean creature that did not need to breathe and only used anaerobic glycolysis (and pyruvate the electron acceptor), letting the lactate waste product diffuse away into the water.
Could a deep ocean creature use some kind of bacteria in its body as a way to generate oxygen?
In circumstances where reduced carbon calories were not limiting but oxygen was, this would be the way to live.
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