Are there any alternatives to oxygen for animal life?

Question

Almost all animal life on Earth uses oxygen, due to its powerful ability to oxidize various compounds

While there are many bacteria which use a different electron acceptor, I can find no examples of any more energetic organisms (like animals or protozoa) using such methods

The oxygen-alternatives must be able to function with carbon-water based life, and be stable enough to make up a usable portion of the atmosphere, like oxygen on Earth. It must also be efficient enough for animal-type life

Is there any substance that could fulfill these criteria?

Answer 1

Nitrate Reduction Cycle

Denitrification and nitrate ammonification are considered the highest-energy-yielding respiration systems in anoxic environments after oxygen has been consumed. The corresponding free energy changes are 7 and 35% lower than that of aerobic respiration, respectively.

[...]

Oxidation of biomass proceeds preferentially with oxygen as electron acceptor, and only after its consumption in deeper sediment layers are alternative electron acceptors such as nitrate, manganese(IV), ferric iron, sulfate, or CO2 reduced (15, 30). After oxygen reduction, reduction of nitrate to N2 (denitrification) or to ammonia (nitrate ammonification) is the highest-energy-yielding process as shown by the following equations calculated with glucose as a representative of biomass (calculated based on the tables in reference 25):

$$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O$$ $$(\Delta6º'=-2,870 \text{kJ per mol of glucose})$$

$$5C_6H_{12}O_6 + 24NO_3^- + 24H^+ \rightarrow 30CO_2 + 12N_2 + + 42H_2O$$ $$(\Delta6º'=-2,670 \text{kJ per mol of glucose})$$

$$C_6H_{12}O_6 + 3NO_3^- + 6H^+ \rightarrow 6CO_2 + 3NH_4^+ + 3H_2O$$ $$(\Delta6º'=-1,870 \text{kJ per mol of glucose})$$

As you can see, nitrate reduction is quite efficient. So if you don't have oxygen you could try using this.

Answer 2

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...

2. 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.

.