Methanogenesis is among the most energy-dense forms of anaerobic respiration. Typically, it requires the environmental presence of hydrogen to reduce CO2; however, hydrogen can be generated from glucose in hydrogenic fermentation, which also produces some metabolic energy.

So, suppose we have an anoxic world where the animal-equivalents breathe in carbon dioxide instead of oxygen, exhale methane, and produce acetate, glyoxylate, and formate (and possibly traces of a variety of other light organic compounds) as additional metabolic wastes; effectively, carbon waste ends up having to be excreted in (the equivalent of) urine a byproduct of normal energy-producing metabolism, parallel to the excretion of nitrogenous wastes from the catabolism of proteins and nucleic acids.

Given that oxygenic phototsynthesis must be suppressed (if it exists at all, it at least must be uncommon, 'cause the whole point is that we want animals to breathe CO2, not oxygen), what does the rest of the ecosystem look like, in terms of basic anabolic / catabolic processes? In particular, what are plants doing, given that CO2 is abundant, methane reasonably common in the air, and animals are pumping out tons of other simple organics along with the methane?

Edit: For reference, the net reactions for acetogenic, glyoxylogenic and formogenic catabolism of glucose are as follows:

$C_6H_{12}O_6 → 2CHCO_2O^- + 2H^+ + 2CH_4$ Glucose ferments into glyoxylate and 2 methanes.

$2C_6H_{12}O_6 + 2H_2O + CO2 → 4CH_3COO^- + 4HCOO^- + 8H^+ + CH4$ Glucose is hydrolyzed and oxidized by carbon dioxide to produce 4 acetates, 4 formates, and methane.

Note that the glyoxylogenic reaction actually doesn't consume CO2--only the mixed-acid acetate / formate pathway; this is because the CO2 used in the glyoxylate pathway is actually endogenously generated during the initial breakdown of glucose via pyruvate. That pathway would be the equivalent of our anaerobic respiration. Both pathways hide a bunch of complexity where hydrogen and sometimes endogenous carbon dioxide are generated along the way, before eventually all of the hydrogen is used up creating various small organics.

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  • $\begingroup$ Well, in the real history of life what plant did was pollute the environment with oxygen; the oxygen was initially soaked up by the available iron (leading to the massive iron oxide deposits which we have been exploiting for thousands of years barely making a dent). When the available iron was exhausted the oxygen pollution lead to the extinction of the pre-oxygenation ecosystem. $\endgroup$ – AlexP Sep 1 '19 at 19:37

In particular, what are plants doing, given that CO2 is abundant, methane reasonably common in the air, and animals are pumping out tons of other simple organics along with the methane?

If one thing is sure, you can't have plants in such ecosystem.

Plant as we know them expel oxygen as byproduct of photosynthesis, and as you state

oxygenic phototsynthesis must be suppressed

If there is any photosynthetizer, it would probably follow the inverse path of methanogenyc respiration instead of

CO2 + 4 H2 → CH4 + 2 H2O + Energy

they would do something along the line of

CH4 + 2 H2O + Energy → CO2 + 4 H2

To store the species so synthesized the pluricellular organisms would need bags (Earth plants produce sugars, which are solid and easier to store), which would make them appear like large bubbles.

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  • $\begingroup$ If your photosynthesizers aren't producing an sugars, then what are the animals eating? And what happens to all of the organic wastes that animals are expelling? $\endgroup$ – Logan R. Kearsley Sep 1 '19 at 23:41

You have flipped the scenario currently in the sunny topside of earth.

Here, animals breathe O2 and use it to oxidize the reduced carbon fixed by plants. The animals breathe out CO2 and hydrogen (as H2O).

Plants scrounge up scarce CO2 and with hydrogen (obtained as H2O) , use the energy of the sun to fix it back into reduced carbon, as carbohydrates.

Flipping this in your scenario, your animals take in CO2 and hydrogen and expel reduced carbon as methane. "Plants" (considered as primary producers) will presumably take up the reduced carbon as methane, scrounge up scarce O2 and use some ambient energy to reform the CO2. Nice that the reduced carbon here is methane; if you are a plant there are better odds of CH4 drifting into your vicinity than glucose.

In your scenario the reduced carbon is methane; in ours we generally eat carbohydrates. It is all reduced carbon.

I think the trick is how much O2 is available. Prevalence of O2 will tip the balance one way or the other, as it presumably did in the early earth and still does in anaerobic / microaerobic environments.

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