How Efficient Could Anaerobic Megafauna Be?

Obviously, if available, atmospheric oxygen is a great source of energy. However, I'm surely not the first worldbuilder who wants an alien species which doesn't depend on it (whether due to having evolved on a largely oxygenless planet, or because of bioengineering that happened once the civilisation got advanced enough).

The question is, even in the best-case scenario (i.e. the most efficient plausible anaerobic metabolism within the constraints below), how inefficient would energy generation be for such an alien? I'm thinking primarily in terms of how much more food would be required for maintaining a level of activity similar to the one mammals, birds, reptiles etc. are capable of, but maybe I'm missing other forms in which inefficiencies would express themselves.

For bonus internet points, I would also like to know whether ability to switch between aerobic and anaerobic energy generation methods would result in a significant reduction of the efficiency of the former and/or latter, and/or carry other drawbacks.

Additional parameters:

• Carbon-based life, and biochemistry not radically different from Terrestrial life (panspermia is in play).
• Obviously we know anaerobic bacteria are a thing, but I'm looking for something at least approximating human size, thus the megafauna criterion.
• The organism should not be obligatorily anaerobic, that is, it should be able to withstand at least 20 kPa of partial O2 pressure without long-term ill effects.
• I'm looking for things that can hypothetically be achieved through evolution and/or genetic engineering, and is inheritable, but not for purely synthetic solutions like adding non-biological devices. Purely biological symbiotes are an edge case (mutualistic bacteria are OK, nanites are not).
• I'd like to know the best-case solutions to the question without employment of blatantly unscientific handwaves before even contemplating adding any.

posted almost 5 years ago

vicky_molokh‭

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