system
Given the following post, respond as if you're commenting on Reddit:
human
What could replace oxygen as a metabolic agent for extraterrestrial life?

I've seen a lot of topics about how carbon could be replaced (by silicon, usually) as a biological building block. What elements could replace oxygen or CO2s (and let's even include water) roles in the metabolic process for, say, life on a planet with no oxygen?
gpt
Great question!  Among other purposes, a major role of oxygen is to serve as a final electron acceptor during aerobic respiration.  In short, because it is a pretty electronegative element, organisms are able to generate energy by shuffling electrons to oxygen gas.

Many organisms on Earth have found ways of using other elements as electron acceptors (i.e., anaerobic respiration).  The energy that organisms acquire from these reactions is related to the electronegativity of the final electron acceptor.  Some common substitutes for oxygen are iron (as ferric iron; Fe^3+ ), nitrogen (as nitrate; NO3^- ), sulphur (as sulfate; SO4^2- ), and even carbon (as CO2 or even as organic compounds).

Anaerobic respiration is very important to biogeochemical cycling in ecological systems.  For example, nitrate reduction and carbon reduction ultimately produce nitrogen gas and methane, respectively, which serve as mechanisms for the export of nitrogen and carbon from ecosystems into the atmosphere.  As a greenhouse gas, methane production from warming arctic wetlands has implications for climate change around the globe -- all because bacteria living in oxygen-free soils use a different electron acceptor to survive!