Sulfate, Nitrate, and Ferric Iron Are Readily Available, Exist in Appreciable

question 146

Multiple Choice

Sulfate, nitrate, and ferric iron are readily available, exist in appreciable amounts, and act as terminal electron acceptors in anaerobic respiratory pathways. If multiple electron acceptors existed before 579 million years ago, then why don't we see evidence of complex multicellularity earlier in Earth's history?

• A)These compounds are very stable as gasses, so they don't readily act as electron acceptors.
• B)The characteristics necessary for complex multicellularity didn't evolve until 575 million years ago.
• C)The ability of those compounds to accept electrons is lower than that of oxygen, so insufficient amounts of energy are liberated.
• D)The ability of these compounds to accept electrons is higher than that of oxygen, but they are scarce in the environment and thus could not support the evolution of complex multicellularity.

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