In what order will bacteria break down the following oxygen-containing compounds: sulfate, carbon dioxide, and nitrate? That is, which sequence represents the source of bound oxygen requiring the least energy?

In what order will bacteria break down the following oxygen-containing compounds: sulfate, carbon dioxide, and nitrate? That is, which sequence represents the source of bound oxygen requiring the least energy?

• A. Nitrate, Carbon dioxide, Sulfate
• B. Sulfate, Nitrate, Carbon Dioxide
• C. Carbon dioxide, Nitrate, Sulfate
• D. Carbon dioxide, Sulate, Nitrate

Answer: (D)

When bacteria extract oxygen from compounds, the ease of removal depends on how tightly the oxygen is bound in each molecule. Nitrate has relatively weaker oxygen–nitrogen bonds compared with the stronger sulfur–oxygen bonds in sulfate and the very strong carbon–oxygen bonds in carbon dioxide. So oxygen can be liberated most readily from nitrate, then from sulfate, and hardest from carbon dioxide. The expected order by increasing energy required to remove oxygen is nitrate first, followed by sulfate, and finally carbon dioxide. The listed options don’t match this sequence, even though bond strength and redox energetics indicate nitrate would be the first source of oxygen, then sulfate, then carbon dioxide.

When bacteria extract oxygen from compounds, the ease of removal depends on how tightly the oxygen is bound in each molecule. Nitrate has relatively weaker oxygen–nitrogen bonds compared with the stronger sulfur–oxygen bonds in sulfate and the very strong carbon–oxygen bonds in carbon dioxide. So oxygen can be liberated most readily from nitrate, then from sulfate, and hardest from carbon dioxide. The expected order by increasing energy required to remove oxygen is nitrate first, followed by sulfate, and finally carbon dioxide. The listed options don’t match this sequence, even though bond strength and redox energetics indicate nitrate would be the first source of oxygen, then sulfate, then carbon dioxide.