Answer:
Both O2 and NO3 are possible electron acceptors.
Step-by-step explanation:
Respiration with oxygen and some times nitrates as a terminal electron acceptor generates more energy than respiration with other terminal electron acceptors.
This plays out in anaerobic form of respiration when oxygen switches to nitrate respiration when oxygen is depleted but that denitrification in their preferred respiration pathway. Their metabolic adaptations allow some species to grow larger than predicted by cell physiology of aerobic organisms due to oxygen limitation. Nitrate is an important macronutrient, and denitrification is the main oceanic nitrate sink. Several postulations too can help to constrain biogeochemical models for marine nitrate cycling.