What happens during endogenous respiration?

• A. BOD is depleted only by external microbial activity; cells don't die.
• B. BOD depleted; bugs consume themselves; 70-80% cell tissue oxidized; ammonia converted to nitrates.
• C. BOD increases due to new cell growth.
• D. Nitrogen is reduced to ammonia.

Answer: (B)

Endogenous respiration is when microorganisms, after the readily available food has been exhausted, switch to oxidizing their own cellular material for energy. This self-consumption causes the BOD that’s measured to drop even though no new external substrate is present. A large portion of the cell mass is oxidized in the process—often cited around 70–80%—which means biomass is reduced as the cells effectively “eat” themselves to survive. If oxygen is available, nitrifying bacteria can also oxidize ammonia to nitrate, so ammonia-to-nitrate formation can occur in the same environment. Taken together, this describes why BOD is depleted, the microbes consume themselves, a large fraction of cell tissue is oxidized, and nitrification can produce nitrate.