Facultative bacteria that may proliferate in anoxic clarifier sludge conditions and may cause nitrogen bubbles are called which type of bacteria?

• A. Nitrifying
• B. Sulfate-reducing
• C. Methanogenic
• D. Denitrifying

Answer: (D)

Denitrifying bacteria drive denitrification, using nitrate as an electron acceptor when oxygen is scarce. In anoxic clarifier sludge, these facultative organisms proliferate and reduce nitrates to nitrogen gas, which can form nitrogen bubbles in the sludge. That gas production aligns with the description of nitrogen bubbles in anoxic conditions. Nitrifying bacteria need oxygen to oxidize ammonia, so they don’t thrive in anoxic sludge. Sulfate-reducing bacteria produce hydrogen sulfide, not nitrogen gas, under anaerobic conditions. Methanogens generate methane in strictly anaerobic environments and aren’t the source of nitrogen bubbles. So the bacteria described are denitrifying.