In a stream where the DO content is zero, an operator should expect to find:

• A. A variety of aquatic life
• B. An abundance of anaerobic bacteria
• C. A sterile environment
• D. An abundance of aerobic bacteria

Answer: (B)

When dissolved oxygen is zero, the environment becomes anoxic and aerobic bacteria can’t carry out respiration since they require oxygen. In these conditions, anaerobic bacteria that use alternative electron acceptors (like nitrate, sulfate, or carbon dioxide) dominate and actively decompose organic matter. That’s why an abundance of anaerobic bacteria is the expected finding. A variety of aquatic life would be unlikely in zero DO, a sterile environment isn’t realistic because microbes persist, and an abundance of aerobic bacteria wouldn’t be present without oxygen.