Which statement best describes algae?

• A. Bacteria that digest waste.
• B. Viruses
• C. Microscopic plants that live in or on water, produce oxygen during the day, use oxygen at night, and affect pH and dissolved oxygen.
• D. Fungi that decompose organic matter.

Answer: (C)

Algae are microscopic plants that live in or on water and photosynthesize, releasing oxygen during the day and consuming oxygen through respiration at night. This daytime oxygen production and nighttime consumption, plus the way their photosynthesis uses carbon dioxide and can shift pH, directly affects dissolved oxygen and pH in the water. The other options describe organisms with different roles: bacteria that digest waste, viruses, and fungi that decompose organic matter, none of which define algae.