They can survive and respire in the absence of oxygen.
Explanation
Respiration is the metabolic process by which organisms convert nutrients into energy. Based on the requirement of oxygen, organisms are classified into two primary categories: aerobes, which require oxygen, and anaerobes, which can function in the absence of oxygen.
Option Analysis
- They require an oxygen-rich environment for survival. is incorrect: Organisms that require an oxygen-rich environment for survival are called aerobes (e.g., most plants and animals). Anaerobes do not require oxygen and, in the case of obligate anaerobes, may even be killed by its presence.
- They can survive and respire in the absence of oxygen. is correct: Anaerobes are defined by their ability to survive and generate energy (respire) in the absence of molecular oxygen. They utilize alternative electron acceptors such as nitrate, sulfate, or carbon dioxide during respiration.
- They primarily include large mammals and birds. is incorrect: Large mammals and birds are obligate aerobes; they strictly require oxygen to sustain their high metabolic rates. Anaerobes are predominantly microscopic organisms, such as bacteria, archaea, and some protozoa.
- They produce water as the sole by-product of respiration. is incorrect: Water is the primary by-product of aerobic respiration (along with carbon dioxide). Anaerobic respiration or fermentation typically produces by-products such as ethyl alcohol, lactic acid, methane, or hydrogen sulfide, depending on the organism and the pathway used.
Key Takeaway
Anaerobes are organisms capable of surviving and respiring without oxygen. They are crucial in processes such as fermentation, biogas production, and the decomposition of organic matter in oxygen-deprived environments (e.g., anaerobic sludge digesters).