Final answer:
Oxygen is toxic to obligate anaerobes, which are organisms that do not need oxygen for growth and might even die when exposed to it.
Step-by-step explanation:
Oxygen is known to be toxic to obligate anaerobes. These organisms do not require oxygen for growth and may even die in its presence. Obligate anaerobes use molecules other than oxygen as terminal electron acceptors. Examples of obligate anaerobes include Clostridium perfringens, which can cause diseases like gas gangrene when infecting wounds. These bacteria are prevalent in environments where anaerobic conditions exist, such as deep sediments, still waters, and the human gut, where they represent a large fraction of microbes.