Facultative anaerobic beings, like the fungi Saccharomyces cerevisiae, a brewing yeast, can survive under oxygen-poor environments carrying out fermentation. However when oxygen is available these beings carry out aerobic respiration. Obligate anaerobic beings are those that cannot survive when oxygen is present.Some fungi, some bacteria (like the agent of botulism Clostridium botulinum, and the agent of tetanus,Clostridium tetani) and some protozoans are examples of obligate anaerobes.