Afalcultative anaerobic organism is able to perform aerobic respiration in the presence of oxygen and fermentation in the absence of oxygen. an example of a falcultative anaerobic organism is brewer's yeast, which is used in the production of alcoholic beverages. if sealed in a vessel containing oxygen gas, water, and sugar (glucose), the yeast will eventually deplete the oxygen gas. as long as glucose is available, however, the yeast can still survive. brewer's yeast use oxygen preferentially because