<P> The basic mechanism for fermentation remains present in all cells of higher organisms . Mammalian muscle carries out the fermentation that occurs during periods of intense exercise where oxygen supply becomes limited, resulting in the creation of lactic acid . In invertebrates, fermentation also produces succinate and alanine . </P> <P> Fermentative bacteria play an essential role in the production of methane in habitats ranging from the rumens of cattle to sewage digesters and freshwater sediments . They produce hydrogen, carbon dioxide, formate and acetate and carboxylic acids; and then consortia of microbes convert the carbon dioxide and acetate to methane . Acetogenic bacteria oxidize the acids, obtaining more acetate and either hydrogen or formate . Finally, methanogens (which are in the domain Archea) convert acetate to methane . </P> <P> Fermentation reacts NADH with an endogenous, organic electron acceptor . Usually this is pyruvate formed from sugar through glycolysis . The reaction produces NAD+ and an organic product, typical examples being ethanol, lactic acid, carbon dioxide, and hydrogen gas (H). However, more exotic compounds can be produced by fermentation, such as butyric acid and acetone . Fermentation products contain chemical energy (they are not fully oxidized), but are considered waste products, since they cannot be metabolized further without the use of oxygen . </P> <P> Fermentation normally occurs in an anaerobic environment . In the presence of O, NADH and pyruvate are used to generate ATP in respiration . This is called oxidative phosphorylation, and it generates much more ATP than glycolysis alone . For that reason, fermentation is rarely utilized when oxygen is available . However, even in the presence of abundant oxygen, some strains of yeast such as Saccharomyces cerevisiae prefer fermentation to aerobic respiration as long as there is an adequate supply of sugars (a phenomenon known as the Crabtree effect). Some fermentation processes involve obligate anaerobes, which cannot tolerate oxygen . </P>

Where do you think the bacteria for fermentation came from