Examples of anaerobic respiration include nitrate reduction, denitrification, sulfate reduction and carbonate reduction. All of these methods use an electron acceptor other than oxygen and have a membrane-bound electron transport system. These anaerobic mechanisms also synthesize adenosine triphosphate, or ATP, through ATP synthase.
In nitrate reduction, organisms use nitrate instead of oxygen as the terminal electron acceptor. As this process is inefficient, organisms that use nitrate reduction do so only in circumstances when oxygen is significantly more available than nitrate. Denitrification is a related process that allows organisms to further reduce nitrate, which can accept up to six more electrons, to nitrogen gas. The toxic byproducts of denitrification are carefully regulated by the organism in a complex process that involves a large number of genes. Due to denitrification's complexity, few organisms are capable of using it. Denitrification also has an effect on the global nitrogen cycle, as most plants are unable to use nitrogen gas and instead rely on nitrate from the soil.
Sulfate reduction is used by organisms that live in environments completely devoid of oxygen. Eight electrons are added to sulfate, which usually becomes hydrogen sulfide. With an even lower energy yield than nitrate, sulfate reduction yields just enough energy to synthesize ATP. Hydrogen gas and small organic acids, such as acetate and lactate, are common sulfate reducers. Carbonate reduction is comparably inefficient, but carbonate is common in nature. This anaerobic process is believed to be one of the oldest, as it was used by members of Archaea.