In aerobic organisms, electrons are shuttled to an electron transport chain, and the final electron acceptor is oxygen. Molecular oxygen is an excellent electron acceptor. Anaerobes instead use less-oxidizing substances such as nitrate (NO 3), fumarate (C 4H 2O 4), sulfate (SO 4), or elemental sulfur (S). These terminal electron acceptors have smaller reduction potentials than O2. Less energy per oxidized molecule is released. Therefore, anaerobic respiration is less efficient than aerobic.
An anaerobic organism or anaerobe is any organism that does not require molecular oxygen for its growth. It may react negatively or even die in the presence of free oxygen. Anaerobic organisms do not use oxygen as a terminal electron acceptor in their respiration process to produce energy, but a less powerful oxidizing agent, such as nitrate, ferric ion, Mn(IV), sulfate or bicarbonate anions. In contrast, an aerobic organism (aerobe) is an organism that requires a sufficiently oxygenated environment to respire, produce its energy, and thrive. Because the anaerobic energy production was the first mechanism to be used by living microorganisms in their evolution and is much less efficient than the aerobic pathway, anaerobes are practically, de facto, always unicellular organisms (e.g. bacteria and archaea (prokaryotes), or protozoans (eukaryotes). However, a minuscule multicellular organism, with an exceptionally rare metabolism and surviving in a hypersaline brine pool in the darkness of the bottom of the Mediterranean Sea, has been recently discovered. Meanwhile, it remains a scientific curiosity, as the much higher energy requirements of most multicellular organisms cannot be met by anaerobic respiration. Most fungi (eukaryotes) are obligate aerobes, requiring oxygen to survive and grow; however, some species, such as the Chytridiomycota that reside in the rumen of cattle, are obligate anaerobes; for these species, anaerobic respiration is used because oxygen would disrupt their metabolism or kill them. The deep seafloor and its underlying unconsolidatedsediments ranks among the largest potential habitats for anaerobic microorganisms on Earth. Moreover, chemoautotroph microbes also thrive around hydrothermal vents, discharging hot water on the oceanseabed near mid-ocean ridges, where anaerobic conditions prevail. These microbes produce energy in the absence of sunlight or oxygen through a process called anaerobic respiration, whereby inorganic compounds and ions such as protons (H), elemental sulfur and its derivatives (SO2−4, S2O2−3), or ferric ions, are reduced to drive oxidative phosphorylation.
Most animals derive most of their energy from aerobic respiration, namely combining the carbohydrates, fats, and proteins with oxygen from air or dissolved in water. Other smaller components of the diet, such as organic acids, polyols, and ethanol (drinking alcohol) may contribute to the energy input. Some diet components that provide little or no food energy, such as water, minerals, vitamins, cholesterol, and fiber, may still be necessary for health and survival for other reasons. Some organisms have instead anaerobic respiration, which extracts energy from food by reactions that do not require oxygen.
Most sulfate-reducing microorganisms can also reduce some other oxidized inorganic sulfurcompounds, such as sulfite (SO 3), dithionite (S 2O 4), thiosulfate (S 2O 3), trithionate (S 3O 6), tetrathionate (S 4O 6), elemental sulfur (S8), and polysulfides (S n). Other than sulfate reduction, some sulfate-reducing microorganisms are also capable of other reactions like disproportionation of sulfur compounds. Depending on the context, "sulfate-reducing microorganisms" can be used in a broader sense (including all species that can reduce any of these sulfur compounds) or in a narrower sense (including only species that reduce sulfate, and excluding strict thiosulfate and sulfur reducers, for example).
Anaerobic respiration in the context of Primary nutritional groups
Primary nutritional groups are groups of organisms, divided according to the sources of energy, carbon, and electrons needed for living, growth and reproduction. The sources of energy can be light or chemical compounds; the sources of carbon can be of organic or inorganic origin; the source of electron can be organic or inorganic.
Anaerobic respiration in the context of Lithotroph
Lithotrophs are a diverse group of organisms using an inorganic substrate (usually of mineral origin) to obtain reducing equivalents for use in biosynthesis (e.g., carbon dioxide fixation) or energy conservation (i.e., ATP production) via aerobic or anaerobic respiration. While lithotrophs in the broader sense include photolithotrophs like plants, chemolithotrophs are exclusively microorganisms; no known macrofauna possesses the ability to use inorganic compounds as electron sources. Macrofauna and lithotrophs can form symbiotic relationships, in which case the lithotrophs are called "prokaryotic symbionts". An example of this is chemolithotrophic bacteria in giant tube worms; or plastids, which are organelles within plant cells that may have evolved from photolithotrophic cyanobacteria-like organisms. Chemolithotrophs belong to the domains Bacteria and Archaea. The term "lithotroph" was created from the Greek terms 'lithos' (rock) and 'troph' (consumer), meaning "eaters of rock". Many but not all lithoautotrophs are extremophiles.
The last universal common ancestor of life is thought to be a chemolithotroph. Different from a lithotroph is an organotroph, an organism which obtains its reducing agents from the catabolism of organic compounds.
