Nitrogen fixation is an extremely energy and electron intensive process, in order to break the triple bond in N2 and reduce it to NH3. It requires a particular enzyme known as nitrogenase , which is inactivated by O2. Thus, nitrogen fixation must take place in an anaerobic environment. Aerobic nitrogen-fixing organisms must devise special conditions or arrangements in order to protect their enzyme. Nitrogen-fixing organisms can either exist independently or pair up with a plant host:.
Assimilation is a reductive process by which an inorganic form of nitrogen is reduced to organic nitrogen compounds such as amino acids and nucleotides, allowing for cellular growth and reproduction.
Only the amount needed by the cell is reduced. Assimilative nitrate reduction is a reduction of nitrate to cellular nitrogen, in a multi-step process where nitrate is reduced to nitrite then ammonia and finally into organic nitrogen. As mentioned above, nitrification is a 2-step process performed by chemolithotrophs using a reduced or partially reduced form of nitrogen as an electron donor to obtain energy.
One group of chemolithotrophs can perform the first part of the nitrification process, ammonia oxidation, while a different group of chemolithotrophs can perform the nitrite oxidation that occurs in the second part of nitrification.
A non-nitrogen compound would serve as the electron acceptor. Denitrification refers to the reduction of NO3- to gaseous nitrogen compounds, such as N2. Denitrifying microbes perform anaerobic respiration, using NO3- as an alternate final electron acceptor to O2. This is a type of dissimilatory nitrate reduction where the nitrate is being reduced during energy conservation, not for the purposes of making organic compounds. This produces large amounts of excess byproducts, resulting in the loss of nitrogen from the local environment to the atmosphere.
Anammox or an aerobic amm onia ox idation is performed by marine bacteria, relatively recently discovered, that utilize nitrogen compounds as both electron acceptor and electron donor. Ammonia is oxidized anaerobically as the electron donor while nitrite is utilized as the electron acceptor, with dinitrogen gas produced as a byproduct. Just like denitrification, the anammox reaction removes fixed nitrogen from a local environment, releasing it to the atmosphere.
Skip to content Chemolithotrophy Chemolithotrophy is the oxidation of inorganic chemicals for the generation of energy. Chemolithotrophy Pathways. Electrons donors Chemolithotrophs use a variety of inorganic compounds as electron donors, with the most common substances being hydrogen gas, sulfur compounds such as sulfide and sulfur , nitrogen compounds such as ammonium and nitrite , and ferrous iron.
Hydrogen oxidizers — these organisms oxidize hydrogen gas H2 with the use of a hydrogenase enzyme. Both aerobic and anaerobic hydrogen oxidizers exist, with the aerobic organisms eventually reducing oxygen to water. Sulfur oxidizers — as a group these organisms are capable of oxidizing a wide variety of reduced and partially reduced sulfur compounds such as hydrogen sulfide H2S , elemental sulfur S0 , thiosulfate S2O , and sulfite SO Sulfate SO is frequently a by-product of the oxidation.
Often the oxidation occurs in a stepwise fashion with the help of the sulfite oxidase enzyme. Nitrogen oxidizers — the oxidation of ammonia NH3 is performed as a two-step process by nitrifying microbes, where one group oxidizes ammonia to nitrite NO2- and the second group oxidizes the nitrite to nitrate NO Chemolithoheterotrophs are a special kind of chemotroph that use inorganic compounds as an energy source and reduced organic compounds as a carbon source.
They are known as mixotrophs. Chemotrophs use fermentation and respiration to obtain energy. Fermentation is restricted to organotrophs. Aerobic Respiration. Anaerobic Respiration. Skip to main content Skip to table of contents. The chemotroph designation is in contrast to phototrophs, which utilize solar energy.
Chemotrophs can be either autotrophic or heterotrophic. Chemoautotrophs generally fall into several groups: methanogens, halophiles, sulfur oxidizers and reducers, nitrifiers, anammox bacteria, and thermoacidophiles. Chemolithotrophic growth could be dramatically fast, such as Thiomicrospira crunogena with a doubling time around one hour.
In chemolithotrophs, the compounds — the electron donors — are oxidized in the cell, and the electrons are channeled into respiratory chains, ultimately producing ATP.
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