Hydrogen oxidizing bacteria


Hydrogen oxidizing bacteria are a group of facultative autotrophs that can use hydrogen as an electron donor.
They can be divided into aerobes and anaerobes. The former use hydrogen as an electron donor and oxygen as an acceptor while the latter use sulphate or nitrogen dioxide as electron acceptors. Some species of both bacteria types have been isolated in different environments, for example in fresh waters, sediments, soils, activated sludge, hot springs, hydrothermal vents and percolating water.
These organisms are able to exploit the special properties of molecular hydrogen thanks to the presence of hydrogenases. The aerobic hydrogen oxidizing bacteria are facultative autotrophs, but they can also have mixotrophic or completely heterotrophic growth. Most of them show greater growth on organic substrates.The use of hydrogen as an electron donor coupled with the ability to synthesize organic matter, through the reductive assimilation of CO2, characterize the hydrogen oxidizing bacteria. Among the most represented genres of these organisms we find: Caminibacter, Aquifex, Ralstonia and Paracoccus.

Examples

Hydrothermal vent bacteria

H2 is an important electron donor in a particular environment: Hydrothermal vents. In this environment hydrogen oxidation represents a significant origin of energy, sufficient to conduct ATP synthesis and autotrophic CO2 fixation so hydrogen oxidizing bacteria are relevant in deep sea habitats. Among the main chemosynthetic reactions that take place in hydrothermal vents, the oxidation of the sulphide and the hydrogen one covers a central role. In particular, for autotrophic carbon fixation, hydrogen oxidation metabolism is more favored than the sulfide/thiosulfate oxidation, although less energy is released. To fix a mole of carbon during the hydrogen oxidation, one third of the energy necessary for the sulphide oxidation is used. This is due to the redox potential of hydrogen, which is more negative than NAD /H. Based on the amount of sulphide, hydrogen and other farm biotics, this phenomenon can be intensified leading, in some cases, to an energy production by oxidation of the hydrogen of 10 -18 times higher than produced one by the sulphide oxidation.

Knallgas bacteria

Aerobic hydrogen oxidizing bacteria, sometimes called Knallgas bacteria, are bacteria that oxidize hydrogen with oxygen as final electron acceptor and source of energy. See microbial metabolism. These bacteria include Hydrogenobacter thermophilus, Cupriavidus necator, and Hydrogenovibrio marinus. There are both Gram positive and Gram negative Knallgas bacteria.
Most grow best under microaerobic conditions because the hydrogenase enzyme used in hydrogen oxidation is inhibited by the presence of oxygen, but oxygen is still needed as a terminal electron acceptor and energy source.
The word means "oxyhydrogen" in German.

Strain MH-110

Ocean’s surface water is characterized by a high concentration of hydrogen. In 1989, for the first time, an aerobic hydrogen oxidizing bacteria was isolated from sea water and the discovery of this strain was very important also because for the first time a hydrogen oxidizing bacteria was identified in normal temperature conditions. Experimentally it has been shown that the MH-110 strain is able to grow in an atmosphere characterized by an oxygen concentration of 40%. This differs from the usual behaviour of hydrogen oxidizing bacteria, which in general thrive strictly under microaerophilic conditions.
This strain is also capable of coupling the hydrogen oxidation with the reduction of sulfur compounds such as thiosulfate and tetrathionate.

Metabolism

Knallgas bacteria are a group of bacteria which are able to fix carbon dioxide using H2 as the electron donor and O2 as the terminal electron acceptor and energy source. Knallgas bacteria stand out from other hydrogen oxydizing bacteria which, although using H2 as an electron donor, are not able to fix CO2, as Knallgas do.
This aerobic hydrogen oxidation, also known as the Knallgas reaction, which releases a considerable amount of energy, determines the generation of a proton motive force :
H2 + O2 H2O ΔGo = -237 kJ/mol
The key enzymes involved in this reaction are the hydrogenases which lead the electrons through the electron transport chain, from hydrogen to the final acceptor, that is O2 which is actually reduced to water, the only product. The hydrogenases, which are divided into three categories according to the type of metal present in the active site, are the enzymes that allow the oxidation of hydrogen. The first evidence of the presence of these enzymes was found in Pseudomonas saccharophila, Alcaligenes ruhlandii and Alcaligenese eutrophus, in which there are two types of hydrogenases: cytoplasmatic and membrane-bound. While the first enzyme takes up hydrogen and reduces NAD+ to NADH for carbon fixation, the second is involved in the generation of the proton motive force. In most Knallgas bacteria only one type of hydrogenase was observed, the one bound to the membrane that provided hydrogen activation.
While these microorganisms are also defined as facultative autotrophs, some are also able to live in completely heterotrophic conditions using organic substances as electron donors; in this case, the hydrogenase activity is less important or completely absent.
However, Knallgas bacteria, growing as chemolithoautotrophs, as soon as they integrate a molecule of CO2 can produce, through the Calvin Benson Cycle or reverse citric acid cycle, biomolecules necessary for the cell:
6H2 + 2O2 + CO2 + 5H2O
A recent study of Alcaligenes eutropha, one of the most representative species of Knallgas bacteria, highlighted that at low concentrations of O2 and consequently with a low ΔH2/ΔCO2 molar ratio, the energy efficiency of CO2 fixation increases until 50%.This is an interesting characteristic of these microorganisms because once assimilated, carbon dioxide is reduced to polyhydroxybutyrate, whose derivatives, being biodegradable, are used in various eco-sustainable applications.

Uses

Given enough nutrients, H2, O2 and CO2, many knallgas bacteria can be grown very quickly in vats using only a small amount of area. This makes it possible to cultivate them as an environmentally sustainable source of food and other products.
Solar Foods is a startup that has sought to commercialize this, using renewable energy to split hydrogen to grow a neutral tasting protein-rich food source for use in products such as artificial meat.Independent studies have also shown that knallgas cultivation is more environmentally friendly than traditional crops.