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. Species of both types have been isolated from a variety of environments, including fresh waters, sediments, soils, activated sludge, hot springs, hydrothermal vents and percolating water.
These bacteria 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 genera of these organisms are Caminibacter, Aquifex, Ralstonia and Paracoccus.

Examples

Hydrothermal vents

H₂ is an important electron donor in 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 form an important part of the ecosystem in deep sea habitats. Among the main chemosynthetic reactions that take place in hydrothermal vents, the oxidation of sulphide and hydrogen holds a central role. In particular, for autotrophic carbon fixation, hydrogen oxidation metabolism is more favored than sulfide or 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 because hydrogen has a more negative redox potential than NADH. Depending on the relative amounts of sulphide, hydrogen and other species, energy production by oxidation of hydrogen can be as much as 10–18 times higher than production by the oxidation of sulphide.

Knallgas bacteria

Aerobic hydrogen-oxidizing bacteria, sometimes called knallgas bacteria, are bacteria that oxidize hydrogen with oxygen as final electron acceptor. 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 is inhibited by the presence of oxygen and yet oxygen is still needed as a terminal electron acceptor in energy metabolism.
The word Knallgas means "oxyhydrogen" in German.

Strain MH-110

Ocean surface water is characterized by a high concentration of hydrogen. In 1989, an aerobic hydrogen-oxidizing bacterium was isolated from sea water. The MH-110 strain is able to grow under normal temperature conditions and in an atmosphere characterized by an oxygen saturation of 40%. This differs from the usual behaviour of hydrogen-oxidizing bacteria, which in general thrive 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 able to fix carbon dioxide using H₂ as their chemical energy source. Knallgas bacteria stand out from other hydrogen-oxidizing bacteria that, although using H₂ as energy source, are not able to fix CO₂, as Knallgas do.
This aerobic hydrogen oxidation, also known as the Knallgas reaction, releases a considerable amount of energy, having a ΔG^o of –237 kJ/mol. The energy is captured as a proton motive force for use by the cell.
The key enzymes involved in this reaction are the hydrogenases, which cleave molecular hydrogen and feed its electrons into the electron transport chain, where they are carried to the final acceptor, O₂, extracting energy in the process. The hydrogen is ultimately oxidized to water, the end product. The hydrogenases are divided into three categories according to the type of metal present in the active site. These enzymes were first found in Pseudomonas saccharophila, Alcaligenes ruhlandii and Alcaligenese eutrophus, in which there are two types of hydrogenases: cytoplasmic 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 the second is found.
While these microorganisms are facultative autotrophs, some are also able to live heterotrophicically using organic substances as electron donors; in this case, the hydrogenase activity is less important or completely absent.
However, knallgas bacteria, when growing as chemolithoautotrophs, can integrate a molecule of CO₂ to produce, through the Calvin–Benson cycle, biomolecules necessary for the cell:
6H₂ + 2O₂ + CO₂ + 5H₂O
A study of Alcaligenes eutropha, a representative knallgas bacterium, found that at low concentrations of O₂ and consequently with a low ΔH₂/ΔCO₂ molar ratio, the energy efficiency of CO₂ fixation increases to 50%. Once assimilated, some of the carbon may be stored as polyhydroxybutyrate.

Uses

Given enough nutrients, H₂, O₂ and CO₂, many knallgas bacteria can be grown quickly in vats using only a small amount of land area. This makes it possible to cultivate them as an environmentally sustainable source of food and other products. For example, the polyhydroxybutyrate the bacteria produce can be used as a feedstock to produce biodegradable plastics in various eco-sustainable applications.
Solar Foods is a startup that has sought to commercialize knallgas bacteria for food production, using renewable energy to split hydrogen to grow a neutral-tasting, protein-rich food source for use in products such as artificial meat. Research studies have suggested that knallgas cultivation is more environmentally friendly than traditional agriculture.