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Knallgas bacteria : metabolism
The aerobic hydrogen oxidizing bacteria, more known as Knallgas bacteria, are a group of bacteria which are able to fix carbon dioxide using H2 as electron donor and energy source and O2 as terminal electron acceptor. Actually, Knallgas bacteria stand out from the 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 formation of proton motive force (PMF).

H2 + O2 $$\longrightarrow$$ H2O                                                          ΔG0 = -237 Kj

The key enzymes involved in this reaction are the hydrogenase which lead the electrons through the electron transport chain, from hydrogen to the final acceptor, that is O2 which is actually reduced in water, the only by-product. The hydrogenases that 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 has been found for the first time in Pseudomonas saccharophila, Alcaligenes ruhlandii and Alcaligenese eutrophus in which there were two types of hydrogenases: cytoplasmatic and membrane-bound. If the first enzyme takes up hydrogen and reduce the NAD+ in NADH for carbon fixation, the second is involved in the generation of the proton motive force. Anyway, in most of the knallgas bacteria only one type of hydrogensase was observed, the one bound to the membrane that provided hydrogen activation.

These microorganisms are also defined as facultative authotrophs, neverthless some of them are also able to live in completly heterotrophic conditions using organic substances as electron donor and energy source, even if in this case the hydrogenase activity takes on less importance or is completely absent.

However, Knallgas bacteria, growing as chemolithoautotrophs, as soon as they integrate every molecule of Co2, they can produce, through the Calvin Benson Cycle or reverse citric acid cycle (TCA cycle), biomolecules necessary for the cell.

6H2 + 2O2 + CO2 $$\longrightarrow$$ (CH2O) + 5H2O

In particular, a recent study carried out on Alcaligenes eutropha, one of the most representative species of Knallgas bacteria,  highlighted that at low concentrations of O2 (about 10 mol %) and consequently with a low ΔH2/ΔCO2 molar ratio (3.3), the energy efficiency of Co2 fixation increasing until of 50%.This is an interesting characteristic of these microorganisms because once assimilated, the carbon dioxide is reduced to polyhydroxybutyrate, from which its derivatives, being biodegradable, are used in various eco-sustainable applications.