Hydrogen oxidation

Hydrogen-oxidizing bacteria are a group of facultative autotrophs that can use hydrogen as an electron donor. They oxidise H₂ (electron donor) and reduce O₂ (electron acceptor) via “knallgas” reaction.

"Knallgas” reaction is the reduction of O₂ with H₂. This reaction yields energy which is used in CO₂ fixation. Hydrogen oxidizing bacteria are also called Knallgas-bacteria. These include Hydrogenobacter thermophilus, Hydrogenovibrio marinus, and Helicobacter pylori. 

There are both Gram positive and Gram negative knallgas bacteria. They can be aerobes and anaerobes. Aerobic bacteria use hydrogen as an electron donor and oxygen as an acceptor while anaerobes use hydrogen as an electron donor and sulphate or nitrogen dioxide as electron acceptors.

Hydrogen is oxidized by a membrane-bound hydrogenase causing proton pumping along with electron transfer to various quinones and cytochromes. In many organisms, a second cytoplasmic hydrogenase is used to generate reducing power in the form of NADH, which is  used to fix carbon dioxide via the Calvin cycle.

Thus, many organisms are capable of using hydrogen (H₂) as a source of energy using hydrogenase enzyme which help in hydrogen oxidation. Most grow best under microaerophilic conditions because the hydrogenase enzyme used in hydrogen oxidation is inhibited by the presence of oxygen. Oxygen is still needed as a terminal electron acceptor. Typically, oxygen levels of about 5-10% support best growth of these bacteria.

Thus, the use of hydrogen as an electron donor and the ability to synthesize organic matter characterize the hydrogen-oxidizing bacteria.

Importance of Hydrogenase enzymes 

Hydrogenase enzyme is crucial for energy generation in hydrogen-oxidizing chemolithotrophs that use molecular hydrogen (H₂) as an electron donor. These enzymes catalyze the oxidation of H₂:

$$\; \; \; \; \; \; \; \; \; \; \; \; \; H₂\to 2{\text{H}}^{+ }+ 2e^{-}$$

The released electrons (e⁻) are then passed into the electron transport chain (ETC). 

As electrons flow through the ETC:

• A proton motive force (PMF) is generated across the membrane.

• This PMF drives ATP synthesis via ATP synthase (oxidative phosphorylation).

In addition, hydrogenase enzymes also contribute to the generation of reducing power (e.g., NADH or NADPH), which is essential for:

• Carbon fixation (e.g., via the Calvin cycle in autotrophs)

• Other biosynthetic reactions

Hydrogen oxidizing bacteria are both gram-positive and gram-negative. The best studied genera of this group of bacteria are Ralstonia, Pseudomonas, Paracoccus, and Alkaligenes; others are Acidovorax, Aquaspirillum, Hydrogenophaga, Hydrogenobacter, Bacillus, Aquifex, and Mycobacterium. Hydrogen-oxidizing bacteria have been isolated from a variety of environments, including fresh waters, sediments, soils, activated sludge, hot springs, hydrothermal vents etc. 

Almost all hydrogen-oxidising bacteria are facultative chemoautotrophs, i.e, they can also grow chemoheterotrophically. This means that the hydrogen-oxidising bacteria can switch between chemoautotrophic and chemoheterotrophic (chemoorganotrophic) modes of metabolism and generally do so whenever required. 

This is a major distinction between hydrogen oxidising bacteria and many sulphur-oxidising bacteria or nitrifying bacteria; most of the isolates from latter two groups are obligate chemoautotrophs.

Hydrogen-oxidizing organisms, such as Cupriavidus necator (formerly Ralstonia eutropha), often inhabit oxic-anoxic interfaces in nature to use hydrogen produced by anaerobic fermentative organisms while still maintaining a supply of oxygen.

Helicobacter pylori 

H. pylori,  is a Gram-negative, microaerophilic bacterium found in the stomach, identified in 1982 by Barry Marshall and Robin Warren. They found that it was present in patients with chronic gastritis and gastric ulcers, conditions that were not previously believed to have a microbial cause. It is also linked to the development of duodenal ulcers and stomach cancer. Over 80 percent of individuals infected with the bacterium are asymptomatic.  

More than 50% of the world’s population harbor H. pylori in their upper gastrointestinal tract.

Helicobacter pylori is a hydrogen oxidizing (H₂-oxidizing) bacterium, also known as a Knall-gas bacterium. It utilizes hydrogenase to oxidize molecular hydrogen, produced by other intestinal bacteria. This oxidation process generates energy for the bacterium, allowing it to respire and meet its metabolic needs. 

 This allows H. pylori to colonize the stomach and contributes to its ability to cause chronic inflammation and gastritis and stomach cancer.

While most H2-oxidizing bacteria can use carbon dioxide to fix carbon, H. pylori does not use the Calvin cycle. Instead, it uses organic carbon from its environment, making it a mixotroph. 

Hydrothermal vents

H₂ is an important electron donor in hydrothermal vents. Hydrogen oxidation represents a significant origin of energy in this environment  sufficient to perform ATP synthesis and autotrophic CO₂ fixation. So hydrogen-oxidizing bacteria form an important part of the ecosystem in deep sea habitats. The oxidation of sulphide and hydrogen is important among the main chemosynthetic reactions that take place in hydrothermal vents.

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. 

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, 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.