Hydrogen oxidation and Methanogenesis (definition and reaction).

 

Hydrogen-oxidizing bacteria are a group of  autotrophs that can oxidise H, and reduce O₂ via “knallgas” reaction, which is the reduction of O₂ with H₂. They use hydrogen as an electron donor and O₂ as electron acceptor.  This reaction yields energy for 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 subsequently used to fix carbon dioxide via the Calvin cycle.

Many organisms are capable of using hydrogen (H₂) as a source of energy. Hydrogenase enzyme helps in hydrogen oxidation, however, hydrogenase enzyme 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. Most hydrogen-oxidizing bacteria thus grow best under microaerophilic conditions.

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. These enzymes catalyze the oxidation of H₂:

H₂→2H⁺+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.  

Almost all hydrogen-oxidising bacteria are facultative chemoautotrophs, i.e, they can also grow chemoheterotrophically (chemoorganotrophically) with organic compounds as energy source.

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 bacteria have been isolated from a variety of environments, including fresh waters, sediments, soils, activated sludge, hot springs, hydrothermal vents and percolating water. 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. It was present in patients with chronic gastritis and gastric ulcers, conditions that were not previously believed to have a microbial cause.

More than 50% of the world’s population harbor H. pylori in their upper gastrointestinal tract. Over 80 percent of individuals infected with the bacterium are asymptomatic.  It can lead to the development of duodenal ulcers and stomach cancer.

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, as an energy source for its respiration and survival. This allows H. pylori to colonize the stomach and contributes to its ability to cause chronic inflammation and gastritis and stomach cancer. 

Source of Hydrogen: H. pylori obtain hydrogen from the fermentative metabolism of other intestinal bacteria. 

Hydrogenase Enzyme: The bacterium contains a specific enzyme called hydrogenase, which is responsible for oxidizing the molecular hydrogen. 

Energy Production: This oxidation process generates energy for the bacterium, allowing it to meet its metabolic needs. 

Role in Virulence: The hydrogen oxidation pathway, provides H. pylori with a high-energy non-carbon substrate. 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. 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. 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 producing  Knallgas bacteria can be used to produce biodegradable plastics. Solar Foods is a startup that uses knallgas bacteria to grow a neutral-tasting, protein-rich food source such as artificial meat. Research studies have suggested that knallgas cultivation is more environmentally friendly than traditional agriculture.

 

 Methanogenesis 

Methanogenesis, or biomethanation, is an anaerobic respiration resulting in the production of methane by the reduction of CO₂ to CH₄ and uses carbon as the terminal electron acceptor, H₂ is commonly used as electron donor however, formate, CO₂ and even certain organic compounds such as alcohol may also be used as electron donors.

Methanogenesis (methane production) is characteristic to a group of obligate anaerobic archaea (archaebacteria) called the methanogens (e.g., Methanobacterium, Methanobrevibacter, Methanococcus, Methanogenium, Methanospirillum, Methanomicrobium, etc.).

Methanogenesis thus involves the anaerobic conversion of carbon compounds to methane and typically follows four steps: hydrolysis, acidogenesis, acetogenesis, and methanogenesis. During hydrolysis, complex organic matter is broken down, followed by acidogenesis to produce volatile fatty acids. Acetogenesis converts these fatty acids into acetate. Finally, methanogens (archaea) utilize acetate, CO₂, and hydrogen gas to produce methane. 

The reduction of CO₂ to methane can be summarised in the following way

1. Hydrolysis:

Complex organic matter (proteins, carbohydrates, lipids) are broken down into simpler, smaller molecules, such as sugars, fatty acids and amino acids, by hydrolytic bacteria e.g., Clostridium spp.

2. Acidogenesis:

Acidogenic bacteria further break down the simpler organic compounds into volatile fatty acids (VFAs), alcohols, H₂, and CO₂. e.g., Bacteroides, Lactobacillus. 

3. Acetogenesis:

Acetogenic bacteria convert the volatile fatty acids, alcohols and other products from the previous steps into acetate, carbon dioxide, and hydrogen gas. Acetogenic bacteria include Syntrophomonas, Syntrophobacter etc.

    Acetogenic bacteria occur in syntrophy with methanogens - hydrogen must be kept at low levels for the reaction to proceed efficiently. The ratio of acetogenic bacteria and methanogens are critical to have a good yield of methane.

4. Methanogenesis:

This is the final step, carried out by methanogenic archaea. They consume the acetate, CO₂, and H₂ produced in the earlier stages to generate methane. 

There are three primary types of methanogenesis, depending on the substrates used by methanogens:

• Hydrogenotrophic methanogenesis: Methanogens reduce CO₂ with hydrogen gas (H₂) to produce methane. eg., Methanobacterium 

CO₂ + H₂ → CH₄ + H₂O (Hydrogenotrophic pathway)

• Aceticlastic methanogenesis: Methanogens cleave acetate into methane and CO₂. eg., Methanosarcina and Methanotricha

CH₃COOH → CH₄ + CO₂ (Acetoclastic pathway)

 

• Methylotrophic methanogenesis: Methanogens convert methylated compounds, such as methanol or methylamines, into methane. eg., Methanomethylovorans 

Methyl compounds → CH₄ (Methylotrophic pathway)

 

Ecological Role of Methanogens:

• Key players in anaerobic ecosystems – they remove end products like H₂ and acetate, allowing upstream fermentative and syntrophic processes to continue.
• Major contributors to global methane emissions, influencing climate change.
• Used in biogas production for renewable energy (methane as fuel).

The process of methanogenesis is crucial for the degradation of organic matter in anaerobic environments, such as wetlands, animal digestive tracts, landfills and anaerobic digesters used in waste treatment. The production of methane is the final step in the decomposition of biomass in most environments. Without methanogenesis, a great deal of carbon (in the form of fermentation products) would accumulate in anaerobic environments.

Biogenic methane can be collected and used as a sustainable alternative to fossil fuels.

Methanogenesis also occurs in the guts of humans and other animals, especially ruminants. In the rumen, anaerobic organisms, including methanogens, digest cellulose into forms usable by the animal. Without these microorganisms, animals such as cattle would not be able to consume grass. The useful products of methanogenesis are absorbed by the gut. Methane is released from the animal mainly by belching. The average cow emits around 250 liters of methane per day. 

Methane is one of the earth’s most important greenhouse gases, with a global warming potential 25 times greater than carbon dioxide. Therefore, the methane produced by methanogenesis in livestock contributes to global warming.