Natural (geological or "white") hydrogen represents one of the most promising zero-emission primary energy sources. However, locating deep underground accumulations and mapping diffuse degassing pathways remains a significant exploration challenge.
Traditional geological survey methods can be substantially augmented by looking at biological response systems. Subsurface hydrogen seeps act as high-yield inorganic energy sources, creating distinct, measurable biomass anomalies in overlying soils, marine sediments, and groundwater. By analyzing the diversity and population density of hydrogen-oxidizing microorganisms, engineers and geoscientists can deploy biogeochemical mapping as a biological sensor network to identify hidden hydrogen reservoirs.
1. The Biogeochemical Detection Mechanism
Hydrogen oxidation is a high-energy chemosynthesis reaction yielding approximately 237 kJ per mole of molecular hydrogen. Because of this high energy return, molecular hydrogen ascending from the Earth's mantle or deep crust alters the ecological balance of the strata it traverses:
- Gas Transport Dynamics: In normal soil conditions, background hydrogen is maintained at low concentrations through interspecies transfer between primary anaerobes (producers) and aerobes (consumers). The soil essentially acts as a bacterial gas filter, preventing the gas from escaping into the atmosphere.
- High-Flux Anomalies: When a deep geological hydrogen stream intersects near-surface media, the hydrogen influx exceeds baseline biological production. This fuels a localized surge in microbial biomass.
2. Geological Hotspots & Sampling Targets
Biogeochemical prospecting focuses on specific geological structures where natural hydrogen generation, migration, and degassing are concentrated:
- Ultramafic and Igneous Rocks: Ophiolites, rift systems, and deep basement faults rich in dissolved and trapped fluid inclusions.
- Ring Subsidence Structures: Surface depressions that indicate active localized subsoil hydrogen degassing.
- Volcanic and Geothermal Systems: Geysers, fumaroles, hot springs, and solfataric seafloor vents.
- Ancient Groundwater Aquifers: Deep, geochemically evolved aquifers where microbial cell density correlates with groundwater age and dissolved hydrogen content.
- Hyperarid and Desert Environments: Systems where metabolically flexible bacteria (such as Actinobacteriota with nickel-iron hydrogenases) sustain themselves by oxidizing trace atmospheric and lithospheric hydrogen ("living on air").
3. Microbial Engine: Functional Classification
Hydrogen-oxidizing microorganisms do not form a single taxonomic group; they are classified by metabolic pathways and oxygen dependence. Their catalytic engine relies on hydrogenase enzymes (membrane-bound and cytoplasmic nickel-iron or iron-hydrogenases):
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Facultative Autotrophs (Knallgas Bacteria):
- Key Genera & Strains: Cupriavidus necator, Hydrogenovibrio marinus, Herbaspirillum autotrophicum, Aquifex aeolicus.
- Metabolic Characteristics: Fix carbon dioxide via the Knallgas reaction (hydrogen oxidation coupled with oxygen); utilize both organic and inorganic substrates.
- Typical Environments: Soils, freshwater lakes, hydrothermal vents, desert crusts.
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Aerobic Hydrogen Oxidizers (Non-Carbon Fixing):
- Key Genera & Strains: Acetobacter (e.g., Acetobacter peroxydans, Acetobacter hansenii), Azotobacter vinelandii.
- Metabolic Characteristics: Utilize hydrogen as an electron donor to reduce quinones and oxygen without fixing carbon dioxide; re-oxidize nitrogenase by-products.
- Typical Environments: Vegetation, decaying organic matter, neutral-to-basic soils.
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Anaerobic Hydrogen Oxidizers:
- Key Genera & Strains: Acetobacterium woodii, Clostridium aceticum, Caldimicrobium rimae.
- Metabolic Characteristics: Anaerobic chemolithotrophic growth; couple hydrogen oxidation with the reduction of sulfate, sulfur, or carbon dioxide to produce acetate.
- Typical Environments: Sewage sludge, anoxic sediments, deep underground strata.
4. Subsurface Engineering Risks: Storage & Biocorrosion
While hydrogen-consuming organisms serve as useful exploration markers, their presence poses severe operational challenges for Underground Hydrogen Storage and deep industrial infrastructure:
- Gas Loss and Souring: Hydrogenotrophic methanogens convert stored hydrogen into methane, while sulfate-reducing bacteria produce toxic, highly corrosive hydrogen sulfide.
- Microbially Influenced Corrosion: Cathodic depolarization driven by hydrogen-consuming biofilms accelerates the degradation of steel well casings, valves, and transport pipelines.
- Thermal Sterility Boundary: Research indicates that underground storage environments with temperatures above 122 °C remain sterile with respect to active hydrogen-consuming methanogens, acetogens, and sulfate reducers, establishing a critical parameter for site selection.
5. Future Research
Interest in hydrogen-oxidising microorganisms is currently growing more and more. Since they have found their application in various fields of biotechnological industry. But first of all the interest to these microorganisms is caused by the fact that they are a reference point for searching and determining the size of the natural hydrogen resource underlying them, because hydrogen is considered as a clean and environmentally friendly fuel of the future, the only by-product of the reaction of conversion of hydrogen into electricity is water. Natural hydrogen has been found in many geological environments, including igneous rocks, ophiolites, rift zones, geysers, hot springs, coal seams, deep faults, "subsidence ring structures", etc.
Considerable evidence is now accumulating on the existence of microbial communities capable of utilising and producing hydrogen in the subsurface.
Although the world of hydrogen-oxidising microorganisms is fairly well understood to date, some questions require further research, for example:
- How hydrogen exposure changes microbial community composition in the long term.
- What impact does an increase in microbial community abundance and activity have on the subsurface hydrogen cycle.
- The rate of microbial community growth in natural environments.
- Developing ways of sampling difficult to access environments, e.g. deep water springs, etc.
Hydrogen concentrations are often kept low by microbial consumption processes, which may hide an active hydrogen economic. Understanding how hydrogen-oxidising microorganisms influence hydrogen concentrations in geological environments will help to answer how much of hydrogen's potential as an energy resource is hidden in the interior of our planet. And answers to the questions of how hydrogen and hydrogen-oxidising microorganisms affect underground subsurface will help in solving problems with the exploration, storage and transport of hydrogen as an energy resource.
