Hydrogenophaga is a genus of Gram‑negative, rod‑shaped bacteria belonging to the family Comamonadaceae within the order Burkholderiales of the class Betaproteobacteria. Species of this genus are characterized by their ability to utilize molecular hydrogen (H₂) as an electron donor for aerobic respiration, a metabolic trait that gives the genus its name (from “hydrogen” + Greek phagein, “to eat”).
Taxonomy
- Domain: Bacteria
- Phylum: Proteobacteria
- Class: Betaproteobacteria
- Order: Burkholderiales
- Family: Comamonadaceae
- Genus: Hydrogenophaga (established 1991)
Morphology and Physiology
- Cells are typically straight rods, 0.5–0.8 µm in width and 1.0–3.0 µm in length.
- Motility is conferred by a single polar flagellum in most species.
- They are aerobic, oxidase‑positive, and catalase‑positive.
- The defining metabolic capability is chemolithoautotrophic growth on H₂; many species can also grow heterotrophically on a range of organic substrates (e.g., sugars, amino acids).
- Growth occurs optimally at neutral to slightly alkaline pH (6.5–8.0) and temperatures between 20 °C and 30 °C, although some isolates tolerate cooler or warmer conditions.
Ecological Distribution
Hydrogenophaga species have been isolated from diverse aquatic and terrestrial habitats, including:
- Freshwater lakes and rivers
- Soil and rhizosphere environments
- Wastewater treatment plants and activated sludge
- Bioreactors employing hydrogen as an energy source
Their capacity to oxidize hydrogen contributes to the regulation of H₂ concentrations in natural and engineered ecosystems.
Notable Species
The genus comprises several officially recognized species, among which the most frequently reported are:
| Species | Authority | Notes |
|---|---|---|
| Hydrogenophaga flava | (ex. Pseudomonas flava) K. Kishimoto et al. 1991 | The type species; exhibits yellow pigmentation. |
| Hydrogenophaga palleronii | (ex. Pseudomonas palleronii) K. Kishimoto et al. 1991 | Isolated from activated sludge. |
| Hydrogenophaga intermedia | K. Kishimoto et al. 1991 | Demonstrates versatile substrate utilization. |
| Hydrogenophaga laconesensis | M. Kuroda et al. 2014 | Isolated from lake sediment in Spain. |
| Hydrogenophaga yajiensis | M. Yoshikawa et al. 2014 | Obtained from a hot spring environment. |
Genomic Characteristics
Whole‑genome sequences are available for several Hydrogenophaga strains. Genomes range from approximately 3.0 to 4.5 Mbp, with a G + C content of 63–68 mol %. Genomic analyses reveal genes encoding:
- Multiple [Ni‑Fe] hydrogenases for H₂ oxidation
- Complete pathways for the tricarboxylic acid (TCA) cycle and oxidative phosphorylation
- Enzymes for the degradation of aromatic compounds, suggesting a role in pollutant biotransformation
Applications and Relevance
- Bioremediation: Certain strains can degrade phenolic and aromatic pollutants, making them candidates for wastewater treatment and soil remediation.
- Biohydrogen Production: Their efficient hydrogen oxidation has been investigated in the design of bioelectrochemical systems, such as microbial fuel cells, where they serve as biocatalysts for energy recovery.
- Ecological Studies: Hydrogenophaga serves as a model organism for studying microbial hydrogen cycling in natural environments.
History
The genus was formally proposed in 1991 following phylogenetic analyses that distinguished hydrogen‑oxidizing Pseudomonas isolates from other members of the genus. The reclassification was based on 16S rRNA gene sequence data and distinctive physiological traits, particularly hydrogen utilization.
References
- This entry is compiled from peer‑reviewed microbiology texts, taxonomic databases (e.g., LPSN, NCBI Taxonomy), and primary research articles describing the isolation, characterization, and genome sequencing of Hydrogenophaga species.