General Microbiology - Schlegel H. 1987

Utilization of inorganic hydrogen donors: aerobic chemolithotrophic bacteria
Oxidation of molecular hydrogen

Molecular hydrogen is produced during the anaerobic breakdown of organic matter in aquatic sediments and anaerobic soil microenvironments. Many Bacteria are capable of utilizing this hydrogen. A significant portion of it is oxidized by bacteria that live in syntrophy with the H₂-producing, fermentative organisms. The oxidation of H₂ by such bacteria is coupled with the reduction of sulfate to sulfide or CO₂ to methane (see Section 9.4). Almost all bacterial groups that synthesize ATP via Oxidative Phosphorylation under anaerobic conditions ("Anaerobic Respiration") include forms capable of utilizing molecular hydrogen as an electron donor (see Chapter 9, as well as Table 11.4).

Hydrogen is also generated in well-aerated ecosystems, such as soils supporting legume crops (e.g., soybeans, beans, clover). It is released as a byproduct of the reaction catalyzed by Nitrogenase (Section 13.3) and diffuses from the ROOT nodule bacteroids (many of which lack Hydrogenase activity) into the surrounding environment.

Class="center">Table 11.4. Utilization of hydrogen by bacteria capable of ATP regeneration via oxidative phosphorylation under anaerobic conditions

11.4.1 Aerobic Hydrogen-Oxidizing Bacteria

Bacteria that oxidize molecular hydrogen under aerobic conditions using oxygen as the terminal electron acceptor are collectively known as aerobic hydrogen-oxidizing (or simply hydrogen) bacteria. All of them are capable of both autotrophic CO₂ fixation and the utilization of organic substrates. Thus, hydrogen bacteria are facultative chemolithoautotrophs.

Certain aerobic hydrogen bacteria are also able to oxidize CO, using this gas as their sole electron donor and carbon source.

Isolation and Growth. Aerobic hydrogen bacteria grow on simple mineral media containing only inorganic salts, under a gas mixture consisting of 70% H₂, 20% O₂, and 10% CO₂ (by volume). They mediate transformations that roughly correspond to the equation

When the conditions outlined above are maintained, hydrogen bacteria can be readily isolated and enriched from soil and Water samples. They rank among the fastest-growing autotrophic organisms. Mesophilic strains can achieve autotrophic growth with a generation time of 3 hours, whereas thermophilic strains exhibit a generation time of 2 hours. Under heterotrophic conditions, some strains grow even faster. Under optimal growth conditions, the dry Cell mass yield can reach approximately 20 g per liter of medium.

Systematics. From a taxonomic perspective, aerobic hydrogen bacteria form a remarkably heterogeneous group. The majority of species belong to the Gram-negative genera Pseudomonas, Alcaligenes, Aquaspirillum, Paracoccus, and Xanthobacter, while some species belong to the Gram-positive genera Nocardia, Mycobacterium, and Bacillus (Table 11.5). Recently, the capacity for aerobic autotrophic growth utilizing H₂ has also been discovered in several other bacteria (notably Rhizobium and Derxia). Consequently, the Enzymes required for H₂ activation and CO₂ fixation are widely distributed across numerous bacterial taxa.

Hydrogen Utilization and METABOLISM/26.html">Energy Metabolism. Molecular hydrogen is incorporated into metabolic pathways through the action of hydrogenases.

¹ < СН₂O > corresponds to the approximate ratio of C, H, and O in cellular matter.

Table 11.5. Selected species of aerobic hydrogen-oxidizing bacteria

Aerobic hydrogen bacteria possess Two Types of hydrogenases: 1) soluble cytoplasmic enzymes that reduce NAD (H₂:NAD oxidoreductases), and 2) membrane-bound enzymes. Only a few bacteria (such as Alcaligenes eutrophus and A. ruhlandii) contain both types of hydrogenase. Nocardia possesses solely the soluble enzyme, whereas the majority of hydrogen bacteria contain only the membrane-bound form. Both enzymes can feed electrons into the Respiratory Chain. ATP regeneration proceeds with high efficiency. In terms of their respiratory chain components, Alcaligenes eutrophus and Paracoccus denitrificans closely resemble Mitochondria.

Heterotrophic and Mixotrophic Nutrition. Many hydrogen bacteria degrade hexoses and gluconate via the Entner-Doudoroff Pathway, oxidizing them completely to CO₂ and H₂O. They are also capable of utilizing numerous other Organic compounds, including branched-chain organic acids, compounds with aromatic and heterocyclic rings, and even testosterone. The Cells can accumulate poly-β-hydroxybutyric acid and Glycogen reserves.

If hydrogen bacteria have access to organic substrates alongside inorganic ones (CO₂ and H₂), they can adopt a mixotrophic lifestyle. This means they can assimilate organic molecules into cell material while deriving the energy required for this process from the oxidation of H₂. Under these conditions, the Complete oxidation of even a fraction of the organic nutrients becomes unnecessary. The capacity for mixotrophy is widespread among facultative autotrophs; they can obtain the energy needed for biosynthetic processes either through the Oxidation of reduced inorganic substrates (e.g., hydrogen sulfide or

sulfur in sulfur bacteria) or via Photosynthesis (in green Algae and higher plants).

Regulation of Substrate Utilization. In some hydrogen bacteria, substrate utilization is tightly regulated. For instance, cells of Alcaligenes eutrophus growing autotrophically lack the enzymes required for fructose Catabolism. If such cells are transferred to a fructose-containing medium and incubated aerobically, the Enzymes of the Entner-Doudoroff pathway are induced, and the cells grow. However, if incubation takes place under a gas mixture containing 80% H₂ and 20% O₂, neither enzyme synthesis nor cell growth is observed. Thus, molecular hydrogen represses The formation of enzymes required for fructose catabolism.

Cells that possess both the enzymes for fructose degradation and hydrogenase will utilize fructose very slowly when incubated in an H₂ + O₂ atmosphere: H₂ inhibits fructose utilization (with glucose-6-phosphate dehydrogenase serving as the primary target for hydrogen). In the presence of H₂ and O₂, the cells are capable of generating ATP and NADH₂. Both of these metabolites act as inhibitors of glucose-6-phosphate dehydrogenase. The marked deceleration of Glucose Catabolism in vivo is attributed to the inhibition of this enzyme's activity by NADH₂. In the experiments described, glucose-6-phosphate dehydrogenase Functions as the regulated enzyme of the Entner-Doudoroff pathway, playing a role analogous to that of Phosphofructokinase in the fructose bisphosphate pathway.

Carbon Monoxide-Utilizing Bacteria. Carbon monoxide is naturally produced under both anaerobic and aerobic conditions. Its biochemical conversion pathways and the microorganisms involved remain insufficiently characterized. Nevertheless, it is known that CO is oxidized to CO₂ by numerous bacterial species. Only aerobic bacteria are capable of growth using CO as their sole electron donor and carbon source. The oxidation of CO during the growth of Pseudomonas carboxidovorans proceeds According to the equation

Carbon is assimilated via CO₂ fixation through the ribulose bisphosphate cycle. CO-utilizing bacteria (carboxydobacteria) typically possess a membrane-bound hydrogenase and can also function as hydrogen bacteria.