About this Research Topic
Among the most ancient and widespread metabolic traits of life is the consumption and production of molecular hydrogen (H2). Three classes of metalloenzymes – [FeFe]-hydrogenase, [NiFe]-hydrogenase, and [Fe]-only hydrogenase – catalyze the reversible oxidation of H2 to electrons and protons. These three enzymes are phylogenetically unrelated and thus have converged evolutionarily to catalyse similar chemistry.
Approximately a third of sequenced microorganisms, spanning at least 70 microbial phyla, encode hydrogenases and are thus are predicted to be capable of metabolising H2. Functionally diverse microorganisms, among them aerobes and anaerobes, heterotrophs and autotrophs, and chemotrophs and phototrophs, oxidise H2 as a low potential electron donor, an activity typically (albeit not exclusively) attributed to distinct lineages of [NiFe]-hydrogenase enzymes. Other bacteria, archaea, and microbial eukaryotes evolve H2 as a diffusible endproduct during fermentative metabolism through the activity of [FeFe]- or [NiFe]-hydrogenases. In many organisms, the ability to metabolise H2 is a facultative trait that is regulated through the expression and maturation of hydrogenases. In such taxa, H2 represents a mechanism for organisms to supplement energy metabolism, thereby expanding their niche space in temporally and spatially variable ecosystems.
The implications of H2 in ecosystem level processes is increasingly being realized in both environmental and biomedical settings. A wide range of ecosystems have now been described where H2 cycling supports the bulk of primary production and where it forms the basis by which species interact, leading to ecologically structured communities. Much of the research on H2 metabolism to date has focused on ecosystems where H2 is present at elevated concentrations due to biological (e.g. anoxic sediments, gastrointestinal tracts) or geological activity (e.g. hydrothermal vents, subsurface systems). More recently, it has been recognised that atmospheric H2 serves as an energy source for the persistence of aerobic soil microorganisms. In parallel, medical microbiologists have shown that H2 metabolism is critical for the virulence of key pathogens, including Helicobacter, Salmonella, and Clostridium species.
This Research Topic will explore, through a series of original articles and reviews, emerging frontiers in microbial H2 metabolism. The topic will broadly cover: (i) the molecular biology, physiology, and biochemistry of H2 metabolism in microorganisms; (ii) the role of H2 metabolism in evolutionary, ecological, medical, and biogeochemical processes; and (iii) the biotechnological opportunities provided or inspired by microbial H2 metabolism.
Keywords: Hydrogen, Hydrogenase, Fermentation, Respiration, Carbon Fixation, Biofuel
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