Limits of Knallgas bacteria for autotrophic CO₂ conversion: physiology, H₂ kinetics and reactor constraints

Aerobic hydrogen-oxidizing bacteria, commonly referred to as Knallgas bacteria, are chemoautotrophic microorganisms that oxidize molecular hydrogen (H₂) while fixing carbon dioxide (CO₂) into cellular material. Historically studied as model systems for lithoautotrophic metabolism, these organisms have regained attention due to their potential for renewable H₂ utilization, land-independent biomass production, and electrochemically coupled CO₂ conversion. Among them, Cupriavidus necator has emerged as a promising industrial platform, while thermophilic and high-affinity H₂-oxidizers such as Kyrpidia spormannii are increasingly explored in bioelectrochemical systems. Despite growing interest, substantial gaps remain between laboratory demonstrations and scalable industrial performance. This review analyzes the physiological characteristics of Knallgas bacteria, including growth kinetics and H₂ oxidation parameters, and evaluates their translation into gas fermentation and oxic microbial electrosynthesis systems. Particular emphasis is placed on key engineering limitations, including H₂ mass transfer, electron partitioning during aerobic metabolism, reactor safety constraints, and electrochemical surface scaling.