Enhanced Biohydrogen Production Through Continuous Fermentation of <em>Thermotoga neapolitana</em>: Addressing By-Product Inhibition and Cell Viability in Different Bioreactor Modes

Hydrogen has emerged as a leading candidate due to its high energy density and potential for climate-friendly production. Unlike fossil fuels, hydrogen can be produced sustainably and without direct greenhouse gas emissions. This study investigates the efficient biogenic production of hydrogen via the thermophilic bacterium Thermotoga neapolitana, focusing on optimising process configurations to maximise yield and productivity. To determine optimal conditions, a 1 l anaerobic bioreactor with online gas analytics was designed. The evaluated processing modes included continuous, fed batch and batch. A maximum hydrogen production rate of 96.1 ± 1.7 Nml·l−1·h−1 was observed in the continuous reactor. The optimal dilution rate was 0.07 h−1. A consistently high cell viability (97%) was also observed across various dilution rates. A detailed carbon balance indicates acetate as the main by-product, closely linked to the hydrogen production pathway. Compared to fed batch and batch, the hydrogen production rate could be increased and hold over longer time. In this way the continuous reactor design showed an additional method to produce hydrogen to the established ones. Fermentative hydrogen production is particularly promising when using carbohydrate containing biomass and biowaste, as it can be considered carbon dioxide neutral.