Microaerobic dark fermentation by purple bacteria as an emerging perspective for biohydrogen production—a review

While in recent years, a lot of research regarding microbial dark fermentation for biohydrogen production has been carried out focusing on improving well-established processes, this review aims to draw attention to the discovery of a new method of process control, i.e., microaerobic dark fermentation in purple non-sulfur bacteria. This new subsegment of research tries to rethink efficiencies of biohydrogen production and substrate conversion based on an increasingly comprehensive understanding of metabolic pathways, particularly in the area of interplay between aerobic and anaerobic metabolism and the control of this state. By combining hydrogen production through dark fermentation and partially activated photofermentation by a partially active nitrogenase (~ 12%), microaerobic fermentation has the potential to increase hydrogen efficiency. At the same time, it allows to maintain advantages of facultative aerobes, such as high-cell densities and growth rates. Studies show that very high yields of up to 8-12 mol H₂ mol^-1 substrate should theoretically be possible with this method, but the yields currently realized are still in the range already achieved with dark fermentation (0.2-1.6 mol H₂ mol^-1 substrate). This review provides an overview of the relevant bacteria, metabolism, cultivation, and challenges to further increase hydrogen productivity using microaerobic dark fermentation by purple bacteria. In addition, the model microorganism Rhodospirillum rubrum is considered in more detail to gain a deeper understanding of the processes at the metabolic level under microaerobic conditions. The review concludes with a detailed proposal of the challenges and opportunities to create a new, exciting perspective for biotechnological hydrogen production by microaerobic dark fermentation.