Exploring the relation between Photoheterotrophic Hydrogen Production and Volatile Fatty Acid Metabolism in Rhodospirillum rubrum

In the past decades, research efforts have intensified to find alternative fuel sources to replace fossil hydrocarbons. Hydrogen gas, with its recyclability, non-polluting characteristics, and high energy content, has emerged as a promising future energy vector. While methods like water electrolysis and hydrocarbon steam reforming are known for dihydrogen production, their cost-effectiveness is hindered by high energy requirements. In this context, the photobiological production of H2 by purple non-sulfur bacteria (PNSB) offers an attractive pathway for renewable energy generation as this environmentally friendly process can integrate waste disposal. In PNSB, H2 generation occurs under anaerobic conditions in the presence of light, utilizing various organic substrates as carbon and electron sources. Depending on growth conditions, dihydrogen production may stem from nitrogen fixation or serve to dissipate reducing power during the assimilation of electron-rich carbon substrates such as volatile fatty acids (VFAs). Hence, a detailed understanding of the mechanisms driving dihydrogen metabolism in PNSB is essential to strike the optimal balance between costs and yields. A significant H2 production was recently observed in the PNSB strain Rhodospirillum rubrum S1H when supplying a VFA mixture containing propionate and butyrate as the sole carbon source. Interestingly, the presence of this VFA mixture appeared to eliminate the need for bicarbonate ions supplementation, facilitating the complete assimilation of butyrate and propionate by inducing H2 production. The present study aims, therefore, to unravel the relation between hydrogen and carbon metabolism in Rs. rubrum when supplied with a propionate and butyrate mixture.