Enhanced photo fermentative H₂ production from cornstalk by acid-tolerant R. capsulatus mutation

Culture pH is very crucial to hydrogen production in the photo-fermentation process. Excessively low pH can directly affect cell growth and hydrogen production by changing bacterial physiological characteristics. Thus, screening acid-tolerant strains that can alleviate acid stress with enhanced hydrogen production is essential. In this work, an acid-tolerant mutant Rhodobacter capsulatus AT11 was isolated by transposon technology, and its photosynthetic characteristic under different initial pH values was investigated. The results indicated that the H₂ yield of AT11 at initial pH of 5.5 was 4682.64 mL·L⁻¹, which was 310.65% higher than that of wild strain JL1, and the mutant gene in AT11 was electron transfer flavoprotein (etf). Then an etf-deletion mutant JL1401 was constructed by deleting etf to verify the effect of etf on H₂ production and investigate the acid-tolerance mechanism. Its H₂ production at initial pH of 5.5 was significantly increased compared to its parental strain JL1. In addition, its photosynthetic characteristic with ammonium additives was studied. Under the condition of 1.8 mM ammonium, hydrogen production was still up to 2637.14 ± 41.46 mL·L⁻¹, while hydrogen production of JL1 was inhibited. Cornstalk hydrolysate was used to evaluate the practical application of the constructed acid-tolerant mutant strains. The hydrogen yield reached 249.3 ± 16.8 mL-H₂·g⁻¹-cornstalk and 285.3 ± 19.1 mL-H₂·g⁻¹-cornstalk by R. capsulatus AT11 and JL1401, which were 2.9-fold and 3.3-fold compared to that of WT strain JL1, respectively.