Microbial protein production under mixotrophic mode: ammonium assimilation pathway and C/N ratio optimization

Abstract

Producing microbial protein (MP) is considered as a promising method to alleviate the demand for the global protein supply. The conversion of carbon dioxide (CO2) and organic carbon into MP by hydrogen oxidizing bacteria (HOB) is considered as a potent solution. In this study, under mixotrophic condition, MP production was 2.4 g/L, higher than the sum of that in autotrophic and heterotrophic mode (1.9 g/L), showing the mutualistic relationship of two pathways. According to functional enzyme analysis, the abundance of glutamate dehydrogenase and glutamine synthetase in the heterotrophic and mixotrophic modes was obviously enhanced (80%−800% increment), indicating that adding soluble organics could enhance both the direct (“NH4+ → L-Glutamate”) and the indirect pathway (“NH4+ → L-Glutamine → L-Glutamate”) of NH4+ assimilation in the mixed HOB consortium. For further enhancing MP production, different C/N ratio was evaluated based on mixotrophic mode, achieving amino acid production of 3.3 ± 0.1 g/L and high protein content (58.1%) in C/N-9 reactor. Corynebacterium was dominant (91.4%). In nitrogen assimilation pathway, the higher abundance of glutamine synthetase than glutamate dehydrogenase suggested that NH4+-N was mainly converted via the indirect pathway. With the increase of C/N ratio, 24–48% increment was obtained on glutamine synthetase, along with the decrement of glutamate dehydrogenase. Thus, high C/N ratio could boost the indirect pathway, while performing the negative effect on the direct pathway.