H2S-laden biogas triggers sulfur amino acids production in microbial protein synthesized during mixed culture fermentation by methane and sulfur oxidizing bacteria

Abstract

Biogas can be converted into high-value bioproducts through fermentation processes. This study proposes a conversion platform where sulfide-rich biogas is utilized as the substrate for the growth of a mixed culture of methane- and sulfur-oxidizing bacteria for the production of high-value and protein-rich microbial biomass. The role of several process variables such as biomethane vs biogas, synthetic vs real biogas, biogas composition and H2S concentration during the fermentation process performed in bubble column reactors was evaluated in terms of biomass growth, yield and volumetric productivity, protein content and amino acids profile as well as microbial community composition and evolution. Despite the presence of 1500 ppm of equivalent H2S in the synthetic biomethane or biogas (CH4:CO2 ratio of 70:30), good performances in terms of biomass growth (559.9 ± 13.3–677.8 ± 46.8 mg of volatile suspended solids (VSS) per liter), biomass yield (0.042 ± 0.004–0.050 ± 0.08 mg VSS per mg of CH4-equivalent chemical oxygen demand (CH4-COD)) as well as protein content (51.9 ± 3.6% − 59.2 ± 3.6% g protein/g VSS) were achieved. The presence of increasing sulfide concentrations until 4000 ppm of equivalent H2S was well tolerated by the mixed culture and increased the production of valuable S-amino acids, i.e., cysteine and methionine (61.3 mg/g biomass and 58.3 mg/g biomass, respectively), which was up to 2.50 and 2.57 times higher than in the absence of sulfide, respectively.