Microbial protein production from sulfide-rich biogas through an enrichment of methane- and sulfur-oxidizing bacteria

This study evaluated the possibility of combining methane oxidizing bacteria (MOB) with sulfur oxidizing bacteria (SOB) to enable the utilization of sulfide-rich biogas for microbial protein production. For this purpose, a MOB-SOB mixed-culture enriched by feeding both methane and sulfide was benchmarked against an enrichment of solely MOB. Different CH4:O2 ratios, starting pH values, sulfide levels and nitrogen sources were tested and evaluated for the two enrichments. The MOB-SOB culture gave promising results in terms of both biomass yield (up to 0.07 ± 0.01 g VSS/g CH4-COD) and protein content (up to 73 ± 5% of VSS) at 1500 ppm of equivalent H2S. The latter enrichment was able to grow also under acidic pH (5.8-7.0), but as inhibited outside the optimal CH4:O2 ratio of 2:3. The obtained results show the capability of MOB-SOB mixed-cultures to directly upcycle sulfide-rich biogas into microbial protein potentially suited for feed, food or biobased product applications.