Carbon-efficient microbial protein production via continuous co-cultivation of methane- and hydrogen-oxidizing bacteria.

Microbial protein is a resource-efficient alternative to conventional protein, and gas-fed systems based on methane- and hydrogen-oxidizing bacteria are attractive because they directly convert gaseous C1 substrates into biomass, without reliance on arable land or organic feedstocks. We examined whether co-cultivating these organisms improves carbon retention by enabling in situ reuse of CO2 released during CH4 oxidation. After selecting a compatible pair (Methylomonas koyamae and Cupriavidus necator), a continuous airlift reactor was operated in four phases with progressively reduced external CO2 supply. Biomass in the co-culture reached 2.1 ± 0.5 g L-1, with protein contents of 50-65% (dry weight). Off-gas CO2 decreased to near zero, corresponding to a marked increase in carbon-use efficiency from 47% to 91%. Amino acid composition and digestibility, expressed as the Digestible Indispensable Amino Acid Score, remained stable across phases, and sensory evaluation indicated a lighter colour and cleaner aroma for the co-culture biomass. This study demonstrates a continuous methane- and hydrogen-oxidizing bacteria process achieving near-complete CO2 recycling and high-quality microbial protein production.