Potential of enriched phototrophic purple bacteria for H₂ bioconversion into single cell protein

Single cell protein (SCP) has emerged as an alternative protein source, potentially based on the recovery of carbon and nutrients from waste-derived resources as part of the circular economy. From those resources, gaseous substrates have the advantage of an easy sterilization, allowing the production of pathogen-free SCP. Sterile gaseous substrates allow producing pathogen-free SCP. This study evaluated the use of an enriched phototrophic purple bacteria (PPB) consortium for SCP production using H₂ and CO₂ as electron and C sources. The influence of pH (6.0-8.5), temperature (15-50 °C) and light intensity (0-50 W·m^-2) on the growth kinetics and biomass yields was investigated using batch tests. Optimal conditions were found at pH 7, 25 °C and light intensities over 30 W·m^-2. High biomass and protein yields were achieved (~ 1 g COD_biomass·g COD_H2consumed^-1 and 3.9-4.4 g protein·g H₂^-1) regardless of the environmental conditions, being amongst the highest values reported from gaseous streams. These high yields were obtained thanks to the use of light as a sole energy source by the PPB consortium, allowing a total utilization of H₂ for growth. Hydrogen uptake rates varied considerably, with values up to 61 ± 5 mg COD·d^-1 for the overall H₂ consumption rates and 2.00 ± 0.14 g COD·g COD^-1·d^-1 for the maximum specific uptake rates under optimal growth conditions. The latter value was estimated using a mechanistic model able to represent PPB growth on H₂. The biomass exhibited high protein contents (>50 % w/w) and adequate amino acid profiles, showing its suitability as SCP for feed. PPB were the dominant bacteria during the experiments (relative abundance over 80 % in most tests), with a stable population dominated by Rhodobacter sp. and Rhodopseudomonas sp. This study demonstrates the potential of enriched PPB cultures for H₂ bioconversion into SCP.