Harnessing pH and light cycles to boost microbial protein production in mixed culture purple non-sulfur bacteria wastewater bioremediation

The application of purple non‑sulfur bacteria (PNSB) in wastewater bioremediation has shown promise, but most studies have focused on agricultural and municipal wastewaters, often employing axenic cultures under controlled conditions. This study explores the use of PNSB-enriched mixed cultures treating high-strength Fischer-Tropsch process water (COD = 5-10 g/L; COD:N up to 59:1) for recovery of microbial protein under microaerobic-natural diurnal light cycles and varying pH conditions over 72 h batch/fed-batch trials. High biomass concentrations (1.1 to 1.4 g VSS /L), pollutant removal (771-1526 mg- COD ∙L -1 ∙d -1 and 16-56 mg- NH4 ∙L -1 ∙d -1 ) and Rhodopseudomonas sp. selectivity (up to 52 %) were achieved, even at high pH and illumination cycling. Acetate was also identified to be the most preferential VFA, followed by propionate. The recovered biomass mostly consisted of protein and was highest in transient conditions like fluctuating pH (56 %) and 12-h light cycling (48 %), compared to baseline conditions (41 %). Lipids comprised 23 % to 37 % of the biomass, mostly consisting of C16-C18 unsaturated fatty acids. Other valuable bioproducts present were carbohydrates (10 % to 13 %) and pigments (<2.5 %). Enrichment of PNSB in the mixed culture directly correlated with pollutant removal rates, biomass accumulation, and pigment content. The results demonstrate that while continuous lighting and pH regulation are preferential, PNSB-rich single-cell protein can be produced through the efficient conversion of high-strength wastewater under natural diurnal light cycles and unregulated pH conditions. Lipids present in the biomass relevant for biodiesel production also provide alternative valorisation pathways. • PNSB mixed culture treated high-strength wastewater under 12-h diurnal cycles. • Treatment rates of 771-1525 mgCOD∙L -1 ∙d -1 and 16-56 mg NH4 ∙L -1 ∙d -1 were achieved. • Recovered biomass mainly comprised protein (41 %-56 %) and lipids (23 %-37 %). • Pollutant removal and biomass concentration directly correlated with PNSB enrichment. • Transient culture (light cycling and pH fluctuation) enhanced biomass protein.