Comprehensive light strategy for regulating photosynthetic bacteria in wastewater treatment systems for single-cell protein production

New protein resources can be developed to alleviate the shortage of protein in the feed industry. Photosynthetic bacteria (PSB) can serve as unique microbial protein sources. Among them, the light environment is a crucial influencing factor in the growth and metabolism of PSB. In this study, the light supply strategies were employed to enhance the growth of Rhodopseudomonas palustris (R. palustris) and the synthesis of single-cell protein (SCP) in the wastewater resource systems of PSB. Additionally, a systematic analysis was made on the correlations between substance synthesis and pollutant degradation under different light supply strategies. The results indicated that a full spectrum of light was more conducive to the accumulation of biomass and protein concentration in R. palustris, compared with the segmented spectra. Under incandescent lamp and white LED irradiation, the highest biomass concentration, daily bacterial production, bacterial yield, and protein concentration reached (1 023.18±201.17) mg/L, (0.32±0.10) g/(L·d), (0.31±0.03) mg/mg and (555.66±9.18) mg/L, respectively. These values increased by 37.26%-43.79%, 108.82%-137.92%, 42.01%-91.85%, and 24.77%-33.47%, respectively, compared with the blue and green light groups (P<0.05). Moderate light intensity was favored to accumulate biomass and protein concentration, while excessive or low intensity was detrimental to the accumulation of high-value products by R. palustris. Specifically, under a light intensity of 120 μmol/(m²·s), the highest biomass concentration, daily bacterial production, bacterial yield, and protein concentration reached (1 646.12±2.47) mg/L, (0.77±0.01) g/(L·d), (0.44±0.05) mg/mg and (803.59±2.62) mg/L, respectively. There was an increase of 60.23%-140.19% in biomass and an increase of 61.90%-94.18% in protein concentration, compared with the 0 μmol/(m²·s) (control) and 240 μmol/(m²·s) groups (P<0.05). Intermittent light exhibited significant advantages over continuous light. An appropriate light cycle was selected to significantly promote the growth and protein synthesis of R. palustris. In the condition of 18 L/6 D, the highest biomass concentration, daily bacterial production, protein concentration, and nitrogen conversion efficiency reached (1 140.56±19.72) mg/L, (0.32±0.02) g/(L·d), (506.53±48.20) mg/L, and (1.77±0.35) mg/mg, respectively. There were increases by 17.06%-93.21%, 54.43%-299.93%, 24.35%-43.88% and 38.78%, respectively, compared with the 3 L/21 D and 9 L/15 D groups (P<0.05). The 3 L/21 D group exhibited the highest protein content at 67.47%, indicating an increase of 21.96%-44.54%, compared with all the other experimental groups (P<0.05). Additionally, under conditions of incandescent light, the light intensity of 120 μmol/(m²·s) and photoperiod of 18 L/6 D, COD and NH₄⁺-N removal efficiencies reached 72.03%-78.40%. Furthermore, there were significant negative correlations between light intensity and spectrum with the protein content and concentration. Conversely, the photoperiod shared a significant positive correlation with protein concentration. Therefore, the photoperiod can be expected to enhance the production of SCP by R. palustris. Its mechanism involves the direct regulation of PSB growth and metabolism by the light/dark cycle. During the light period, PSB pigments capture photons through photoreactions, driving electron transfer, generating electrical energy, and ultimately converting it into ATP; Simultaneously, the Calvin cycle efficiency is enhanced, leading to the accumulation of cell growth and SCP. During the dark period, PSB cells focus on the division process, increasing overall biomass. This research can provide innovative insights to enhance the SCP synthesis in wastewater systems using PSB.