Effects of light intensity on soluble microbial products produced by Synechocystis sp. PCC 6803 and associated heterotrophic communities

Characterization of a newly isolated self-flocculating microalga Bracteacoccus pseudominor BERC09 and its evaluation as a candidate for a multiproduct algal biorefinery

Microalgae have the highest capability to fix the atmospheric carbon and wastewater-derived nutrients to produce high-value bioproducts including lipids and carotenoids. However, their lower titers and single-product-oriented biomass processing have made the overall process expensive. Hence, increased metabolite titer and processing of the biomass for more than one product are required to ensure the commercial robustness of the algal biorefinery. In this study, a newly isolated algal strain was identified as Bracteacoccus pseudominor BERC09 through phylogenetic analysis based on the 18S rRNA gene sequence. Basic characterization of the strain revealed its promising potential to produce carotenoids and lipids. The lipids and carotenoid biosynthesis pathways of BERC09 were further triggered by manipulating the abiotic factors including nitrogen sources (NaNO₃, KNO₃, NH₄Cl, Urea), nitrogen concentrations (0.06-0.36 gL^-1), light intensity (150 μmolm^-2s^-1 to 300 μmolm^-2s^-1), and light quality (white and blue). Resultantly, 300 μmolm^-2s^-1 of blue light yielded 0.768 gL^-1 of biomass, 8.4 mgg^-1 of carotenoids, and 390 mgg^-1 of lipids, and supplementation of 0.36 gL^-1 of KNO₃ further improved metabolism and yielded 0.814 gL^-1 of biomass, 11.86 mgg^-1 of carotenoids, and 424 mgg^-1 of lipids. Overall, the optimal combination of light and nitrogen concurrently improved biomass, carotenoids, and lipids by 3.5-fold, 6-fold, and 4-fold than control, respectively. Besides, the excellent glycoproteins-based self-flocculation ability of the strain rendered an easier harvesting via gravity sedimentation. Hence, this biomass can be processed in a cascading fashion to use this strain as a candidate for a multiproduct biorefinery to achieve commercial robustness and environmental sustainability.

Controlling average number of photons received per biomass to promote the growth of Synechocystis sp. PPC 6803

To investigate the actually received light of cells in the photo bioreactor, a light attenuation model of Synechocystis sp. PCC 6803 was established. The relationship between the average number of photons received per biomass (APRPB) and the growth of cell was analyzed. The results demonstrated, Cornet model was accurately fitted with the light attenuation of Synechocystis sp. PCC 6803 and the cell growth rate was affected by APRPB. When the value of APRPB is 3.2 μmol g^-1 s^-1, the cell have the maximum light efficiency. A maximum specific growth rate of 0.05 h^-1 was achieved with APRPB from 3.2 to 12.8 μmol g^-1 s^-1. After 156 h cultivation, compared to cells cultured under constant light [light intensity: 100 and 1800 μmol/(m² s)], the DCW under controlled light intensity (light intensity increasing with the cell density) was higher by 79.1% and 20.0%, respectively. This study indicated that APRPB could be used as a light intensity regulation criterion to improve cell production despite different types of reactor and cell density, which provided a theoretical basis for improving the biomass yield of Synechocystis sp. PCC 6803 or other photosynthetic auto-trophic organism.