Internal and external spectral light conversion amplifying growth/bio-products formation of Dunaliella salina

Abstract

Rhodamine-6-G (R6G) was used to investigate the effect of internal/external light conversion on Dunaliella salina in a 1000 mL double-layer flat plate photobioreactor (DLF-PBR) and a 900 mL cylindrical photobioreactor (C-PBR). External spectrum conversion was examined in the DLF-PBR where the outer layer was filled with four different dye solutions (water/ethanol as solvents, dye at 25/50 ppm) to select the appropriate dye level and solvent type. The highest biomass (2.1 g L^-1) and productivity (116 mg L^-1day^-1) were obtained at low dye level in ethanol, while the highest productivities of storage materials (lipid:7.5, starch:18.73 mg L^-1day^-1) belonged to high level of dye in water. To benefit from conventional bioreactors with lower cost, internal conversion was carried out in C-PBR by addition of dye into medium aiming to compare growth and product formation with/without spectrum conversion. The highest level of biomass (3.49 g L^-1) was achieved by light conversion in C-PBR. Additionally, the effect of adaption of cells to dye was investigated in the C-PBR. A long lag-phase was observed in the case of non-adapted cells while this considerably reduced in the case of adapted cells. Moreover, a two-stage cultivation was studied in the C-PBR in which mixed red-blue LEDs exposure allowing enhanced growth without dye was used in the 1^st-stage and this was followed by addition of R6G and shifting to fluorescent exposure in the 2^nd-stage, which resulted in 146 and 293% higher productivities of lipid and starch, respectively, compared to control. The highest cell contents of lipid and starch (9.91, 22.15% g g^-1DCW) also corresponded to this strategy.