Lab-Scale Testing of a Two-Stage Continuous Culture System for Microalgae

This research describes an innovative two-stage turbidostat culture system that supported sustained exponential growth at different rates and lipid accumulation in the halophilic chlorophyte Dunaliella sp. The first stage was operated as a nitrogen-replete cycloturbidostat, with a 14:10-h light:dark cycle at a growth rate of ∼1.5/day, and its effluent fed an 8-fold larger second-stage culture that then grew at ∼0.2/d. Turbidity-triggered pumps supplied medium consistently throughout the photoperiod only. Cell densities were about double in stage 2 compared to stage 1. They varied minimally throughout the day in stage 2, but in stage 1 they decreased by 50% in the first half of the photoperiod due to cell division beginning mid-photoperiod. Cell sizes increased through the first half of the photoperiod in stage 1, consistent with the phases of cell division, and indicative that the turbidity signal is more strongly influenced by cell size than density. Nitrogen-induced fluorescence transients confirmed nitrogen-limitation of stage 2, but not stage 1. Rapid light curves indicated reduced photosynthetic efficiency and capacity in stage 2. Nile Red fluorescence measured by flow cytometry was 3 times higher in stage 2 cells compared to stage 1 cells, demonstrating elevated neutral lipid content per cell. This system needs to be optimized for biomass and lipid yields, tested with other species, and scaled up for outdoor testing.