Hydrodynamics and photosynthesis performance of Chlorella fusca (Chlorophyta) grown in a thin-layer cascade (TLC) system

Abstract

The thin-layer cascade (TLC) system is an open system for microalgae cultivation composed of a retention tank connected by pump and pipes to a horizontal exposed area that consists of an upper basin and a TLC. Light and hydrodynamics are different among compartments, so overall photosynthetic activity can be influenced by the retention time of the cells in each compartment. We established 2 settings with different retention times in the cascade and tank to compare the photosynthetic activity of Chlorella fusca (Chlorophyta) among compartments. Changes in the retention time were achieved using 2 layer thicknesses in the cascade: 8 and 18 mm. Retention time in the cascade represented about 16 and 34% of the duration of a whole system cycle when H1 (8 mm thickness) and H2 (18 mm thickness) units, respectively, were used. These retention periods were lower than those in the tank (67 and 49%, respectively) but higher than those in the basin (12% for both H1 and H2). Photosynthetic activity was measured in situ as relative electron transport rate (rETR) using a pulse-amplitude modulated fluorometer. In both setups, the highest rETR was reached in the cascade. The increase of the layer thickness was a good option to avoid photoinhibition. We suggest estimating the mean rETR of the whole system considering the retention time, since it can better reflect overall growth because it takes into account the time that the cells spend in each compartment. These results are useful for optimization of photosynthetic activity and growth of outdoor microalgae mass cultures in TLCs for biotechnological purposes.