Changes in photosynthesis, growth and biomass composition in outdoor Chlorella g120 culture during the metabolic shift from heterotrophic to phototrophic cultivation regime

Abstract

Up to now, scarce information has been available regarding major photobiochemical changes that accompany the metabolic shift from heterotrophy to phototrophy in microalgae cells. In the present trials the trophic conversion from heterotrophic to phototrophic growth regime was studied in the microalga Chlorella vulgaris g120 grown in outdoor thin-layer cascades. The crucial question was whether Chlorella g120 can undergo the photoacclimation of its photosynthetic apparatus under natural irradiance. As for comparison the phototrophic strain Chlorella vulgaris R-117 was cultured in parallel. Various in-situ and ex-situ monitoring techniques namely oxygen production and chlorophyll fluorescence were tested to characterize physiological response of the Chlorella g120 culture during photoacclimation and correlate it with growth. We show that Chlorella g120 can undergo the metabolic shift from heterotrophic to phototrophic growth regime, but its conversion is. A rather high respiration rate was interpreted as a strategy to dissipate the unused light energy. Chlorella g120 was found as the strain characterized by a small-antenna size strain (low chlorophyll/cell ratio < 1%) which should be favorable for light utilization in dense cultures. The high carotenoid/chlorophyll ratio (= 0.63) may be of interest from biotechnological point of view for carotenoid (namely lutein) production. The build-up of dissolved oxygen (DO) concentration and high electron transport rate measured in-situ indicated high photosynthetic activity of outdoor microalgae cultures. These variables are usable on-line as reliable markers as they reflect the physiological status of microalgae culture. Highly productive cultures, like Chlorella R-117 have high photosynthesis which is inevitably accompanied by the build-up on high DO gradients. To maintain high biomass production in such strains we have to find an interplay among photosynthetic activity, gradients of DO concentration, biomass density and culture layer thickness that can secure high growth rate (and productivity).