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In this work, the performance of a vertical multiplate photobioreactor is analyzed and presented. The photobioreactor consisted of 20 vertical plates (1 m 2 each) connected by manifolds and a working volume of 1300 L. The total area occupied (footprint) was 10 m 2 , while the illuminated area was 40 m 2 , therefore the ratio of illuminated area to volume ratio was about 30 m -1 . The performance of the photobioreactor was evaluated using a culture of Synechocystis PCC 6803, circulated by a centrifuge pump. The results showed that the amount of light captured by the photobioreactor at a plate spacing of 0.5 m was 90.2 % of the light incident on the horizontal surface, while at a plate spacing of 1.0 m, 50.3 % was captured. The corresponding biomass yield, calculated based on the ground area occupied by the reactor, was 26.0 g m -2 day -1 and 7.2 g m -2 day -1 , when the plates were spaced at 0.5 m and 1.0 m respectively. Therefore, the light conversion efficiency calculated based on the ground area was significantly higher in the configuration with a plate spacing of 0.5 m, reaching 5.43 % based on PAR (photosynthetically active radiation), and 2.44 % based on solar radiation, giving a value 3.7 higher than when the plates were spaced 1.0 m apart. It was concluded that the light conversion efficiency might be further improved by reducing the plate spacing while also reducing the culture light path., Competing Interests: Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Giuseppe Torzillo reports financial support was provided by European Commission., (Copyright © 2022 The Authors. Published by Elsevier B.V. All rights reserved.)

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). • Heterotrophic Chlorella g120 undergoes metabolic shift to phototrophic growth. • Various techniques/variables were verified for in-situ/ex-situ culture monitoring. • Growth was slow due ineffective photosynthetic performance. • High respiration rate can represent a strategy to dissipate the unused light energy. • High carotenoid/chlorophyll ratio in biomass may have biotechnological advantage. [ABSTRACT FROM AUTHOR]