Photosynthetic activity measured in-situ in microalgae cultures grown in large-scale raceway ponds

Abstract Two microalgae, Scenedesmus sp. and Chlorella sp. (Chlorophyceae), robust and well-growing species, with a potential for biostimulating activities, were cultured in raceway ponds (RWPs) placed in a greenhouse. The objective of this case study was to monitor the performance of microalgae cultures in-situ at various depths as concerns photosynthetic activity and physico-chemical variables (irradiance, temperature, dissolved oxygen concentration) including biostimulating activity. The data (photochemical yield and electron transport rate monitored by Chl fluorescence and photosynthetic oxygen production) both in-situ and ex-situ revealed that (i) even in thin cultures (0.5-1 g dry weight L⁻¹), the active photic layer in the culture was only about 1 cm indicating that most of the culture was ˊphotosyntheticallyˋ in the dark and (ii) nevertheless, even at high dissolved oxygen concentrations of about 200 %sat and higher the cultures retained relatively high actual photochemical yield Y(II) of about 0.35 and higher when monitored in-situ. The presented work can be used as exemplary data to optimize the growth of microalgae cultures in large-scale raceway ponds by understanding the interplay between culture depth and cell concentration.