The microalga Scenedesmus sp. (Chlorophyceae) was cultured in a raceway pond (RWP) placed in a greenhouse. The objective of this case study was to monitor the photosynthesis performance and selected physicochemical variables (irradiance, temperature, dissolved oxygen concentration) of microalgae cultures in situ at various depths of RWP. The data of actual photochemical yield Y(II), the electron transport rate monitored by in vivo chlorophyll fluorescence and photosynthetic oxygen production both in situ and ex situ revealed that (i) even in diluted cultures (0.6 g DW L⁻¹), the active photic layer in the culture was only about 1 cm, indicating that most of the culture was “photosynthetically” inactive; (ii) the mechanism of non-photochemical quenching may not be fast enough to respond once the cells move from the surface to the deeper layers; and (iii) even when cells were exposed to a high dissolved oxygen concentration of about 200% sat and higher, the cultures retained a relatively high Y(II) > 0.35 when monitored in situ. The presented work can be used as exemplary data to optimize the growth regime of microalgae cultures in large-scale RWPs by understanding the interplay between photosynthetic activity, culture depth and cell concentration.

Nutrient and micropollutant removal, and bioactivity were studied in cultures of the green microalga Tetradesmus obliquus MACC-677 grown in centrate from municipal wastewater (WW). Two outdoor units, a thin-layer cascade (TLC) and a thin-layer raceway pond (TL-RWP), were tested for microalgal culturing in batch and semi-continuous regimes where their photosynthetic performance was monitored. The results revealed that the T. obliquus cultures grew well, showing a high specific growth rate µ of 0.31 day⁻¹ and 0.25 day⁻¹ when grown in WW in TLC and TL-RWP, respectively. The cultivation trials showed high nutrient removal efficiency for ammonium nitrogen (98.5%) as well as orthophosphate (89%), the most abundant forms of N and P occurring in municipal WW. The removal of selected pharmaceuticals and endocrine disruptors (e.g., ibuprofen, amitriptyline, bisphenol A, etc.) was also assessed. Ibuprofen was the most abundant micropollutant detected in the centrate, with concentrations up to 5000 ng L⁻¹ and fast removal during the cultivation. The biomass produced in the centrate revealed antimicrobial activity against plant pathogens, including fungi, oomycota, and bacteria. These findings have shown that the culturing of T. obliquus can be considered a suitable way to contribute to a circular economy, to remove nutrients and micropollutants from municipal WW from which biomass extracts can be further used for plant protection in agriculture.

Microalgae cultures were used for a WW treatment to remediate nutrients while producing biomass and recycling water. In these trials, raceway ponds (RWPs; 1 and 0.5 ha) were located next to a municipal (WW) treatment plant in Mérida, Spain. The ponds were used for continuous, all-year-round microalgae production using WW as a source of nutrients. Neither CO2 nor air was supplied to cultures. The objective was to validate photosynthesis monitoring techniques in large-scale bioreactors. Various in-situ/ex-situ methods based on chlorophyll fluorescence and oxygen evolution measurements were used to follow culture performance. Photosynthesis variables gathered with these techniques were compared to the physiological behavior and growth of cultures. Good photosynthetic activity was indicated by the build-up of dissolved oxygen concentration up to 380% saturation, high photochemical yield (Fv/Fm = 0.62–0.71), and relative electron transport rate rETR between 200 and 450 μmol e⁻ m⁻² s⁻¹ at midday, which resulted in biomass productivity of about 15–25 g DW m⁻² day⁻¹. The variables represent reliable markers reflecting the physiological status of microalgae cultures. Using waste nutrients, the biomass production cost can be significantly decreased for abundant biomass production in large-scale bioreactors, which can be exploited for agricultural purposes.