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.

The application of microalgae in wastewater treatment has recently been at the forefront of interest due to the increasing concern about environmental protection and economic sustainability. This work aimed to study two chlorophyte species, Chlorella vulgaris and Scenedesmus acutus, co-cultured outdoors in centrate of municipal wastewater as a nutrient source. Two different thin-layer units were used in these trials — thin-layer cascade (TLC) and thin-layer raceway pond (TL-RWP), suitable for this purpose due to their high biomass productivity and better culture transparency when using muddy wastewater. The units were operated in batch, and subsequently in semi-continuous growth regime — and monitored in terms of photosynthetic performance, growth, nutrient removal rate, and bioactivity. The results showed that the co-cultures grew well in the centrate, achieving the maximum biomass densities of 1.3 and 2.1 g DW L⁻¹ in TLC and TL-RWP, respectively, by the end of the batch regime and 1.9 and 2.0 g DW L⁻¹ by the end of the semi-continuous regime. Although TL-RWP grown cultures showed faster growth, the TLC-one revealed better nutrient removal efficiencies batch wise than the culture grown in TL-RWP — removing up to 48% of total nitrogen and 43% of total phosphorus. Conversely, the latter was more efficient under the semi-continuous regime (54% and 42% consumption of total nitrogen and phosphorus, respectively). In the harvested biomass, an important antimicrobial activity (specifically antifungal) was detected. In this sense, the in-vitro growth of the oomycete Pythium ultimum was inhibited by up to 45% with regard to the control. However, no biostimulating activity was observed. The present findings confirm the possibility of using these two species for biomass production in municipal wastewater centrate using highly productive thin-layer systems. This technology can be a valuable contribution to circular economy since the produced biomass can be re-applied for agricultural purposes.

The use of wastewater (WW) for cultivation contributes to the sustainability of microalgae production due to the reduced costs of cultivation. The main objective of this work was to study growth, physiological performance and bioactivity of the microalgae strain Chlorella vulgaris MACC-1 grown in two nutrient sources – inorganic BG-11 medium and centrate from municipal WW. For the comparison, two thin-layer cultivation units – thin-layer cascade and a novel, thin-layer raceway pond – were used. The cultures grew well in both units showing good photosynthetic activity. The germination index of watercress seeds, as well as the auxin-like activity in mung bean and cytokinin-like activity in wheat growth tests were used to evaluate the biostimulant potential. The slight increase on the germination index was determined in C. vulgaris cultures grown in BG-11, but the biomass revealed no biostimulant activity when cultivated in WW. On the other hand, the antibacterial and antifungal activities determined by antagonism bioassay using dual cultures were significantly higher when grown in WW. We expect that the antimicrobial activities may be induced by WW-microbes and the biostimulating effect could probably be suppressed by the presence of some inhibiting substances. The results revealed a clear interplay among ambient irradiance intensity, growth rate, maximum quantum yield of PSII, Fv/Fm and oxygen production/respiration.