| Summary: | The aim of the present study was to assess the potential of producing four microalgal strains using secondary-treated urban wastewater supplemented with centrate, and to evaluate the biostimulant effects of several microalgal extracts obtained using water and sonication. Four strains were studied: Chlorella vulgaris UAL-1, Chlorella sp. UAL-2, Chlorella vulgaris UAL-3, and Chlamydopodium fusiforme UAL-4. The highest biomass productivity was found for C. fusiforme, with a value of 0.38 ± 0.01 g·L−1·day−1. C. vulgaris UAL-1 achieved a biomass productivity of 0.31 ± 0.03 g·L−1·day−1 (the highest for the Chlorella genus), while the N-NH4+, N-NO3−, and P-PO43− removal capacities of this strain were 51.9 ± 2.4, 0.8 ± 0.1, and 5.7 ± 0.3 mg·L−1·day−1, respectively. C. vulgaris UAL-1 showed the greatest potential for use as a biostimulant—when used at a concentration of 0.1 g·L−1, it increased the germination index of watercress seeds by 3.5%. At concentrations of 0.5 and 2.0 g·L−1, the biomass from this microalga promoted adventitious root formation in soybean seeds by 220% and 493%, respectively. The cucumber expansion test suggested a cytokinin-like effect from C. vulgaris UAL-1; it was also the only strain that promoted the formation of chlorophylls in wheat leaves. Overall, the results of the present study suggest the potential of producing C. vulgaris UAL-1 using centrate and wastewater as well as the potential utilisation of its biomass to develop high-value biostimulants.
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