Biostimulant Extracts from Tetradesmus obliquus Cultivated in Rice Parboiling Effluent Improve Rice Germination and Generate PUFA-rich Residues in a Circular Biorefinery Strategy
The valorization of agro-industrial effluents (AIEs) as cultivation media for microalgae represents a promising strategy for integrating wastewater remediation with bioproduct generation. The increasing demand for sustainable and climate-resilient agricultural systems, combined with the need to reduce microalgal production costs, has intensified interest in circular biorefinery approaches capable of converting industrial residues into bioactive compounds. In this context, Tetradesmus obliquus demonstrates strong potential for the production of agricultural biostimulants and nutritionally valuable metabolites. In this study, T. obliquus biomass cultivated in Bold’s basal medium (BBM) and rice parboiling AIE underwent biochemical characterization and aqueous extraction. Germination and seed vigor assays evaluated the biostimulant effects of the extracts on four rice cultivars ( Oryza sativa L.) under standard and cold-stress conditions, while the residual biomass underwent lipid profiling by gas chromatography–mass spectrometry. AIE-derived biomass showed higher total carbohydrate content (197.12 mg g⁻ 1 ) than BBM biomass (121.33 mg g⁻ 1 ), whereas BBM biomass presented higher chlorophyll levels (5.37 mg g⁻ 1 ). Amino acid profiles also differed between cultivation systems; BBM extracts contained higher proportions of glutamic acid, valine, and glycine, whereas AIE extracts showed enrichment in alanine, proline, and histidine, indicating metabolic adaptation to effluent-based cultivation. Despite these compositional differences, both extracts promoted comparable biostimulant effects, particularly at treatments 3L·100 kg⁻ 1 seed, achieving germination rates above 97% in IRGA 424 RI relative to the control. These findings demonstrate that T. obliquus cultivated in AIE preserves bioactive functionality and supports circular biorefinery strategies integrating wastewater remediation, agricultural biostimulant production, and lipid valorization for potential aquaculture applications.