Lipid production by Tetradesmus obliquus M42 and valorization of the deoiled biomass for toxic dyes removal

This study aimed to evaluate the biomass and fatty acids production in the newly isolated, acidophilic chlorophyte, namely Tetradesmus obliquus M42, and to valorize the deoiled biomass as an adsorbent to concomitantly reduce the concentration and toxicity of synthetic dyes (methylene blue, crystal violet, and Congo red) in aqueous solutions. To this end, we evaluated: (i) the effects of temperature, pH, nitrogen, phosphate and inorganic carbon on the biomass production and lipid yield; (ii) the dye removal process, as a function of deoiled biomass dose, pH, initial dye concentration, and contact time, and (iii) the dye toxicity reduction, using the inhibition of bioluminescence and viability of Phaeodactylum tricornutum. While initial pH 5 supported rapid growth of M42 and CO2 consumption, the most promising conditions for both biomass (1.36 g L−1) and lipid yield (680 mg L−1) were at pH 8 under nitrogen limitation and inorganic carbon supplementation. Lipid profile, dominated by monounsaturated fatty acids, met the requirements for biodiesel standards. Deoiled biomass (2.5 g L−1) removed methylene blue faster (62% in 1.3 h) than crystal violet (87% in 1.7 h) and Congo red (61% in 5.5 h) at pH 9, and the adsorption mechanism was mainly attributed to electrostatic interactions. The toxicity of the dyes was significantly reduced, although the two biological models responded differently. From a circular economy perspective within a microalgae-based biorefinery, the use of M42 could offer a dual benefit: high lipid production as a biodiesel feedstock and deoiled biomass reuse to mitigate the environmental impact of toxic dyes.