Effect of methanol, ethanol, DMSO and hexane on Chlorella vulgaris in the context of phytohormone delivery

Phytohormones have the potential to improve microalgae culture productivity. Still, not every phytohormone is water-soluble, and most have to be dissolved in organic solvents before being delivered to the cells. Unfortunately, most studies neglect or do not report the effect of the solvent. This is a problem as the solvent itself can have an effect, and not accounting for it would lead to confounding and potentially erroneous interpretations. Consequently, this work evaluates the effects of classical solvents (methanol, ethanol, DMSO, and hexane) in the context of phytohormone delivery. Thus, Chlorella vulgaris was exposed to different dosages of those solvents. The cultures were monitored for three days using optical density to evaluate growth, the OJIP test to assess photosynthetic apparatus status, and flow cytometry to measure viability. Individual outputs and integrated biomarkers were used to analyze the results. Overall, ethanol triggers cell flocculation for the tested concentrations (as low as 0.1%vol) and lysis above 0.5%vol. Methanol also led to flocculation above 0.5%vol, but started to impair viability from 0.1%vol on. DMSO and hexane appear as viable solvents, with no deviation from control for 0.25 and 0.1%vol dosage. Their dosages can be increased to 1%vol while still claiming admissibly perturbed cultures, as absolute values remain in line with control, but dispersion increases. Finally, a validation run was carried out. DMSO-delivered salicylic acid was used to modify Chlorella vulgaris lutein production. Results confirmed the efficiency of salicylic acid and the safety of DMSO at the chosen dosage.