Abstract

Torrefaction of biomass produces torrefied biomass, non-condensable gases and condensable gases, which can be cooled to form a torrefaction condensate (TC). It contains assimilated organic carbon and compounds capable of inhibiting microbial growth. This study evaluated whether TC produced from aspen wood chips torrefied at 225 °C could be incorporated into cultures of the red microalga Porphyridium marinum and how TC exposure affected growth, biomass composition, and exopolysaccharide (EPS) production. TC was added to established cultures at 0.5–2.5 mL/L. Harvested biomass and purified EPS were characterised by chromatographic, colorimetric, and antioxidant assays. TC caused immediate concentration-dependent growth inhibition followed by partial recovery. However, purified EPS yield was lower than the control (202.7 mg/L) in all TC treatments (118.4–185.5 mg/L). The biomass lipid fraction reached 47.47% at 0.5 mL/L, compared to 15.47% in the control, and decreased as TC dosage increased. EPS protein and sulphate contents were in the ranges of 2.08–2.89% and 8.54–9.56%, respectively, compared to 1.67% and 8.80% in the control. FTIR spectra indicated the preservation of principal EPS functional groups, whereas antioxidant activity was generally weak and assay-dependent. These findings demonstrate the tolerance and compositional acclimation of P. marinum to low aspen–TC loadings, but not improved biomass or EPS productivity. This study suggests a new route for integrating thermochemical conversions with cultivation of red microalgae; however, it requires detailed detoxification, process optimisation, and further investigation before biorefinery implementation.

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