Enhancing pigment accumulation in Coelastrella thermophila var. globulina via olive-mill waste biostimulant addition during photosynthetic biogas upgrading

This study investigated the potential of hydrothermal carbonization (HTC) aqueous phase fractions derived from olive mill waste to stimulate the metabolism of the green alga Coelastrella thermophila var. globulina, with the aim of enhancing pigment production, biomass productivity, and CO2 biofixation. Three different HTC fractions were evaluated: raw solution (RS), solid hydrochar fraction (HC), and the resulting supernatant after HC separation (SU). Regardless of the tested condition, complete CO2 removal occurred after only 60 h of incubation. The addition of 100 mg L−1 of HC supported the highest carbohydrates accumulation of 53 ± 7% w:w, while the total carotenoids and chlorophyll content significantly increased regardless of the fraction evaluated without hindering the biomass production. Subsequent optimization of the RS concentration revealed a positive correlation between increased dosage and the production of chlorophyll a, chlorophyll b, and total carotenoids, which reached 837 ± 95 µg g−1dcw, 601 ± 98 µg g−1dcw and 1115 ± 93 µg g−1dcw, respectively, by the addition of 3 mL L−1 of RS, while maintaining a high biomass productivity of 1.27 ± 0.05 g L−1 d−1. Indeed, this study confirmed the potential of olive-mill waste HTC aqueous phase supplementation as a sustainable and cost-effective strategy to enhance pigment production from C. thermophila var. globulina.