A circular bioprocess for integrated microalgal CO₂ capture and valorisation in mushroom cultivation

Mushroom farming is a rapidly expanding industry, yet conventional cultivation practices contribute significantly to carbon dioxide (CO 2 ) emissions, highlighting the need for sustainable carbon mitigation strategies. This study presents the first bicarbonate-based microalgal carbon capture process using in situ CO₂ emissions from mushroom cultivation, enabling circular bioresource utilisation. Oyster mushrooms were grown in a controlled growth chamber, and the emitted CO₂ was converted into a bicarbonate-enriched medium to cultivate two microalgal species: Vischeria cf. stellata and Porphyridium purpureum . Culture pH was optimised at small scale, with optimal pH (pH 7) selected for scale-up in 8 L photobioreactors. P. purpureum consistently outperformed V. stellata in mushroom-derived bicarbonate media, achieving over 85 % inorganic carbon removal, threefold higher biomass yield, and significantly increased lipid, protein, and phycobiliprotein (phycoerythrin and phycocyanin) accumulation. The enhanced pigment production suggests an adaptive response to high salinity and alkalinity. In contrast, V. stellata showed higher productivity in synthetic bicarbonate media and experienced growth inhibition and extended lag phases in mushroom-based media at scale, likely due to salt stress . This work demonstrates the feasibility of scalable, circular carbon capture and utilisation within mushroom farming and highlights the critical role of algal strain selection and stress tolerance for optimising biomass productivity and high-value product generation. • First demonstration of microalgal carbon capture integrated with mushroom cultivation. • Bicarbonate-based CO₂ utilisation enabled microalgal growth and bioproduct synthesis. • Porphyridium purpureum achieved >85 % inorganic carbon removal efficiency. • High-value bioproducts included lipids, proteins, phycoerythrin, and phycocyanin. • Process scalability validated in 8 L photobioreactors.