Simultaneous enhancement of biomass and lipid production of Monoraphidium sp. QLZ-3 in a photobioreactor by using walnut shell extracts

To utilize waste walnut shells and to enhance the production of biomass and lipid by microalgae, cultivation of Monoraphidium sp. QLZ-3 in a photobioreactor under different proportions of walnut shell extracts (WSE) was investigated. The highest biomass productivity (534.7 mg L-1 d(-1)) and lipid content (55.42%) were obtained using 40% WSE as part of the growth medium. Meanwhile, application of WSE increased the protein content and nutrient removal efficiency and upregulated the transcription levels of lipid biosynthetic genes but decreased the carbohydrate and gamma-aminobutyric acid (GABA) contents. WSE also increased the levels of reactive oxygen species (ROS) and glutathione (GSH). After supplementation of WSE, further evidence indicated that GABA could further promote biomass, lipid and GABA accumulation by regulating ROS signalling and increase the levels of lipogenic gene expression and GSH. The systemic GABA content was 24.22 mg g(-1). Collectively, the cultivation of microalgae in WSE has great feasibility for biofuel and GABA production and solid waste treatment, and the present study provides insights into the regulatory mechanism of cytosolic GABA in the modulation of cell growth, lipid synthesis and oxidative stress in QLZ-3 by the addition of WSE.