Toward energy-efficient microalgal biorefineries: Microwave-assisted dilute acid pretreatment of wet Nannochloropsis oceanica

• Microwave-assisted pretreatment with acid hydrolysis to process wet N. oceanica . • Low biomass-to-pretreatment solution ratio (1:15 w/w) favoured lipid extraction • High biomass-to-pretreatment solution ratio (1:30 w/w) favoured protein recovery. • Dilute acid pretreatment significantly enhanced lipid, protein, and sugar recovery. • SEM imaging confirmed extensive cell wall disruption under acid pretreatment. The extraction of valuable bioproducts from microalgae is often limited by the recalcitrant structure of the cell wall and the energy-intensive nature of biomass drying. This study investigates the integration of microwave-assisted pretreatment with dilute sulfuric acid hydrolysis for the recovery of lipids, proteins, and sugars from wet Nannochloropsis oceanica biomass, emphasizing the influence of biomass-to-pretreatment solution ratios (1:15 to 1:30, w/w). Results indicate that both pretreatment temperature and biomass-to-pretreatment solution ratio significantly affect bioproduct recovery. Microwave-assisted acid pretreatment at 140 °C achieved the highest yields of lipid after hexane extraction of 142.2 g/kg under 1:15 condition, as well as protein and sugar yields reaching 272.1, and 75.1 g/kg biomass, under the 1:30 condition. While biomass-to-pretreatment solution ratio of 1:15 (high biomass concentration) favoured lipid extraction, biomass-to-pretreatment solution ratio of 1:30 (low biomass concentration) enhanced protein yields. Scanning electron microscopy confirmed substantial cell wall disruption under acid pretreatment, correlating with improved extraction efficiency. Energy analysis showed that the nominal microwave energy input (11.25-22.50 MJ/kg biomass) was substantially lower than the theoretical minimum thermal energy required for drying (36.10-74.66 MJ/kg biomass), confirming the energetic advantage of direct wet processing. This wet processing strategy, combined with low-polarity solvent extraction, presents a scalable and energy-efficient alternative to conventional biomass drying and processing route, offering a pathway toward cascading microalgal biorefineries for multi-product generation.