Harnessing Stress: Conventional and Unconventional Strategies for Enhancing Microalgal Productivity in Sustainable Biorefineries

Review Harnessing Stress: Conventional and Unconventional Strategies for Enhancing Microalgal Productivity in Sustainable Biorefineries Chia-En Chuang 1,†, Yu-Han Chien 1,†, Sheng-Yung Lin 1,†, Adi Kusmayadi 2, Jia-Hui Zhou 1, Chiao Ching Chang 1, Chen Rui Zhang 1 and Yoong Kit Leong 1,3,* 1 Department of Chemical and Materials Engineering, Tunghai University, Taichung 407, Taiwan 2 Department of Mechanical Engineering, Politeknik Negeri Indramayu, Indramayu 45252, Indonesia 3 Research Center for Smart Sustainable Circular Economy, Tunghai University, Taichung 407, Taiwan * Correspondence: [email protected] † These authors contributed equally to this work. Received: 18 January 2025; Revised: 2 March 2025; Accepted: 6 March 2025; Published: 2 April 2025 Abstract: Microalgae are versatile platforms for producing biofuels and high-value metabolites, such as lipids, proteins, and carotenoids. Numerous stress strategies have been adopted to improve microalgal cultivation and biomolecule yield. This review examines how conventional stress factors (light and salinity) and unconventional treatments (electric field treatment) influence microalgal growth and metabolite accumulation. Light intensity, spectrum, and photoperiod significantly affect photosynthesis, biomass yield, and carotenoid biosynthesis, with moderate intensities found to enhance efficiency. However, excessive levels may induce photoinhibition. Salinity stress induces activation of antioxidant systems and lipid accumulation, optimizes biofuel properties. However, excessive high salinity can impair the growth of microalgae. In this review, we focused on the electric field treatment as a potential strategy for enhancing microalgal productivity, representing a major novelty of the study. Unlike traditional stress factors that primarily induce adaptive metabolic shifts, electric field treatment offers a unique and an understudied approach for modulating cellular physiology. Electric treatment technology offers an energy-efficient method for stimulating cell differentiation and enhancing lipid and pigment production while reducing environmental effects. Integrating these stress factors may be an attractive approach for controlling over microalgal metabolism, supporting sustainable and scalable biorefinery applications.