Extremophilic Microalgae and Cyanobacteria as Platforms for Climate Resilience, Circular Resource Utilization and Bioprospecting.

The untapped genetic diversity of extremophilic microalgae provides distinctive physiological and biochemical traits with potential to support Sustainable Development Goal (SDG) 6 (Clean Water and Sanitation), SDG 7 (Affordable and Clean Energy), and SDG 15 (Life on Land). This review examines the diversity, stress-adaptation mechanisms, and biotechnological potential of extremophilic microalgae, with emphasis on their roles in resource recovery, carbon utilization, and bioproduct generation. Their ability to maintain cellular functions under extreme salinity, pH, temperature, radiation, and nutrient conditions enables their cultivation in environments that may be unsuitable for conventional production systems and facilitates the accumulation of lipids, pigments, carbohydrates, proteins, and other bioactive metabolites. These characteristics create opportunities to couple wastewater treatment, carbon capture and utilization, and biomass valorization within circular bioprocesses. However, laboratory-scale demonstrations should not be equated with established environmental or commercial benefits, as strain-specific physiological requirements, energy demand, process control, harvesting, downstream processing, and resource inputs can constrain scale-up. The review therefore emphasizes the need to evaluate extremophilic microalgal systems using integrated techno-economic and life-cycle approaches rather than biological productivity alone. Future advances will depend on combining multi-omics, metabolic engineering, adaptive cultivation, process optimization, and predictive modeling to identify robust strains and cultivation strategies. Such integration could accelerate the translation of extremophile-based bioprocesses into resource-efficient and economically credible applications for environmental sustainability.