Main conclusion <p><b>Thermophilic cyanobacteria and microalgae have a set of coordinated structural and molecular changes that allow them to survive under elevated temperatures. All these features make are their thermostable enzymes, strong stress response systems, and high-capacity carbon-fixation systems that make these organisms interesting candidates of biotechnological use and sustainability.</b></p> Abstract <p>Cyanobacteria and thermophilic microalgae are a unique group of extremophiles that can survive high temperatures and complex environments. Their morphological, physiological, and evolutionary characteristics enable them to survive in hot springs, arid soils, geothermal environments, and hydrophilic ecosystems. This involves production of heat-stable enzymes, osmolytes, pigments, and protective biomolecules, and increased thermostability of phycobilisomes, reliable repair of photosystem II components, and structural changes of photosystems. Microalgae and cyanobacteria exhibit remarkable morphological plasticity, transforming between unicellular, colonial, and filamentous forms while producing specialized cells like heterocysts, spores, and dormant vegetative cells to survive in various environments. Further, their ecological resilience is enhanced by adaptations to oxidative stress, nutrient&#xa0;limitation, UV radiation, and desiccation. These organisms have great potential for industrial biotechnology, particularly biofuels, bioprocessing, carbon capture, bioremediation, and the synthesis of high-value compounds, due to their unique thermostable enzymes, heat-stable pigments, and carbon fixation efficiency. This review highlights current understanding of the phylogeny, stress adaptation mechanisms, and ecological significance of thermophilic microalgae and cyanobacteria, emphasizing their growing importance in sustainable biotechnology.</p> Graphical abstract <p></p>