Physiological and Biochemical Mechanisms of Marine Algae in Response to Environmental Stress: Adaptive Strategies and Applications in Circular Bioeconomy—A Narrative Review and Conceptual Synthesis

Marine algae sustain primary productivity, nutrient cycling, and coastal carbon regulation, yet warming, salinity fluctuations, acidification, and nutrient/metal loading increasingly challenge their physiology, and evidence linking adaptation to biotechnological value remains fragmented. This narrative review synthesizes physiological, biochemical, and transcriptomic studies (2016–2026) on marine micro- and macroalgae exposed to thermal, saline, light/oxidative, nutrient, heavy-metal, and acidification stress; a few non-marine extremophile taxa are included and flagged as mechanistic comparators. Across taxa, algae rely on recurrent strategies—osmolyte accumulation, antioxidant/photoprotective pigments, fatty-acid desaturation, metal chelation, and phytohormone-mediated (ABA, SA, MeJA) signalling coupled to ribosome-biogenesis upregulation—that appear to restore redox and osmotic homeostasis. These phenotypes underpin biofuel, bioremediation, blue-carbon, and bioplastic applications, ranked on a Technology Readiness Level scale from wastewater bioremediation and aquaculture co-culture (TRL 6–7) to biodiesel and blue-carbon crediting (TRL 3) and photoprotection-trait transfer (TRL 2). We propose a reaction-level conceptual framework in which ribosome-biogenesis control and ABA/SA/MeJA signalling act as a testable stress-integration hub, presented explicitly as a hypothesis rather than an established mechanism. Stress tolerance and circular-economy value may be mechanistically linked, but most evidence derives from short-term, single-species laboratory exposures, remains taxonomically narrow, and macroalga-specific comparative laboratory data are scarce.