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.

Livestock wastewater represents both an environmental challenge and a potential nutrient source for microalgal bioprocesses. This study developed a sequential upstream–downstream strategy integrating wastewater-based cultivation of Chlorella vulgaris with ultrasound-assisted carotenoid recovery. A simplex-centroid mixture design was first used to evaluate slaughterhouse wastewater, swine wastewater, and Bold’s Basal Medium as cultivation components. Within the investigated mixture domain, 100% swine wastewater produced the highest cell concentration, with predicted and experimentally validated values of 5.08 × 10⁶ and 5.41 × 10⁶ cells mL⁻¹, respectively. Nutrient removal reached up to 97% for NH₄⁺–N, 40% for PO₄³⁻–P, and 92% for COD, depending on wastewater formulation. Subsequently, ultrasound-assisted carotenoid extraction was optimized using response surface methodology. The model-selected operating condition was 25% amplitude and 10 min; experimental testing yielded a spectrophotometric lutein-equivalent content of 116.80 mg kg⁻¹ and an extract purity index of 2.39, compared with model estimates of 118.47 mg kg⁻¹ and 2.42, respectively. The study demonstrates the laboratory-scale feasibility of experimentally linking wastewater formulation with subsequent pigment recovery while identifying analytical, energetic, and scale-up limitations requiring further investigation.