This thesis provides original empirical findings on the twin-role potential of halotolerant microalgae for wastewater bioremediation and crop biofertilisation within a circular bioeconomy framework. Regarding point (a), the influence of salinity is crucial; cultivating microalgae in ionic media (Na⁺ and Cl⁻ at 1–2 g/L) significantly boosted metabolic efficiency, accelerating nitrate and phosphate reduction by 90% or more. For point (b), exceptional bioremediation and biodesalination capabilities were demonstrated: at low ionic strength, Scenedesmus obliquus removed 96.2% of nitrates and 93.6% of phosphates, while Chlorella vulgaris achieved 57.3% chloride uptake. At full-marine strengths, Porphyridium purpureum absorbed 67.2% of chloride, proving robust biodesalination potential. Addressing point (c), greenhouse trials confirmed the high effectiveness of mechanically processed, nutrient-rich post-growth biomass as a biofertiliser and biostimulant across 13 plant species. Factorial assays validated that microalgae treatments performed on par with or better than synthetic options, with C. vulgaris increasing tomato height by 33.7% and radish root extension by 41.3%. Finally, for point (d), regional material flow analysis and systems modelling evaluated the strategy's integration within a Water-Energy-Food-Climate (WEFC) nexus in Saudi Arabia. Scaled systems could sequester 758,000 tonnes of CO₂ annually, generate 192 GWh/year of bioenergy, and cut synthetic fertiliser use by 75%. Identifying Riyadh, Makkah, and the Eastern Region as Tier 1 zones, the study proposes a seven-phase deployment schedule (2025–2060) aligned with Saudi Vision 2030 and Net-Zero 2060. This strategy targets an 80% national wastewater reuse rate, 830,000 annual tonnes of CO₂ offsets, and 150,000 green jobs, driving a novel Regional Circularity Index from 0.24 to 0.75 by 2040. Ultimately, this reframes microalgae as central agents of sustainability and economic diversification in arid regions.
