Cultivation and physiological characterization of a desert-derived Halospirulina isolate

Here, we describe a filamentous Halospirulina isolate ( Halospirulina saudiensis ) obtained from water-clay microhabitat the Empty Quarter desert, الربع الخالي (ar-Rubʿ al-Khālī), Saudi Arabia which grows in saline conditions. We present its fully sequenced genome, the first for the genus, and characterize its growth dynamics as well as biochemical composition under a range of cultivation conditions. Protein, carbohydrate, lipid, and phycocyanin content varied with cultivation regime but were largely stable. H. saudiensis reached biomass concentrations of up to 9.83 g L ^-1 at pH 7, 35 °C and continuous 325 µmol photons m ^-2 s ^-1 . Variable climate simulations in lab-scale photobioreactors revealed preference for warmer season cultivation under modeled outdoor conditions. Carotenoid analysis revealed a pigment profile enriched in canthaxanthin and other ketocarotenoids, distinguishing it from industrial Limnospira and positioning its value for neutraceuticals and feed additives. Genome analysis identified a carotene ketolase ( crtO ) homolog consistent with other cyanobacteria that accumulate ketocarotenoids. Phycocyanin content was heavily dependent on culture health and varied with cultivation pH, irradiance, reaching maximum values of 67.3 ± 0.8 mg gDW ^-1 (6.73 %). Extracted phycocyanin showed marginal thermal stability compared to that from L. platensis . The findings suggest that H. saudiensis could be a promising source of biomass, ketocarotenoids, and natural pigments, cultivated in saline conditions with elevated temperature and irradiance.