Phycoremediation potential of Microcystis aeruginosa and Auxenochlorella pyrenoidosa for salinity reduction: mechanisms and isotherm modeling

The treatment of saline and brackish water remains a major challenge, driving the search for low-energy biological alternatives. Accordingly, this study evaluated the salinity-reduction capacity of Microcystis aeruginosa and Auxenochlorella pyrenoidosa, isolated from brackish water in the vicinity of Lake Qarun, Fayoum Governorate, Egypt, as potentially sustainable biological agents for salt removal from saline waters. The isolates were evaluated under varying NaCl concentrations (10, 20, and 30 g/L) and inoculation ratios 1:6.67, 1:10, and 1:20 (v/v) over a 12-day treatment period. Physicochemical and molecular characterizations were conducted using total dissolved solids, electrical conductivity (EC), and pH measurements, Fourier-transform infrared (FTIR), scanning electron microscopy (SEM)–energy-dispersive X-ray spectrometer (EDX), and adsorption isotherm modeling. M. aeruginosa achieved a maximum salt removal of 17.41% (3.01 g/L) at 20 g/L NaCl with a 1:6.67 inoculation ratio, while A. pyrenoidosa removed 2.67 g/L (15.62%). Corresponding EC reductions reached 3.91 and 3.62 mS/cm, respectively. SEM–EDX and FTIR analyses revealed distinct, species-specific structural and molecular responses, with M. aeruginosa exhibiting greater biomass-associated Na+ retention and a higher apparent ion-removal capacity than A. pyrenoidosa. In both species, salt removal was concentration dependent and involved a combination of surface-level interactions and intracellular or biomass-associated ion retention processes, thereby underscoring their potential as sustainable, complementary approaches to conventional desalination technologies.