Combining the Salt-Tolerant Plants Limonium algarvense Erben and Polygonum maritimum L. Promotes Synergistic Bioactivities for Enhanced Cosmetic Applications.

The demand for natural and sustainable ingredients in dermo-cosmetic formulations has promoted the use of coastal salt-tolerant plants as novel bioresources. This study evaluated aqueous extracts of Limonium algarvense and Polygonum maritimum, two salt-tolerant coastal plants and their mixtures (3:1, 1:1, 1:3, w/w) for antioxidant, metal-chelating, enzyme-inhibitory, antimicrobial and cytotoxic properties. Chemical profiling revealed diverse phenolic signatures dominated by catechin, myricetin and quercetin derivatives, which are known contributors to antioxidant and enzyme-modulating activity. L. algarvense showed the strongest superoxide-scavenging activity (EC50 = 219 µg/mL), while P. maritimum was the most effective hyaluronidase inhibitor (EC50 = 42.9 µg/mL). The 1:3 mixture displayed the most pronounced synergistic behaviour, notably in hydroxyl-radical scavenging (EC50 = 62.7 µg/mL; SE = 0.08) and antibacterial activity against Escherichia coli (MIC = 39.68 µg/mL; SE = 0.31). The 3:1 mixture showed enhanced elastase inhibition (EC50 = 33.0 µg/mL; SE = 0.76) and increased copper-chelating capacity (EC50 = 533 µg/mL, SE = 0.52). All extracts maintained cell viability above 90%, supporting their safety for topical applications. Together, these results indicate that L. algarvense and P. maritimum deliver complementary and synergistic activities relevant to dermo-cosmetic innovation, emphasising their potential as sustainable, multifunctional ingredients compatible with saline agriculture and low-freshwater production systems.