Uncovering the Potential of Nitrogen and Salt Stress for Enhanced ꞵ-Carotene Production and Antioxidant Capacity in Plant Pathogenic Alga Cephaleuros

A pure strain of Cephaleuros alga, designated Cephaleuros Cp.1, was successfully isolated directly from a citrus leaf lesion. This study investigated factors influencing both biomass and carotenoid accumulation in this green filamentous alga. Different nitrogen sources, NaCl stress, and trace elements in HSM, BBM, and Bristol media were compared. The autotrophic condition with HSM medium clearly offered the highest green biomass. Interestingly, Cephaleuros Cp.1 remained green in HSM using NH4Cl as the nitrogen source but visibly changed to an orange hue due to the accumulation of β-carotene in BBM containing NaNO3. This color change, along with the lower biomass and more intense yellow color when using nitrate, was the first reported in Cephaleuros, implying that nitrate may cause stress in the alga. Similar phenomena were clearly observed when NaCl was applied to HSM and BBM; on the other hand, Hutner’s trace elements and trace metal solution had no significant effect. These findings suggest, for the first time, a link between stress conditions and the accumulation of β-carotene in Cephaleuros Cp.1. TLC revealed β-carotene as the main carotenoid accumulated by this alga. The accumulation was further enhanced by both nitrogen deficiency and salt stress. However, these stresses also led to a decrease in algal biomass. This study is the first to report free radical scavenging activity linked to β-carotene in Cephaleuros. Among the tested cultures, BBM exhibited the strongest activity (EC50 1.40 mg/mL). These findings hold promise for future applications of Cephaleuros as a source of natural β-carotene with antioxidant properties.