Enhancing phycocyanin productivity and thermostability in Synechocystis sp. AUPL1 using Ulva lactuca hydrolysates and ulvan polysaccharides

Increasing the productivity and thermostability of phycocyanin (PC) are key challenges in its industrial production. This study utilized aqueous and acidic hydrolysates derived from the seaweed Ulva lactuca for fed-batch mixotrophic cultivation of Synechocystis sp. AUPL1. The acidic hydrolysate contained higher levels of nutrients and reducing sugars (0.08 mg mL⁻¹) compared to the aqueous hydrolysate (0.03 mg mL⁻¹). Supplementing the cyanobacterial culture with 3 % (v/v) acidic hydrolysate every three days significantly increased biomass productivity, PC content, and PC productivity, achieving approximately 2-, 3-, and 6.5-fold improvements, respectively, compared to the autotrophic control. Additionally, ulvan polysaccharide, extracted from the same seaweed, was tested as a natural preservative to prevent PC degradation at elevated temperatures. Ulvan at 3 % (w/v) significantly extended the PC half-life at 60 °C to 293.21 min, compared to 101.65 min in the control. Thermodynamic analysis confirmed the thermostabilizing effect of the PC-ulvan mixture, revealing that the thermal degradation process was endothermic and non-spontaneous. This study underscores the potential of U. lactuca biomass as a low-cost, sustainable nutrient source for enhancing PC productivity, while also demonstrating that ulvan effectively improves PC thermostability.