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.