Effects of anthropogenic calcium enrichment and phosphorus limitation on Phaeocystis globosa bloom formation

Phaeocystis globosa, a bloom-forming haptophyte, dominates phosphorus limited (P-limited) coastal environments where calcium concentrations are also increasingly elevated due to anthropogenic inputs. However, the interplay between P-limitation and calcium enrichment in shaping its bloom dynamics remains poorly understood. This study investigates the physiological and transcriptomic responses of P. globosa under varying calcium concentrations ([Ca²⁺]) (400, 800 and 1500 mg/L) in both P-replete (36.20 μmol/L) and P-limited (13.75 μmol/L) conditions. Results show that the response of P. globosa to elevated [Ca²⁺] is strongly dependent on phosphorus availability. Under P-limitation, 400–800 mg/L [Ca²⁺] stimulates colony expansion by enhancing Ca²⁺-ATPase activity, maintaining calcium homeostasis, sustaining photosynthesis, promoting glycosaminoglycan (GAG) and exopolysaccharide (EPS) synthesis. These adaptations bolster colony matrix production and increase bloom potential. Conversely, high [Ca²⁺] (1500 mg/L) induced oxidative stress, suppressed polysaccharide production, GAG biosynthesis and impaired colony formation, particularly under P-limitation. Transcriptomic analyses revealed that P-limited cells activated diverse phosphorus-scavenging strategies including phospholipid remodelling, and inorganic pyrophosphate hydrolysis, although these mechanisms were compromised under high [Ca²⁺]. Together these results demonstrate a synergistic effect of P-limitation and calcium enrichment in promoting P. globosa bloom formation. Understanding this calcium‑phosphorus interplay is crucial for anticipating harmful algal bloom (HAB) trajectories in response to increasing nutrient shifts and anthropogenic calcium inputs in coastal systems.