Extracellular enzymatic activities of octocorals and scleractinian corals under environmental stress

Extracellular enzymes, released by coral holobionts (coral host, symbiotic dinoflagellates and associated microorganisms) are involved in nutrient cycling and can serve as diagnostic indicators of coral health and reef ecosystem functionality. For example, α-glucosidases (α-Glu), Leucine-aminopeptidases (LAP) and alkaline phosphatases (APA), hydrolyze large molecules into assimilable nutrients containing carbon, nitrogen and phosphorus, respectively. This study investigated the extracellular activity (EEA) of these three enzymes in octocoral and hexacoral species under different environmental conditions. Results revealed that EEA from mucus-associated microbes was low, while entire coral holobionts exhibited significant activity. Furthermore, under identical environmental conditions and substrate concentrations, LAP activity was the highest, followed by APA and α-Glu, suggesting nitrogen and phosphorus limitation rather than carbon. Heat and light stress significantly influenced enzyme activities, with APA showing the strongest increase, reflecting an increased demand for phosphorus and adaptive strategies to mitigate phosphorus limitation. Finally, all three EEAs were much lower in octocorals than in hexacorals. By investigating the mechanisms controlling enzymatic activities in corals, this research contributes to a deeper understanding of coral physiology and nutrient metabolism in response to changing environmental conditions.