Heat-stressed corals and the important role of picoplankton-derived nitrogen
摘要
Carbon (C) and nitrogen (N) are essential nutrients for coral–Symbiodiniaceae associations, yet global change can disrupt C and N acquisition by corals, affecting their resilience under stress. We investigated how two octocorals (Sarcophyton glaucum, Lobophytum sp.) and two hexacorals (Stylophora pistillata, Turbinaria reniformis) assimilate nitrogen from three 15N-labelled sources—dissolved free amino acids (DFAAs), Synechococcus (picoplankton), and Artemia salina nauplii (microplankton)—supplied at 1 µM N, under control (26 °C) and heat stress (30 °C) conditions. Corals were also incubated with natural pico-nanoplankton assemblages, with concentrations measured via flow cytometry. In addition, we measured the rates of photosynthesis and respiration, to estimate the relative contribution of autotrophy to the corals’ respiratory needs. Across all species, Synechococcus was the most efficiently assimilated N source, with uptake increasing under heat stress. Estimates of heterotrophic carbon assimilation (using C:N ratio) coupled with respiratory measurements showed that Synechococcus can provide 30–70% of the corals’ daily respiratory carbon demand, especially in the most heterotrophic species. Octocorals exhibited lower C and N assimilation rates than hexacorals, likely due to lower N needs and consistent with their overall lower metabolic requirements. Notably, S. glaucum displayed great flexibility in compensating reduced autotrophy through heterotrophy under heat stress. Incubations with natural plankton assemblages revealed increases in pico-nanoplankton concentrations in the presence of corals, likely stimulated by coral mucus release. These results demonstrate that picoplankton contributes significantly to both carbon and nitrogen budgets during heat stress and highlight species-specific nutritional strategies that may shape coral resilience under future climate conditions.