Abstract :
[en] Understanding the physiological response of hermatypic scleractinians to higher pCO2 is critical to predict the future of coral reefs. Our aim is to investigate key aspects of the coral holobiont physiology: calcification, photosynthesis efficiency and nutrients assimilation. Two Acroporidae (Acropora digitifera and Acropora muricata) and two Pocilloporidae (Pocillopora damicornis and Seriatopora hystrix) are studied in original, patented chemostats (total volume: 1.25L). They allow to maintain scleractinians in controlled and monitored environment (temperature 26±0.1 °C, salinity 34±0.1, total alkalinity 2.40±0.02 meq.kg-1,[N] 1±0.5 µmol.L-1,[P] 0.4±0.2 µmol.L-1,12h/12h light/dark phases, 250 µE.m-1.s-2). Four replicates for each species are placed in two contrasted pH (8.05+-0.02 & 7.80+-0.02) during a 24h period. Complete balance of photosynthesis, respiration, calcification, CO2 fluxes and nutrients assimilation is calculated every four hours. PSII complex efficiency is also estimated by pulse amplitude modulation (PAM) fluorometry.
Under current pCO2, we observe equilibrium between net photosynthesis and dark respiration for each species: primary production by zooxanthellae is high enough to sustain the metabolism of the holobiont, including its high calcification rate. At short term and under higher pCO2, calcification rate is not affected, but we observe a clear change of carbon fluxes, that is, a decrease in net photosynthesis, and higher dark-phase respiration rate. Nutrients assimilation, which occurs primarily during the light-phase, is also affected: a decrease is observed both for N and P. Experiments after a longer acclimation phase are planned to assess if there is any recovery of the equilibrium after adjustment of symbioses to the lower pH conditions.
