| Summary: | Global trends of increasing atmospheric CO2, warming, and eutrophication enrich seawater with CO2, either directly, through gas absorption, or indirectly, through microbial processes. This enrichment alters the speciation of inorganic carbon, increasing seawater [H+] and [HCO3–], and decreasing the seawater carbonate saturation state and buffering capacity. Because corals use [HCO3–] as carbon source, CO2 enrichment may increase coral photosynthesis. The CO2 induced increase in [H+] on the other hand, may increase the energy required for the upregulation of the calcifying fluid pH resulting in a decrease in photosynthesis. Here, I tested the effects of CO2 enrichment on the photochemical efficiency of the reef building coral Acropora millepora (Ehrenberg, 1834) at an ambient temperature of ~25 ℃ using pulse-amplitude modulated fluorometry. Nine coral fragments were acclimated to a daily light cycle increasing to ~70 µmol m–2 s–1 at midday and then exposed to increasing seawater pCO2 during the following 16 days. A saturation pulse analysis was conducted every thirty minutes to assess the photochemical efficiency of the coral fragments. This time series was interrupted daily between 0200 and 0230 hours to conduct a fast sequence of saturation pulse analyses during the induction of photosynthesis and subsequent recovery. These measurements failed to demonstrate that CO2 enrichment affects the maximum photochemical efficiency (Fv/Fm), the effective photochemical efficiency (∆F/Fm’), or the maximum excitation pressure over PSII (Qm) of the coral symbiont adding to the existing evidence that CO2 enrichment does not affect coral photosynthesis. Furthermore, the time-series measurements revealed diel variations in the Fv/Fm ratio that point to the existence of an alternate respiratory pathway, chlororespiration, and the induction of state transitioning: a nocturnal decreases in the Fv/Fm ratio followed by a sharp increase in Fv/Fm at the onset of low irradiance at the start of the day.
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