Effects of experimental CO2 enrichment on the PSII photochemical efficiency of Symbiodinium sp. in Acropora millepora

Enrichment of seawater with CO2 decreases the concentration of the carbonate ion while increasing that of hydrogen and bicarbonate ions. We use pulse-amplitude-modulation (PAM) fluorometry to investigate whether, in the absence of warming, and in sub-saturating light, these changes affect the PSII photochemical efficiency of _Symbiodinium_ sp. in the reef-building coral _Acropora millepora_. We assessed this experimentally with 30-min-interval saturation pulse analyses at 25 °C, a daily peak in the intensity of the photosynthetically active radiation (PAR) at ~65 µmol quanta m–2 s–1, and a seawater _p_CO2 that we gradually increased over nine days from ~496 to ~1290 μatm by injection of CO2-enriched air. Nine 14-day time series, which, except one, were recorded at the growing apices of a coral branch, revealed diel oscillations in the PSII photochemical efficiency characterized by a steep nocturnal decrease followed by a steep increase and peak in the morning, a daily minimum at midday (∆F/Fm’,midday), and a daily maximum at the onset of darkness at 19:00 h (Fv/Fm,19:00 h). An inadvertent shift in the position of one of the PAM fluorometer measuring heads revealed differences between the basal part and the growing coral apices of a coral branch in ∆F/Fm’midday and Qm. In ambient seawater (Control) _Symbiodinium_ sp. exhibited a gradual decrease, over the course of the experiment, in ∆F/Fm’,midday, Fv/Fm,19:00 h, and the slope of the linear regression between the relative electron transport rate and the intensity of PAR (rETR/PAR). Although two of three successive experiments indicated that CO2 enrichment counteracted these trends, statistical analyses failed to confirm an influence of _p_CO2 on ∆F/Fm’,midday, Fv/Fm,19:00 h, and Qm, rendering this experiment inconclusive.