Effects of Experimental CO₂ Enrichment on the PSII Photochemical Efficiency of 𝘚𝘺𝘮𝘣𝘪𝘰𝘥𝘪𝘯𝘪𝘶𝘮 sp. in 𝘈𝘤𝘳𝘰𝘱𝘰𝘳𝘢 𝘮𝘪𝘭𝘭𝘦𝘱𝘰𝘳𝘢

Abstract

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–²s–¹, and a seawater 𝘱CO₂ 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/F²ₘ’,midday), and a daily maximum at the onset of darkness at 19:00 h (Fᵥ/F²ₘ,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/F²ₘ’midday and Q²ₘ. In ambient seawater (Control) 𝘚𝘺𝘮𝘣𝘪𝘰𝘥𝘪𝘯𝘪𝘶𝘮 sp. exhibited a gradual decrease, over the course of the experiment, in ∆F/F²ₘ’,midday, Fv/F²ₘ,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 𝘱CO₂ on ∆F/F²ₘ’,midday, Fv/F²ₘ,19:00 h, and Q²ₘ, rendering this experiment inconclusive.