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In oxygenic photosynthesizing organisms, it has been noticed on a number of occasions that photosynthetic performance was lower in the afternoon than in the morning, at the same light intensities. This hysteresis phenomenon is called the 'afternoon depression' and has been observed in phytoplankton, macroalgae, and higher plants. Here we characterize, with high temporal resolution, in situ diel courses of oxygen evolution and chlorophyll fluorescence yields in 3 Indo-Pacific corals (Favia favus, Goniopora lobata, Plerogyra sinuosa) and 2 Caribbean symbiotic corals (Montastrae faveolata and Porites astreoides) using a 3-chamber submersible respirometer and a SCUBAbased fast repetition rate fluorometer. In contrast to all previously published cases, the oxygen measurements revealed an unexpected hysteresis, with higher photosynthetic rates occurring in the afternoon than in the morning. The oxygen diel patterns were highly consistent in all organisms examined, including the 3 corals and 2 macroalgae (Ulva sp. and Gracilaria sp.) from the Red Sea. Surprisingly, the diurnal patterns of the quantum yields of photochemistry in Photosystem II (PSII), assessed from variable fluorescence (quantum yield of photochemistry in PSII measured under ambient light, ∆F '/F m ') exhibited much higher variability and often showed a hysteresis pattern opposite to that of oxygen. In most organisms the values of ∆F '/F m ' and the deduced rates of photosynthetic electron transport were higher in the morning than in the afternoon; however, the opposite trend with lower values of ∆F '/F m ' in the morning was also observed. Lower values of ∆F '/F m ' were always accompanied by higher quantum yields of non-photochemical quenching, consistent with the energetic balance within the primary photosynthetic reactions. The direction of the diurnal hysteresis in variable fluorescence appears to be species-specific and may vary even within the same species, reflecting microscale variability in bio-optical properties and gross photosynthesis of the corals.

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