DiPerna, Stephanie, Hoogenboom, Mia, Noonan, Sam, and Fabricius, Katharina (2018) Effects of variability in daily light integrals on the photophysiology of the corals Pachyseris speciosa and Acropora millepora. PLoS ONE, 13 (9). e0203882.

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Abstract

Phototrophic sessile organisms, such as reef corals, adjust their photosynthetic apparatus to optimize the balance of light capture versus protection in response to variable light availability (photoacclimation). In shallow marine environments, daily light integrals (DLI) can vary severalfold in response to water clarity and clouds. This laboratory study investigated the responses of two coral species to fluctuations in DLI. Corals were exposed to four contrasting DLI treatments: 'highlight' (potentially photoinhibiting conditions, 32 mol photons m(-2) d(-1)), 'lowlight' (potentially lightlimiting conditions, 6 mol photons m(-2) d(-1)), and two ` variable light' treatments that alternated between high and low conditions every 5 days. In the variable treatments, the shadetolerant coral Pachyseris speciosa displayed cycles of rapid declines in maximum quantum yield during highlight and subsequent recoveries during lowlight, showing photoacclimation at a time scale of 3 +/- 5 days. In contrast, the shallowwater coral Acropora millepora showed slow (> 20 days) photoacclimation, and minimal changes in photosynthetic yields despite contrasting light exposure. However, growth (change in buoyant weight) in A. millepora was significantly slower under variable light, and even more so under lowlight conditions, compared with highlight conditions. The responses of yields in P. speciosa match their preference for lowlight environments, but suggest a vulnerability to even short periods of highlight exposure. In contrast, A. millepora had better tolerance of highlight conditions, however its slow photoacclimatory responses limit its growth under low and variable conditions. The study shows contrasting photoacclimatory responses in variable light environments, which is important to identify and understand as many coastal and midshelf reefs are becoming increasingly more turbid, and may experience higher variability in light availability.

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