Effects of variability in daily light integrals on the photophysiology of the corals Pachyseris speciosa and Acropora millepora

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 several-fold 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: 'high-light' (potentially photoinhibiting conditions, 32 mol photons m-2 d-1), 'low-light' (potentially light-limiting 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 shade-tolerant coral Pachyseris speciosa displayed cycles of rapid declines in maximum quantum yield during high-light and subsequent recoveries during low-light, showing photoacclimation at a time scale of 3-5 days. In contrast, the shallow-water 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 low-light conditions, compared with high-light conditions. The responses of yields in P. speciosa match their preference for low-light environments, but suggest a vulnerability to even short periods of high-light exposure. In contrast, A. millepora had better tolerance of high-light 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.

Fig 1. Photosynthetic potential, light stress and non-photochemical quenching under constant high and low DLI, and under two variable DLI treatments.

Mean maximum quantum yield (F_v/F_m), excitation pressure on PSII (Q_m) and non-photochemical quenching (NPQ) of Pachyseris speciosa (A, C, E)—and Acropora millepora (B, D, F), over the 20-days experiment in high DLI treatment (HL, orange), low DLI treatment (LL, purple), variable DLI 1 treatment (VL1, blue dashed lines) and variable DLI 2 treatment (VL2, green dashed lines). Values represent means over 16 colonies per treatment per species, with shaded areas representing standard error.

Fig 2. Effects of constant and variable light on pigment concentrations.

Concentration of chlorophyll a (μg cm^-2) and total carotenoids (μg cm^-2) in Pachyseris speciosa (N = 5–6 nubbins/treatment) and Acropora millepora (N = 16 nubbins/treatment) under high DLI (white), low DLI (black), and variable DLI (VL1, light gray, and VL2, dark gray) treatments at the end of the 20-days experiment. Tukey HSD post-hoc results from one-way ANOVA comparison superimposed. Error bars represent standard error.

Fig 3. Photosynthesis-irradiance curves describing the contrasting photosynthetic features of the study species.

Mean light-dependent oxygen production or consumption (μmol O₂ cm^-2 h^-1) for Pachyseris speciosa (A) and Acropora millepora (B) at the end of the 20-day experiment in the high DLI (HL, solid grey), low DLI (LL, solid black), and variable DLI (VL1, dashed light gray, and VL2, dashed dark gray) treatments. N = 2–3 colonies/treatment/species.

Fig 4. Effects of constant versus variable light on daily net oxygen production.

Net daily production, Pn (μmol O₂ cm^-2 d^-1), for Pachyseris speciosa and Acropora millepora in high DLI (HL, white), low DLI (LL, black), variable DLI treatment ending in high-light (VL1, light gray) and variable DLI treatment ending in low-light (VL2, dark gray) derived from P-I curves. Error bars represent standard error, N = 2-3/treatment/species.

Fig 5. Relative growth of Acropora millepora between constant and variable DLI treatments.

Mean percent change of Acropora millepora buoyant weight in the high DLI (HL, white), low DLI (LL, black), and two variable DLI treatments (VL1, light grey & VL2, dark grey) after 12 days. Tukey HSD post-hoc results from one-way ANOVA comparisons superimposed. Error bars represent standard error, n = 16 per treatment.