Abstract
Corals and their photosymbionts experience inherent changes in light along depth gradients, leading them to have evolved several well-investigated photoacclimation strategies. As coral calcification is influenced by light (a process described as LEC—‘light-enhanced calcification’), studies have sought to determine the link between photosynthesis and calcification, but many puzzling aspects still persist. Here, we examine the physiology of Euphyllia paradivisa, a coral species found at a wide range of depths but that is strictly mesophotic in the Red Sea; and also examines the coupling between photosynthesis and LEC by investigating the response of the coral under several controlled light regimes during a long-term experiment. E. paradivisa specimens were collected from 40 to 50 m depth and incubated under three light conditions for a period of 1 year: full-spectrum shallow-water light (approx. 3 m, e.g. shallow-light treatment); blue deep-water light (approx. 40 m, e.g. mesophotic-light treatment) or total darkness (e.g. dark treatment). Net photosynthesis remained similar in the shallow-light-treated corals compared to the mesophotic-light-treated corals, under both low and high light. However, calcification increased dramatically with increasing light intensity in the shallow-light-treated corals, suggesting a decoupling between these processes. Photoacclimation to shallow-water conditions was indicated by enhanced respiration, a higher density of zooxanthellae per polyp and lower chlorophyll a content per cell. The dark-treated corals became completely bleached but did not lower their metabolism below that of the mesophotic-light-treated corals. No Symbiodinium clade shift was found following the year-long light treatments. We conclude that E. paradivisa, and its original symbiont clade, can adapt to various light conditions by controlling its metabolic rate and growth energy investment, and consequently induce LEC.
Highlights
Recent studies have demonstrated that coral reefs below recreational SCUBA diving limitation, commonly referred to as mesophotic coral ecosystems (MCEs), host a thriving community of flora and fauna that has remained almost completely unexplored [1,2,3,4,5]
Euphyllia paradivisa was selected as the model coral for the different light condition experiments because it is abundant in the Gulf of Eilat/Aqaba (GoE/A), found along a broad mesophotic depth gradient (36–72 m depth), easy to fragment into individual polyps and, most importantly, strictly mesophotic in this area [14]
It was shown in a previous study that the Red Sea E. paradivisa is able to survive at shallow depths at a high rate [14], even compared to corals that are naturally distributed along the 8 transplantation depth range (i.e. [18,40,41])
Summary
Recent studies have demonstrated that coral reefs below recreational SCUBA diving limitation (greater than 30 m depth), commonly referred to as mesophotic coral ecosystems (MCEs), host a thriving community of flora and fauna that has remained almost completely unexplored [1,2,3,4,5]. In contrast with the shallow reefs (0–30 m depth), where corals are usually exposed to high solar energy (up to 2000 mmol photons m22 s21), MCEs (30– 150 m depth) are characterized mainly by a low light intensity (up to 150 mmol photons m22 s21) and narrower (bluish) light spectrum Corals typically found along the shallow reefs have been shown to possess several mechanisms that enable them to cope with the intense light regime: a high density of UV-absorbing mycosporine-like amino acids (MAAs) [8,9], and xanthophylls for non-photochemical quenching [10]. Very few MCE corals have been examined in this respect [11]
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