Photoacclimation and induction of light-enhanced calcification in the mesophotic coralEuphyllia paradivisa

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.

Photosynthesis and Bio-Optical Properties of Fluorescent Mesophotic Corals

“…Conducting the metabolic measurements under blue light resulted in smoother P-E curves (Supplementary Figure 3), which may represent a more natural performance of mesophotic corals. Most of the previously reported values for P-E derived parameters that we were able to compare to our measurement aligned with the current results (Cooper et al, 2011;Nir et al, 2014;Eyal et al, 2019). The results indicate that mesophotic corals usually present relatively low compensation irradiance ranging between 15 and 96 µmol photons m −2 s −1 as well as low saturating irradiances, ranging between 28 and 80 µmol photons m −2 s −1 leaving a narrow window of light intensities which enable photosynthesis.…”

“…Our measured values of Symbiodiniaceae density are higher than previously reported values in mesophotic corals (Bongaerts et al, 2011;Cooper et al, 2011), however this parameter can greatly vary between species, depth, and light availability [reviewed by Roth (2014)]. Chlorophyll concentration values determined in this study, align with previously reported values ( Lesser et al, 2010;Cooper et al, 2011;Eyal et al, 2019). The latter result also indicates that the brighter color of the yellow morph of G. minor is probably the result of a higher expression of FPs rather than a low algal density or low chlorophyll concentration.…”

“…We therefore sought to determine whether a specific fluorescent signal would indeed attract symbionts that differ genetically. The genetic identity of Symbiodiniaceae revealed no significant effect of the fluorescence morph, and all corals harbored Symbiodiniaceae from the genus Cladocopium, as previously described in other coral species at the mesophotic reefs of the GoE/A (Nir et al, 2011;Einbinder et al, 2016;Eyal et al, 2019), as well as at other mesophotic reefs worldwide (Ziegler et al, 2015;Goulet et al, 2019). We therefore conclude that despite the possibly of serving as general Symbiodiniaceae attractant, specific fluorescence emissions do not attract specific Symbiodiniaceae genotypes.…”

Depth alone is an inappropriate proxy for physiological change in the mesophotic coralAgaricia lamarcki

“…Coral growth rates can vary with increasing depth (Baker & Weber, 1975), and this in turn can leave isotopic signals (Patzold, 1984). Further, coral growth rates can vary with light exposure, independent of changes in photosynthesis (Eyal et al ., 2019). Growth signals can also correlate with light exposure in skeletal carbon fractionation (Shimamura et al ., 2008).…”

Response of fluorescence morphs of the mesophotic coral Euphyllia paradivisa to ultra-violet radiation

“…Despite being found only at depths greater than 36 m in Eilat, this species has demonstrated both a low mortality rate and an ability also to prosper in shallow depths and shallow light environments for prolonged periods of time following transplantation 20 . When the photoacclimatization potential of this species to a shallow light environment was tested 21 , the corals lost their original mesophotic ability to efficiently utilize low-light intensities, but were still able to withstand higher light intensities before suffering from photoinhibition (Ben-Zvi et al . ; in preparation).…”