Abstract
Mesophotic coral ecosystems (MCEs) are light-dependent coral-associated communities found at 30–150 m depth. Corals inhabiting these deeper reefs are often acclimatized to a limited and blue-shifted light environment, enabling them to maintain the relationship with their photosynthetic algal symbionts (family Symbiodiniaceae) despite the seemingly suboptimal light conditions. Among others, fluorescent proteins produced by the coral host may play a role in the modulation of the quality and spectral distribution of irradiance within the coral tissue through wavelength transformation. Here we examined the bio-optical properties and photosynthetic performances of different fluorescence morphs of two mesophotic coral species Goniopora minor and Alveopora ocellata, in order to test the photosynthesis enhancement hypothesis proposed for coral fluorescence. The green morph of G. minor and the low fluorescence morph of A. ocellata exhibit, in their natural habitats, higher abundance. The morphs also presented different spectral reflectance and light attenuation within the tissue. Nevertheless, chlorophyll a fluorescence-based, and O2 evolution measurements, revealed only minor differences between the photosynthetic abilities of three fluorescence morphs of the coral G. minor and two fluorescence morphs of A. ocellata. The fluorescence morphs did not differ in their algal densities or chlorophyll concentrations and all corals harbored Symbiodiniaceae from the genus Cladocopium. Thus, despite the change in the internal light quantity and quality that corals and their symbionts experience, we found no evidence for the facilitation or enhancement of photosynthesis by wavelength transformation.
Highlights
Stony corals are sessile organisms and are considered to be highly dependent on the photosynthates derived from their algal symbionts as their main energy source (Muscatine, 1990)
Ten colonies of the mesophotic scleractinian coral A. ocellata were collected from the reef in front of the Interuniversity Institute for Marine Sciences in Eilat (IUI; 29◦30 N, 34◦55 7 E) and seven colonies of G. minor were collected from the Dekel Beach site (29◦32 N, 34◦56 56 E), Gulf of Eilat/Aqaba (GoE/A), northern Red Sea
Two fluorescence morphs are described for A. ocellata: a low fluorescence morph (Figure 1D; λem = 520 nm) and a high fluorescence morph (Figure 1E; λem = 520 nm), both of which present the same green emission peak, with the distinction that the low fluorescence morph appears red under both illuminations due to chlorophyll fluorescence of its symbionts
Summary
Stony corals are sessile organisms and are considered to be highly dependent on the photosynthates derived from their algal symbionts (family Symbiodiniaceae; LaJeunesse et al, 2018) as their main energy source (Muscatine, 1990). Coral photoacclimatization to MCEs is manifested at the coral-host level as changes in colony morphology (Kaniewska et al, 2008; Nir et al, 2011), skeletal features that modify the internal light environment (Enríquez et al, 2005; Kahng et al, 2012), changes in the populations of endosymbiotic Symbiodiniaceae expressed as a shift in their genetic identity (Cooper et al, 2011; Bongaerts et al, 2013; Einbinder et al, 2016) and modifications in the composition of photosynthetic pigments and structure of the photosynthetic complex (Einbinder et al, 2016). Mesophotic corals may rely more on heterotrophy rather than autotrophy as their main strategy for acquiring energy (Mass et al, 2007; Lesser et al, 2010)
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