Abstract
Photosynthetic coral reef structures extend from the shallow sundrenched waters to the dimly lit, “twilight” mesophotic depths. For their resident endosymbiotic dinoflagellates, primarily from the genus Symbiodinium spp., this represents a photic environment that varies ~15 fold in intensity and also differs in spectral composition. We examined photosynthesis in the scleractinian coral Stylophora pistillata in shallow (3 m) and mesophotic settings (65m) in the northern Red Sea. Symbiodinium spp. in corals originating from the mesophotic environment consistently performed below their photosynthetic compensation point and also exhibited distinct light harvesting antenna organization. In addition, the non-photochemical quenching activity of Symbiodinium spp. from mesophotic corals was shown to be considerably lower than those found in shallow corals, showing they have fewer defenses to high-light settings. Over a period of almost four years, we extensively utilized closed circuit Trimix rebreather diving to perform the study. Phylogenetic analysis showed that shallow corals (3m) transplanted to a deep reef environment (65 m) maintained their initial Symbiodinium spp. community (clade A), rather than taking on deep low-light clades (clade C), demonstrating that shallow S. pistillata acclimate to low-light mesophotic environments while maintaining their shallow photosynthetic traits. Mesophotic corals exhibited static depth-related chlorophyll content per cell, a decrease in PSI activity and enhanced sigmoidal fluorescence rise kinetics. The sigmoidal fluorescence rise kinetics we observed in mesophotic corals is an indication of energy transfer between photosynthetic units. We postulate that at mesophotic depths, a community of adapted Symbiodinium spp. utilize a unique adaptation to lower light conditions by shifting their light harvesting to a PSII based system, where PSII is structured near PSI, with additional PCP soluble antenna also trapping light that is funneled to the PSI reaction center. In this study, we provide evidence that mesophotic Symbiodinium spp. have developed novel adaptive low-light characteristics consisting of a cooperative system for excitation energy transfer between photosynthetic units that maximizes light utilization.
Highlights
Scleractinian corals build the framework of the world’s tropical reefs and their growth relies primarily on photosynthetic dinoflagellate endosymbionts from the genus Symbiodinium (Jones et al, 1998)
non-photochemical quenching (NPQ) activity was measured in S. pistillata (3 and 65 m), and was up to five times higher in shallow corals than in the mesophotic corals (Figure 3 and Table 1)
Using the Fluorescence Induction and Relaxation (FIRe) approach (Kolber et al, 1998), we found that mesophotic PSII traps energy better than shallow PSII; and its variable fluorescence magnitude is higher than the shallow symbionts (Table 1)
Summary
Scleractinian corals build the framework of the world’s tropical reefs and their growth relies primarily on photosynthetic dinoflagellate endosymbionts from the genus Symbiodinium (Jones et al, 1998). These photosymbionts reside within the corals endoderm and produce photosynthates that are translocated to the host (Muscatine et al, 1975; Falkowski and Dubinsky, 1981). Light intensity decays exponentially with depth (Gordon, 1989) and the mesophotic reef environment, known as the “twilight zone,” is characterized by light fluxes that vary seasonally between 5 and 100 μmol photons m−2 s−1. There have been studies that have reported no significant changes in the Symbiodinium chlorophyll content at depths below 25 m (Titlyanov et al, 2002; Mass et al, 2007; Lesser et al, 2010)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
