Mesophotic coral depth acclimatization is a function of host-specific symbiont physiology

Maren Ziegler,Cornelia M Roder,Claudia Bã¼Chel,Christian R Voolstra

doi:10.3389/fmars.2015.00004

Abstract

Mesophotic coral ecosystems receive increasing attention owing to their potential as deep coral refuges in times of global environmental change. Here, the mechanisms of coral holobiont photoacclimatization over a 60 m depth gradient in the central Red Sea were examined for the four coral genera Porites, Leptoseris, Pachyseris, and Podabacia. General acclimatization strategies were common to all host-symbiont combinations, e.g. Symbiodinium cell densities and photoprotective to light-harvesting pigment ratios both significantly decreased with water depth. Porites harboured Symbiodinium type C15 over the whole 60 m depth range, while Pachyseris and Podabacia had limited vertical distributions and hosted mainly Symbiodinium type C1. Symbiodinium type C15 had generally higher xanthophyll de-epoxidation rates and lower maximum quantum yields than C1, and also exhibited a strong photoacclimatory signal over depth that relates to the large distribution range of Porites. Interestingly, the coral host had an effect on Symbiodinium pigment composition. When comparing Symbiodinium type C1 in Podabacia and Pachyseris, the s-carotene chl a-1, the peridinin chl a-1, and diadinoxanthin chl a-1 ratios were significantly different between host species. Our data support a view that depth acclimatization of corals in the mesophotics is supported by Symbiodinium physiology, which in turn is host-specific.

Highlights

Light in the ocean decreases exponentially with depth and it is one of the most important factors shaping coral communities (Falkowski and Dubinsky, 1981; Dubinsky et al, 1984)
Diadinoxanthin and diatoxanthin are components of the xanthophyll cycle in dinoflagellates, and together with ß-carotene (ß-car) comprise the PP pigments (Jeffrey and Haxo, 1968; Brown et al, 1999). ß-carotene is found in both photosystems together with chl a where it mainly acts as a sink for excess light energy, and aids in stabilizing the photosystem structure (Kirk, 1994; Frank and Cogdell, 1996; Fromme et al, 2006)
Some general trends of light acclimatization could be observed for all central Red Sea corals, and these trends were similar to those reported from studies conducted in other oceans

Summary

Introduction

Light in the ocean decreases exponentially with depth and it is one of the most important factors shaping coral communities (Falkowski and Dubinsky, 1981; Dubinsky et al, 1984). While light limitations constraint most Symbiodinium-bearing corals to depths shallower than 90 m, photosynthetically active corals can be found as deep as 145 m in the Red Sea owing to the transparency of the water column (Schlichter et al, 1986). This is because corals rely on light energy for the photoautotrophic carbon production through their dinoflagellate endosymbionts from the genus Symbiodinium (Muscatine and Porter, 1977; Muscatine et al, 1981). In Symbiodinium, light-harvesting (LH) and photoprotective (PP) pigments form part of the photosynthetic apparatus. Diadinoxanthin (ddx) and diatoxanthin (dtx) are components of the xanthophyll cycle in dinoflagellates, and together with ß-carotene (ß-car) comprise the PP pigments (Jeffrey and Haxo, 1968; Brown et al, 1999). ß-carotene is found in both photosystems together with chl a where it mainly acts as a sink for excess light energy, and aids in stabilizing the photosystem structure (Kirk, 1994; Frank and Cogdell, 1996; Fromme et al, 2006)

Methods

Results

Conclusion