Acclimatization of symbiotic corals to mesophotic light environments through wavelength transformation by fluorescent protein pigments.

Edward G Smith,Yoni Sharon,Cecilia D'Angelo,Dan Tchernov,Joerg Wiedenmann

doi:10.1098/rspb.2017.0320

Abstract

The depth distribution of reef-building corals exposes their photosynthetic symbionts of the genus Symbiodinium to extreme gradients in the intensity and spectral quality of the ambient light environment. Characterizing the mechanisms used by the coral holobiont to respond to the low intensity and reduced spectral composition of the light environment in deeper reefs (greater than 20 m) is fundamental to our understanding of the functioning and structure of reefs across depth gradients. Here, we demonstrate that host pigments, specifically photoconvertible red fluorescent proteins (pcRFPs), can promote coral adaptation/acclimatization to deeper-water light environments by transforming the prevalent blue light into orange-red light, which can penetrate deeper within zooxanthellae-containing tissues; this facilitates a more homogeneous distribution of photons across symbiont communities. The ecological importance of pcRFPs in deeper reefs is supported by the increasing proportion of red fluorescent corals with depth (measured down to 45 m) and increased survival of colour morphs with strong expression of pcRFPs in long-term light manipulation experiments. In addition to screening by host pigments from high light intensities in shallow water, the spectral transformation observed in deeper-water corals highlights the importance of GFP-like protein expression as an ecological mechanism to support the functioning of the coral–Symbiodinium association across steep environmental gradients.

Highlights

Light plays a fundamental role in structuring communities of photosynthetic species [1,2,3,4]
Communities in deeper water must possess mechanisms to cope with a light environment that is characterized by reduced spectral composition and low intensity
Corals living on deeper reefs inhabit an environment characterized by low light intensities and limited spectral composition of the incident irradiance [6,21]

Summary

Introduction

Light plays a fundamental role in structuring communities of photosynthetic species [1,2,3,4]. We argue that in corals the intra-tetrameric FRETmediated, blue-to-orange wavelength transformation by pcRFPs represents an adaptation of the coral host to blue, low-intensity light environments, which benefits the symbionts located deeper in the tissue This redistribution is critical because common low light acclimatization strategies of corals and their symbionts, namely increased numbers of zooxanthellae and/or higher pigment content [12,27,36], create a packaging effect [37]. We experimentally demonstrate that spectral transformation by pcRFPs can exploit the absorption window of zooxanthellae pigments to facilitate enhanced light penetration in the coral tissue to sustain photosynthesis in deeper symbiont layers. Corals in the deep water simulation received an irradiance of approximately 25 mmol photons m22 s21 of blue light from a 54 W Narva T5 fluorescent lamp (Tunze, Penzberg, Germany; electronic supplementary material, figure S1). The proportion of red fluorescent colonies was recorded and calculated relative to the total number of coral colonies

Results

Findings

Discussion