A community change in the algal endosymbionts of a scleractinian coral following a natural bleaching event: field evidence of acclimatization.

A M Jones,J C Mieog,R Berkelmans,M.J.H Van Oppen,W Sinclair

doi:10.1098/rspb.2008.0069

Abstract

The symbiosis between reef-building corals and their algal endosymbionts (zooxanthellae of the genus Symbiodinium) is highly sensitive to temperature stress, which makes coral reefs vulnerable to climate change. Thermal tolerance in corals is known to be substantially linked to the type of zooxanthellae they harbour and, when multiple types are present, the relative abundance of types can be experimentally manipulated to increase the thermal limits of individual corals. Although the potential exists for this to translate into substantial thermal acclimatization of coral communities, to date there is no evidence to show that this takes place under natural conditions. In this study, we show field evidence of a dramatic change in the symbiont community of Acropora millepora, a common and widespread Indo-Pacific hard coral species, after a natural bleaching event in early 2006 in the Keppel Islands (Great Barrier Reef). Before bleaching, 93.5% (n=460) of the randomly sampled and tagged colonies predominantly harboured the thermally sensitive Symbiodinium type C2, while the remainder harboured a tolerant Symbiodinium type belonging to clade D or mixtures of C2 and D. After bleaching, 71% of the surviving tagged colonies that were initially C2 predominant changed to D or C1 predominance. Colonies that were originally C2 predominant suffered high mortality (37%) compared with D-predominant colonies (8%). We estimate that just over 18% of the original A. millepora population survived unchanged leaving 29% of the population C2 and 71% D or C1 predominant six months after the bleaching event. This change in the symbiont community structure, while it persists, is likely to have substantially increased the thermal tolerance of this coral population. Understanding the processes that underpin the temporal changes in symbiont communities is key to assessing the acclimatization potential of reef corals.

Highlights

Coral reefs owe their success to the symbiosis between reef-building corals and intracellular, phototrophic dinoflagellates of the genus Symbiodinium that supply up to 95% of the coral host’s energy requirements (Muscatine 1990)
The balance of the symbiosis shifted from a predominant association between A. millepora and Symbiodinium type C2 to a predominance of type D and to a lesser extent to predominance of type C1
We propose that symbiont shuffling is a more likely explanation for the observed shift in symbiont communities than switching because (i) all 14 colonies that harboured low levels of D-type symbionts prior to the bleaching event survived and changed from C2 to D predominance, (ii) single-stranded conformation polymorphism (SSCP) analysis is known to lack the sensitivity to detect symbiont types at a relative abundance of less than 5–10% and (iii) cloning and sequencing a subset of samples before bleaching revealed D and C1 below the detection limits of SSCP, the presence of which predicted their appearance after bleaching if shuffling was the mechanism of change

Summary

Introduction

Coral reefs owe their success to the symbiosis between reef-building corals and intracellular, phototrophic dinoflagellates of the genus Symbiodinium (zooxanthellae) that supply up to 95% of the coral host’s energy requirements (Muscatine 1990). This acclimatization mechanism has been shown to occur in at least one population of Acropora millepora on the Great Barrier Reef after transplantation to a different thermal environment (Berkelmans & van Oppen 2006).

Results

Conclusion