Estimating the potential for adaptation of corals to climate warming.

Robert Desalle,Nikolaus B M Császár,Ray Berkelmans,Peter J Ralph,Madeleine J H Van Oppen,Richard Frankham

doi:10.1371/journal.pone.0009751

Abstract

The persistence of tropical coral reefs is threatened by rapidly increasing climate warming, causing a functional breakdown of the obligate symbiosis between corals and their algal photosymbionts (Symbiodinium) through a process known as coral bleaching. Yet the potential of the coral-algal symbiosis to genetically adapt in an evolutionary sense to warming oceans is unknown. Using a quantitative genetics approach, we estimated the proportion of the variance in thermal tolerance traits that has a genetic basis (i.e. heritability) as a proxy for their adaptive potential in the widespread Indo-Pacific reef-building coral Acropora millepora. We chose two physiologically different populations that associate respectively with one thermo-tolerant (Symbiodinium clade D) and one less tolerant symbiont type (Symbiodinium C2). In both symbiont types, pulse amplitude modulated (PAM) fluorometry and high performance liquid chromatography (HPLC) analysis revealed significant heritabilities for traits related to both photosynthesis and photoprotective pigment profile. However, quantitative real-time polymerase chain reaction (qRT-PCR) assays showed a lack of heritability in both coral host populations for their own expression of fundamental stress genes. Coral colony growth, contributed to by both symbiotic partners, displayed heritability. High heritabilities for functional key traits of algal symbionts, along with their short clonal generation time and high population sizes allow for their rapid thermal adaptation. However, the low overall heritability of coral host traits, along with the corals' long generation time, raise concern about the timely adaptation of the coral-algal symbiosis in the face of continued rapid climate warming.

Highlights

Tropical coral reefs are among the most biodiverse ecosystems on the planet and of immense economic value to an estimated 500 million people worldwide [1]
We investigate the degree of genetic determination of phenotypes in the two populations at their respective time of assessment, under the particular environmental conditions experienced
Algal symbionts belonging to Symbiodinium clade D (Magnetic Island) and type C2 (Orpheus Island) displayed a substantially higher potential than their coral host Acropora millepora for genetic adaptation to bleaching conditions (32uC) (Table 2)

Summary

Introduction

Tropical coral reefs are among the most biodiverse ecosystems on the planet and of immense economic value to an estimated 500 million people worldwide [1]. The success of stony corals as carbonate depositing reef structures depends on their functional association with dinoflagellate symbionts of the algal genus Symbiodinium. This association is based on a closely linked nutrient cycling which promotes a near one-to-one ratio of algal photosynthesis and coral calcification rates [2]. Warming of tropical seas has already pushed many coral species close to their upper thermal limit [6]. Sea surface temperatures (SSTs) that exceed summer maxima by 1–2uC over a few weeks usually cause severe bleaching of corals due to the loss of algal symbionts and/or algal pigments [7]. A third of all reef-building coral species is threatened by an increased risk of extinction [8]. An estimated 50% of species are at high risk of extinction [8]

Methods

Results

Conclusion