Understanding the flexibility of the Coral-Algal symbioses on the Great Barrier Reef (Australia) as a mechanism to respond to environmental change

Reef corals form an obligate symbiotic relationship with dinoflagellates of the genus Symbiodinium (zooxanthellae); without these endosymbiotic algae the coral host cannot survive. Extended periods of elevated sea water temperature (and/or high light) can lead to coral bleaching (reviewed in Hoegh-Guldberg 1999), which is the expulsion of the microalgae or their pigments by the host (a breakdown of the symbiosis), and hence can cause massive coral death and reef degradation. Mass coral bleaching has increased in incidence since the 1980’s and was first recorded on all the major coral reefs around the world in 1998. The genus Symbiodinium comprises an extraordinarily heterogeneous group of dinoflagellates. So far, seven phylogenetic clades have been identified based on nuclear ribosomal DNA (A-G), and within each clade sub-clades (strains or species) are present. In many cases, the juvenile coral takes up zooxanthellae from the environment to establish the symbiosis, and can associate with a wide range of algal strains. Multiple strains can co-exist within a single host. The role of symbiont identity and diversity in terms of fitness of the coral colony has only been poorly studied. Previous studies have shown that cultured zooxanthella strains can differ considerably in physiological characteristics, such as photoacclimatory responses to changes in irradiance and responses to high temperatures. However, it is unclear how these data from zooxanthella cultures relate to the natural, symbiotic situation. Our recent transplant work shows, for the first time, that a change in zooxanthellae from clade C to clade D can cause coral colonies to become more tolerant to high temperatures (Berkelmans and van Oppen, in prep.). However, there are indications that corals associated with clade D grow more slowly than corals with clade C (Berkelmans, unpubl.; Little et al., unpubl.). This suggests that switching zooxanthella strain implies a trade-off between growth and survival, which will have important consequences for the coral’s ability to compete for space and recover from disturbances. This project aims to understand to what extent reef corals can acclimatise or adapt to increasing seawater temperatures by changing their algal endosymbionts, and to examine what the consequences of associating with a heat-tolerant symbiont strain are in terms of other physiological characteristics, such as growth. The results of this study will enhance our understanding of the resilience of coral reefs to temperature disturbance (and hence, the impacts of climate changes) and will lead to improved conservation management.