Abstract
<p>Coral reef ecosystems worldwide are in decline as a result of climate change and other anthropogenic impacts. Prolonged exposure to extreme temperatures are predicted to occur more frequently in the future, resulting in coral bleaching and/or mortality. Different coral colonies within a species often show variation in both bleaching and mortality when exposed to temperature stress, but whether this is a result of genetic differences in the coral host, or its algal symbiont communities, is not clear. To distinguish these influences, the effect of different temperatures (20, 26, 30 and 32ºC) on four different genotypes of Pocillopora damicornis (identified using microsatellites) containing different Symbiodinium communities (identified using quantitative PCR), was investigated. Corals grew most rapidly at control temperatures (26ºC, p<0.001), but this effect varied in strength across genotypes. Extreme temperatures had a negative effect on growth in three of the four genotypes (p<0.016). There was differential mortality in the highest temperature (32ºC), with genotypes 1 and 3 showing the earliest mortality on day 52, while genotypes 2 and 4 experienced total mortality by day 66. In the 30ºC treatment, mortality occurred on day 80, with genotype 4 having the highest mortality (100%), followed by genotype 2 (67%), 1 (44%) and 3 (7%). Genotype 3 was the only coral to show mortality at the cold temperature (20ºC), with 7% of colonies having total mortality and 78% of colonies having at least some partial mortality. Quantitative PCR analysis of the algal symbionts (Symbiodinium spp.) in these corals revealed genotypes 1, 2 and 4 were initially dominated by Symbiodinium C1b-c, while genotype 3 was dominated by Symbiodinium D1. However, when exposed to 30ºC, all genotypes became dominated by D1 by day 72. All corals experienced a decrease in symbionts at 30ºC. However, the density of C1b-c symbionts decreased by an average of 98% across genotypes 1, 2 and 4 when the temperature was raised to 30ºC, suggesting that the shift to D-dominance is most likely due to the expulsion of C1b-c, with D1 symbionts in genotype 3 decreasing by an average of 61% at this temperature. Exposure to cold temperatures resulted in a large increase in densities of clade C symbionts, with genotype 4 having a 350% increase in the number of symbionts per host cell. Conversely, cold temperatures caused an 80% decrease in D1 symbionts in genotype 3, compared to initial levels. Photochemical efficiency of symbionts also varied across temperatures and coral host genotype. Fv/Fm values for the clade C-hosting genotypes were similar at all temperatures except at 20ºC, where genotype 1 values were significantly lower than those of genotypes 2 and 4 at 20ºC and 26ºC. Genotypes 2 and 4 showed no photochemical response to cold temperatures relative to the control. Clade C symbionts had a strong negative response to high temperatures, with Fv/Fm values significantly lower compared to controls. Fv/Fm values recovered at 30ºC for C1b-c symbionts towards the end of the experiment, likely due to the expulsion of C1b-c symbionts and the resulting dominance by D1. At high temperatures, D1 symbionts in genotype 3 had similar Fv/Fm values as the control, but lower values at 20ºC. Together, these data suggest that variability within coral genotypes plays a significant role in thermal tolerance. This variation is further influenced by the algal symbiont community, with complex interactions occurring between the host genotype and symbiont identity. The data presented in this study supports the growing volume of scientific literature that suggests that coral host genotype is an important component in the coral holobiont’s thermal tolerance. These data also show that, while D1 symbionts are more tolerant of high temperatures, corals hosting these symbionts may not survive indefinitely if the host genotype is itself thermally sensitive. This information may help restoration efforts designed to increase the resilience of coral reefs to climate change, by identifying coral genotypes and host-symbiont combinations best suited to the prevailing thermal environment.</p>