Coral Bleaching Response Research Articles

Patterns of Gene Expression in a Scleractinian Coral Undergoing Natural Bleaching

Coral bleaching is a major threat to coral reefs worldwide and is predicted to intensify with increasing global temperature. This study represents the first investigation of gene expression in an Indo-Pacific coral species undergoing natural bleaching which involved the loss of algal symbionts. Quantitative real-time polymerase chain reaction experiments were conducted to select and evaluate coral internal control genes (ICGs), and to investigate selected coral genes of interest (GOIs) for changes in gene expression in nine colonies of the scleractinian coral Acropora millepora undergoing bleaching at Magnetic Island, Great Barrier Reef, Australia. Among the six ICGs tested, glyceraldehyde 3-phosphate dehydrogenase and the ribosomal protein genes S7 and L9 exhibited the most constant expression levels between samples from healthy-looking colonies and samples from the same colonies when severely bleached a year later. These ICGs were therefore utilised for normalisation of expression data for seven selected GOIs. Of the seven GOIs, homologues of catalase, C-type lectin and chromoprotein genes were significantly up-regulated as a result of bleaching by factors of 1.81, 1.46 and 1.61 (linear mixed models analysis of variance, P < 0.05), respectively. We present these genes as potential coral bleaching response genes. In contrast, three genes, including one putative ICG, showed highly variable levels of expression between coral colonies. Potential variation in microhabitat, gene function unrelated to the stress response and individualised stress responses may influence such differences between colonies and need to be better understood when designing and interpreting future studies of gene expression in natural coral populations.

The effect of species and colony size on the bleaching response of reef-building corals in the Florida Keys during the 2005 mass bleaching event

Understanding the variation in coral bleaching response is necessary for making accurate predictions of population changes and the future state of reefs in a climate of increasing thermal stress events. Individual coral colonies, belonging to inshore patch reef communities of the Florida Keys, were followed through the 2005 mass bleaching event. Overall, coral bleaching patterns followed an index of accumulated thermal stress more closely than in situ temperature measurements. Eight coral species (Colpophyllia natans, Diploria strigosa, Montastraea cavernosa, M. faveolata, Porites astreoides, P. porites, Siderastrea siderea, and Stephanocoenia intersepta), representing >90% of the coral colonies studied, experienced intense levels of bleaching, but responses varied. Bleaching differed significantly among species: Colpophyllia natans and Diploria strigosa were most susceptible to thermal stress, while Stephanocoenia intersepta was the most tolerant. For colonies of C. natans, M. faveolata, and S. siderea, larger colonies experienced more extensive bleaching than smaller colonies. The inshore patch reef communities of the Florida Keys have historically been dominated by large colonies of Montastraea sp. and Colpophyllia natans. These results provide evidence that colony-level differences can affect bleaching susceptibility in this habitat and suggest that the impact of future thermal stress events may be biased toward larger colonies of dominant reef-building species. Predicted increases in the frequency of mass bleaching and subsequent mortality may therefore result in significant structural shifts of these ecologically important communities.

Bleaching response of corals and their Symbiodinium communities in southern Africa

The high-latitude coral communities of southern Africa suffered minimal impacts during past mass bleaching events. Recent reports indicate an increase in bleaching frequency during the last decade, yet the actual levels of thermal stress and contributing factors in these bleaching events, and the degree of acclimatisation or adaptation on these reefs are poorly understood. During the 2005 warm-water anomaly in the southern Indian Ocean we conducted bleaching surveys and collected samples for genotyping of the algal symbiont communities at 21 sites in southern Mozambique and South Africa. Coral bleaching reached unprecedented levels and was negatively correlated with both latitude and water depths. Stylophora pistillata and Montipora were the most susceptible taxa, whereas three common branching corals had significantly different bleaching responses (Stylophora > Acropora > Pocillopora). Temperature records indicated that localised strong upwelling events coupled with persistent above-average seawater temperatures may result in accumulated thermal stress leading to bleaching. Symbiodinium in 139 scleractinian corals belonged almost exclusively to clade C, with clade D symbionts present in only 3% of the colonies. Two atypical C subclades were present in Stylophora and Pocillopora colonies and these were more abundant in shallow than deeper sites. Taxon-specific differences in bleaching responses were unrelated to different clades of algal symbionts and suggest that Symbiodinium C subtypes with diverse thermal tolerance, coupled with acclimatisation and morphology of the host colony influence the bleaching response. Additionally, the predominance of putatively thermal-sensitive Symbiodinium in southern African corals may reflect a limited experience of bleaching and emphasises the vulnerability of these reefs to moderate levels of thermal stress.

EFFECTS OF CLIMATE AND SEAWATER TEMPERATURE VARIATION ON CORAL BLEACHING AND MORTALITY

Coral bleaching due to thermal and environmental stress threatens coral reefs and possibly people who rely on their resources. Here we explore patterns of coral bleaching and mortality in East Africa in 1998 and 2005 in a region where the equatorial current and the island effect of Madagascar interact to create different thermal and physicochemical environments. A variety of temperature statistics were calculated, and their relationships with the degree-heating months (DHM), a good predictor of coral bleaching, determined. Changes in coral cover were analyzed from 29 sites that span >1000 km of coastline from Kenya to the Comoros Islands. Temperature patterns are influenced by the island effect, and there are three main temperature environments based on the rise in temperature over 52 years, measures of temperature variation, and DHM. Offshore sites north of Madagascar that included the Comoros had low temperature rises, low DHM, high standard deviations (SD), and the lowest relative coral mortality. Coastal sites in Kenya had moderate temperature rises, the lowest temperature SD, high DHM, and the highest relative coral mortality. Coastal sites in the south had the highest temperature rises, moderate SD and DHM, and low relative coral mortality. Consequently, the rate of temperature rise was less important than background variation, as reflected by SD and kurtosis measures of sea surface water temperature (SST), in predicting coral survival across 1998. Coral bleaching responses to a warm-water anomaly in 2005 were also negatively related to temperature variation, but positively correlated with the speed of water flow. Separating these effects is difficult; however, both factors will be associated with current environments on the opposite sides of reefs and islands. Reefs in current shadows may represent refugia where corals acclimate and adapt to environmental variation, which better prepares them for rising temperature and anomalies, even though these sites are likely to experience the fastest rates of temperature rise. We suggest that these sites are a conservation priority and should be targeted for management and further ecological research in order to understand acclimation, adaptation, and resilience to climate change.

Seasonal and local spatial patterns in the upper thermal limits of corals on the inshore Central Great Barrier Reef

Experimental studies of the upper thermal limits of corals from Orpheus Island, an inshore reef in the central Great Barrier Reef, show that Acropora formosa has a 5-day 50%-bleaching threshold of between 31 and 32 °C in summer, only 2 to 3 °C higher than local mean summer temperatures (29 °C). Summer bleaching thresholds for Pocillopora damicornis and A. elseyi were 1 °C higher (between 32 and 33 °C). The winter bleaching threshold of Pocillopora damicornis was 1 °C lower than its summer threshold, indicating that seasonal acclimatisation may take place. This seasonal difference raises the possibility that at least some corals may be capable of short-term thermal acclimatisation. Neither P. damicornis nor A. elseyi showed habitat-specific (reef flat versus reef slope) differences in bleaching thresholds. Further, colonies of P. damicornis collected from sites 3 km apart also showed no difference in bleaching threshold despite populations of this species responding differently at these two sites during a natural bleaching event. The bleaching thresholds determined in this study are best considered as the maximum tolerable temperatures for local populations of these species because they were determined in the absence of additional stressors (e.g. high light) which often co-occur during natural bleaching events. We consider the 5-day 50% bleaching thresholds determined in these experiments to be fair indicators of upper thermal limits, because >50% of a sample population died when allowed to recover in situ. We found a delay of up to a month in the bleaching response of corals following thermal stress, a result that has implications for identifying the timing of stressful conditions in natural bleaching events.