Abstract
Thermal tolerance tests on Acropora millepora, a common Indo-Pacific hard coral, have shown that adult corals can acquire increased thermal tolerance by shuffling existing type C to type D Symbiodinium zooxanthellae when subjected to increased seawater temperatures. We report here dimethylsulphoniopropionate (DMSP) concentrations in A. millepora and examine links between DMSP concentrations, zooxanthellae clade, and bleaching tolerance. DMSP analysis on native and transplanted corals from three locations in the Great Barrier Reef indicated that the lower thermal tolerance in type C zooxanthellae coincided with variable DMSP concentrations, whilst the more thermal tolerant type D zooxanthellae had more stable areal DMSP concentrations as seawater temperatures increased. Our results suggest this increased thermal tolerance in type D zooxanthellae may reflect the ability of these coral symbionts to conserve their antioxidant DMSP levels to relatively constant concentrations, enabling the coral to overcome the build-up of oxygen free radicals in the cytoplasm of A. millepora. A conceptual diagram illustrates how the antioxidants DMS (P) participate in the bleaching process by scavenging oxygen free radicals and form DMSO, thus moderating coral bleaching and increasing thermotolerance.
Highlights
Coral bleaching has been more regularly observed in the Great Barrier Reef (GBR) GBR over more than two decades [1,2,3,4,5] with bleaching events becoming more severe in the mid-1980s and late 1990s, culminating in two extreme mass coral bleaching episodes in 1998 and 2002 severely affecting hundreds of coral reefs and potentially threatening the health and integrity of coral reefs around the world [1,6,7]
It is thought that climate change is the main cause of these mass coral bleaching episodes, but human impacts such as increased nutrient levels, suspended sediments from land runoff and dredging can exacerbate the bleaching response
Magnetic and Keppel Island (Keppels) Transplant colonies had overall higher zooxanthellae densities compared to colonies from the other three locations (Figure 3), at 32 °C where the average densities of these two colonies were 650,645 zooxanthellae cm−2 and 369,734 zooxanthellae cm−2, respectively [22]
Summary
Coral bleaching has been more regularly observed in the Great Barrier Reef (GBR) GBR over more than two decades [1,2,3,4,5] with bleaching events becoming more severe in the mid-1980s and late 1990s, culminating in two extreme mass coral bleaching episodes in 1998 and 2002 severely affecting hundreds of coral reefs and potentially threatening the health and integrity of coral reefs around the world [1,6,7].It is thought that climate change is the main cause of these mass coral bleaching episodes, but human impacts such as increased nutrient levels, suspended sediments from land runoff and dredging can exacerbate the bleaching response. Photosynthesis in any plant cell including zooxanthellae reduces molecular oxygen to water and in the process oxygen free-radicals and other cellular intermediates are produced [10,11]. Superoxide is generated in the mitochondria, nuclei and chloroplasts of plant cells [10,12]. This radical causes lipid peroxidation, mitochondria dysfunction, ATP depletion, membrane damage and leakage, essentially resulting in cell death [10,11]. This process occurs naturally, temperature and light stress can exacerbate the rate of radical formation [7,13].
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