Coral reefs form in clear, nutrient-poor waters. The metabolic energy supporting coral reefs comes from the symbiosis between coral polyps and endosymbiotic photosynthetic algae. Coral bleaching, which occurs when coral polyps eject their algae (thereby losing their source of reduced carbon), has been noticed with increasing frequency over the past decade, and thus the health of coral reefs may be declining. The predominant causes of coral bleaching are thought to be thermal stress and increased solar irradiance. The physiological responses of coral polyps and algae to these stresses during bleaching events remain unclear. Brown and colleagues have examined physiological responses to thermal stress in a coral that exhibits seasonal variation in within-colony susceptibility to bleaching.
Brown and colleagues observed that colonies of Goniastrea asperagenerally receive more sunlight on the west-facing surfaces than the east-facing surfaces in the afternoon, which is when tidal emersion and maximal thermal stress occur. During periods of elevated sea surface temperature, corals bleached mostly on the eastern side. This was surprising because earlier in the year when water temperatures were cooler, bleached areas were confined to the western sides of corals! The authors hypothesized that acclimatization to high light environments protected west-facing corals from bleaching by conferring increased tolerance to thermal stress.
To investigate this hypothesis, the researchers drilled cores from east-and west-facing sides of individual corals and measured a range of physiological indices, including algal densities, host photoprotective pigments and biomarkers for oxidative and thermal stress. These measurements were made in intact coral cores as well as in isolated coral polyps and algal cells and were measured in specimens in the field and specimens acclimated to normal and elevated temperatures in the laboratory.
Algal densities of intact coral cores were higher in west cores in field-collected and laboratory-acclimated conditions, with the largest increases observed in cores acclimated to high temperature. Algal production of stress proteins increased during acclimation to high temperatures, but there were no differences in biomarkers for oxidative and thermal stress found in isolated algae between east and west cores in any acclimation treatment. Thus, although the concentration of algae cells per polyp changed between coral sides, there were no physiological differences between east and west core algal cells.
Isolated polyps from field-collected east cores had higher levels of oxidative damage and irreversible protein damage, and polyps from west cores had higher levels of antioxidant enzyme and photoprotective pigments. Acclimation to high temperature resulted in large increases in the levels of all biomarkers, but there were differences in the magnitude of responses of east and west cores. The levels of oxidative damage increased most in east cores, while levels of antioxidant enzymes and stress proteins increased the most in west cores.
The results of this study support the hypotheses that acclimatization to high light levels confers resistance to thermal stress, and that this resistance is due to physiological changes in the coral polyp. West-facing corals increased algal densities and upregulated production of photoprotective pigments, antioxidant enzymes and stress proteins to resist oxidative and thermal stresses. Because physiological properties of algal cells were not observed to change, the authors concluded that although both partners in the symbiosis conferred stress tolerance, the coral polyps were solely responsible for regulating resistance to bleaching-related environmental stresses.