Irradiance Level Only Moderately Affects Thermal Bleaching in the Stony Coral <i>Stylophora pistillata</i>
Light is considered an important co-factor in causing thermal bleaching in photosymbiotic corals. To quantify the effects of light strength on thermal bleaching, colonies of the stony coral <i>Stylophora pistillata</i> were experimentally subjected to a gradual increase in temperature (1...
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| Main Authors: | , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
MDPI AG
2025-06-01
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| Series: | Oceans |
| Subjects: | |
| Online Access: | https://www.mdpi.com/2673-1924/6/2/32 |
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| Summary: | Light is considered an important co-factor in causing thermal bleaching in photosymbiotic corals. To quantify the effects of light strength on thermal bleaching, colonies of the stony coral <i>Stylophora pistillata</i> were experimentally subjected to a gradual increase in temperature (1 °C per 4 days) under two irradiance levels: 100 and 500 µmole quanta m<sup>−2</sup> s<sup>−1</sup>. Corals kept under the same irradiance levels at a constant temperature of 26 °C were used as controls. The apparent photochemical yield ΔF/Fm′ of Photosystem II of the coral symbionts was monitored daily as an indicator for the onset of thermal bleaching, the onset of bleaching being defined as a steep decrease in ΔF/Fm′. In heat-treated corals incubated under the high irradiance of 500 µmole quanta m<sup>−2</sup> s<sup>−1</sup>, the onset of bleaching occurred 26 days after the start of the heat ramp, at a temperature of 33 °C. ΔF/Fm′ in corals incubated under the low irradiance of 100 µmole quanta m<sup>−2</sup> s<sup>−1</sup> started to drop 1 day later at the same temperature. Before and after the observed drop in ΔF/Fm′, coral samples were taken for analysis of symbiont densities and levels of chlorophyll-a. At the onset of bleaching, symbiont densities and chlorophyll-a levels in heat-treated corals were not different from those of corals kept under control conditions. Three days after the onset of bleaching, symbiont densities and levels of chlorophyll-a in heat-treated corals had substantially decreased in comparison to controls. Under low irradiance, symbiont density and chlorophyll-a content were 84% and 76% lower than controls, respectively, whereas under high irradiance, symbiont density and chlorophyll-a content were 41% and 46% lower. These data suggest that damage to the photosystem in coral symbionts is the root cause of thermal bleaching in symbiotic corals, followed later by a collapse of the symbiosis. The role of light in augmenting thermal bleaching was only moderate, with a five-fold reduction in irradiance causing only a 1-day delay in bleaching. These results suggest that temperature is the main driver of bleaching in the studied coral. |
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| ISSN: | 2673-1924 |