Effect of increased irradiance and thermal stress on the symbiosis of Symbiodinium microadriaticum and Tridacna gigas |
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| Institution: | 1. Department of Biological Sciences, National University of Singapore, Kent Ridge, Singapore 117543, Republic of Singapore;2. The Tropical Marine Science Institute, National University of Singapore, Kent Ridge, Singapore 119227, Republic of Singapore;3. Natural Sciences and Science Education, National Institute of Education, Nanyang Technological University, 1 Nanyang Walk, Singapore 637616, Republic of Singapore;1. Australian Centre for Pacific Islands Research, Faculty of Science, Health, Education and Engineering, University of the Sunshine Coast, Maroochydore, Queensland 4556, Australia;2. Aquasearch, 6-10 Elena Street, Nelly Bay, Magnetic Island, Queensland 4819, Australia;1. Department of Biological Sciences, National University of Singapore, Kent Ridge, Singapore 117543, Republic of Singapore;2. The Tropical Marine Science Institute, National University of Singapore, Kent Ridge, Singapore 119227, Republic of Singapore;3. St. John''s Island National Marine Laboratory, National University of Singapore, Kent Ridge, Singapore 119227, Republic of Singapore;4. Natural Sciences and Science Education, National Institute of Education, Nanyang Technological University, 1 Nanyang Walk, Singapore 637616, Republic of Singapore;1. Department of Earth Sciences, Royal Holloway University of London, Egham, Surrey TW20 0EX, United Kingdom;2. Center for Applied Geoscience, Geomicrobiology, Eberhard Karls University of Tübingen, 72074 Tübingen, Germany;3. Department of Geosciences, Isotope Geochemistry, Eberhard Karls University of Tübingen, 72074 Tübingen, Germany;4. Earth Science Institute, The Hebrew University of Jerusalem, Israel;5. Institute of Geosciences, Goethe University Frankfurt, 60438 Frankfurt/Main, Germany |
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| Abstract: | The impetus for this study was the mass bleaching event of giant clams in 1997–1998 at several reefs in the Great Barrier Reef (GBR, Australia). From September until December 1999, the study investigated the effects of high light intensities and increased temperature accompanied by a nutrient limitation on the metabolism of Tridacna gigas, to test, if these environmental changes can induce bleaching. In a 50-day trial similar conditions of the mass bleaching event were imitated stressing clams, collected from Orpheus Island and Nelly Bay, by a two- to threefold light intensity and an increased temperature of 4–6 °C. The objectives of the experiments were to determine whether high light intensities can induce changes in chlorophyll content or alter zooxanthella cell sizes and populations in the tissue. After 50 days of exposure to high light intensity the mantle tissue of stressed clams exhibited a decreased number of zooxanthellae per unit area from 19.8 ± 0.8 (× 107·cm–2) to 0.2 ± 0.2 (× 107·cm–2) (mean ± CL). Additionally, the average cell size of zooxanthellae were downsized from 7.4 ± 0.1 μm to 5.3 ± 0.1 μm (mean ± CL). Subsequently, the chlorophyll content of both, chl a and chl c1, declined as well, chl a from 192 ± 4 to 0.1 ± 0.1 μg·ml–1 and chl c1 from 145 ± 6 to 0 μg·ml–1 (mean ± CL). This study shows that increased light intensity and temperature are the main causes for bleaching in giant clams. Thus, the study confirmed the four major aspects involved in bleaching: (1) loss of symbiotic algae, (2) decrease of chl a/c1 in the remaining symbiotic algae, (3) retention of small zooxanthellae in the tissue and (4) release of ammonium (NH4+) into the water column while nutrient uptake of ammonium was largely blocked. |
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