By Trevor Rivers, Erin Macri, and Adrienne Miller under the advisement of Gisele Muller-Parker
Shannon Point Marine Center, Western Washington University, Bellingham, WA 98225-9181

INTRODUCTION
Coral reef bleaching, the phenomenon of corals losing their symbiotic zooxanthellae and/or photosynthetic pigments, is caused by elevated temperature and UV-B radiation. Bleaching can, for the coral, cause reduced skeletal growth, reproductive deterioration, and if severe enough, host mortality (Glynn 1996). For the symbiont, exposure to elevated temperature and UV-B radiation causes a depression of photosynthesis (Lesser 1996).

Objectives

• To determine the relative effects of elevated temperature and UV-B radiation on productivity of zooxanthellae remaining in the coral Montastraea faveolata, as well as those expelled.
• To determine whether bleaching is the result of zooxanthellar expulsion from the host or loss of photosynthetic pigments

METHODS AND MATERIALS
Coral plugs (2.1 cm dia.) were collected from 3 different colonies of Montastraea faveolata at depths of 15 ±1 m from North Perry Reef, Lee Stocking Isl., Bahamas (Figure1 and Figure 2). Corals received either 29 °C (ambient reef temperature) or 31°C with one of the following light treatments:
 

Experimental Setup

• Corals were placed in individual containers in an outdoor temperature-controlled incubator for a natural photoperiod.
• Window screen and mylar sheets were used to obtain the desired radiation parameters for each treatment. Cellulose acetate lids were used to prevent seawater evaporation (Figure 3).

• 3 coral plugs from each treatment were sampled every 24 hours for up to 120 hours.
• Coral plugs were sampled at dusk (end of photoperiod)
• Expelled Zooxanthellae (EZ) for a 24-hour period were collected from the coral containers by centrifuging all of the seawater.
• Concentrated cells were used to obtain zooxanthellae cell densities and cells for use in ¹⁴C experiments.
• Animal Zooxanthellae (AZ) were water-pikked from the skeleton and the homogenate was processed as described in Muller-Parker et al. (1994) to obtain zooxanthellae cell densities and cells for use in ¹⁴C experiments.

• Zooxanthellae were incubated with ¹⁴C for 30 min. in a photosynthetron (CHPT Inc.) at predetermined irradiances (0-1180 mmol/m²/s) and at experimental temperatures (Figure 4).
• P_max and alpha parameters were obtained from photosynthetic rates using the hyperbolic tangent equation (Jassby and Platt 1976).

RESULTS
in situ  biomass parameters

• The zooxanthellar density remains fairly constant over time (Figure 5).
• The photosynthetic capacity does not decrease over time (Figure 5).
• The in situ zooxanthellae did not deteriorate over the course of the experiment, indicating our data is not affected by changes in the natural population.

Zooxanthellae Density

Bleaching (loss of zooxanthellae) takes place in corals exposed to the enhanced treatment only, and only after 48 hours of exposure to this treatment. Enhanced UVB alone does not result in coral bleaching at ambient reef temperature (Figure 6).

Productivity
• At ambient reef temperature, photosynthesis of zooxanthellae was unaffected by UVB radiation (Figure 7).
• Although bleaching occurred in corals exposed to the enhanced treatment, P_max and a were unaffected in the zooxanthellae remaining in the animal (AZ) in this treatment (Figure 7).
• P_max and a follow the same trends in all treatments (Figure 7).
• Expelled zooxanthellae (EZ) have lower P_max and a parameters than animal zooxanthellae (AZ), indicating damage (Figure 8).
• Productivity of EZ decreases with increasing UVB (Figure 8).

Summary
• Bleaching occurred only in the enhanced treatment, after 48 hours of exposure. Only expelled zooxanthellae were damaged, indicating that the animal protects zooxanthellae from exposure to UVB radiation.
• UVB radiation does not have a significant effect on zooxanthellae at in situ reef temperatures.

Results obtained at 31°C

Zooxanthellae Density

Bleaching takes place after 48 hours in all treatments, suggesting that high temperature plays a larger role in bleaching than UVB radiation (Figure 9).

Productivity
• At 31^oC, all treatments resulted in a marked decrease in photosynthetic parameters of zooxanthellae in the coral. The largest decrease was seen in the Enhanced treatment, followed by the Reef+UV treatment (Figure 10).
• P_max and a follow the same trend in all treatments (Figure 10).
• Expelled zooxanthellae (EZ) have lower P_max and a values than animal zooxanthellae (AZ), indicating greater damage to zooxanthellae upon expulsion from the host (Figure 11).
• Productivity decreases with increasing UVB (Figure 11).

Summary
• Bleaching of corals occurred after 48 hours of exposure to 31^oC, in all treatments. Zooxanthellae remaining in the coral (AZ) and those expelled by the coral (EZ) were damaged. These results indicate that the animal protects zooxanthellae from UVB radiation but does not protect them at elevated temperature.
• UVB radiation has a significant effect on zooxanthellae at elevated temperature.

Conclusions

• Temperature plays a larger role than UVB radiation in coral bleaching.When combined with exposure to high temperature, UVB radiation becomes more important, resulting in a synergistic effect of the two stressors.
• The animal protects the algae from UVB radiation, but does not protect the coral from temperature stress.
• P_max and a follow the same trend in all treatments. This suggests that reductions in productivity of zooxanthellae with enhanced UVB and elevated temperature involve general cellular damage (including damage to both the light-harvesting photosynthetic apparatus and to enzymes involved in carbon fixation).

• Zooxanthellae are better off in the host during high-stress situations (high light and temperature) because the host offers protection from UVB radiation.
• Residual zooxanthellae in the host are more likely to be the source of zooxanthellae for recovery after a bleaching event, since these are less damaged by stressors than are expelled zooxanthellae.

Jassby, A.D., and T. Platt. 1976. Mathematical Formulation of the Relationship Between Photosynthesis and Light for Phytoplankton. Limnol. Oceanogr. 21:540-547.

Lesser, M.P. (1996). Elevated Temperatures and Ultraviolet Radiation Cause Oxidative Stress and Inhibit photosynthesis in Symbiotic Dinoflagellates. Limnol Oceanogr. 41:271-283.

ACKNOWLEDGEMENTS
The authors would like to thank: Pis Giséle Muller-Parker, Suzanne Strom, and Jack Hardy; NSF grant C-RUI DBI 97-11075; the Shannon Point Marine Center Staff; Scientific Technical Services, the Caribbean Marine Research Center, Maria Bynangle, Emily Peterson, Carl Schmidt, and Teresa Steely.