Volume 8, Number 20: 18 May 2005
In the introduction to their intriguing review of the thermal aspects of coral bleaching, Smith et al. (2005) note that "photoinhibition of photosynthesis and photodamage to photosystem II of the zooxanthellae, with the consequent increase in the production of damaging reactive oxygen species (ROS), have been implicated as the cause of thermal bleaching (Brown, 1997; Fitt et al., 2001; Lesser, 2004; Tchernov et al., 2004)." At the end of their review, they additionally report that the "thermal bleaching of many corals is ultimately the result of the destruction of photosynthetic pigments by ROS," and that the production by the zooxanthellae of one particular ROS, hydrogen peroxide, "may be a signal that triggers a response in the host cell to eject the zooxanthellae or shed the host cell from the coral."
These facts resonate with other findings we have reviewed on our website and suggest that the ongoing rise in the air's CO2 content may ultimately provide the solution to the worldwide problem of heat-induced coral bleaching. This concept originates from research conducted in the terrestrial realm, which reveals, in the words of Ren et al. (2001), that "elevated CO2 can enhance the capacity of plants to resist stress-induced oxidative damage."
In the case of ozone pollution, the primary problems occur in the leaf mesophyll, where ozone dissolves into the wet surfaces of exposed cell walls. There, reactions of ozone with water and solutes in the apoplasm lead to the formation of several ROS, including hydrogen peroxide (H2O2), hydroperoxide, superoxide, hydroxyl radicals and singlet oxygen (Foyer et al., 1994; Kangasjarvi et al., 1994; Wohlgemuth et al., 2002), all of which substances promote oxygen toxicity (Podila et al., 2001). However, in a FACE study of this phenomenon in aspen and paper birch seedlings exposed to ambient air, ozone-enriched air, CO2-enriched air or air enriched with both ozone and CO2, Oksanen et al. (2003) found that H2O2 accumulation only occurred "in ozone-exposed leaves and not in the presence of elevated CO2," adding that "CO2 enrichment appears to alleviate chloroplastic oxidative stress." Similarly, in a study of mature holm and white oak trees that had been growing near natural CO2 springs in central Italy for 30 to 50 years, Schwanze and Polle (1998) found that they exhibited significant reductions in their amounts of lipid peroxidation.
To see if such ROS-fighting properties of elevated CO2 might be operative in the aquatic realm, Yu et al. (2004) grew the marine microalgae Platymonas subcordiformis in the laboratory at ambient levels of atmospheric CO2 and UV-B radiation flux density, as well as at elevated levels of 5000 ppm CO2 and/or UV-B radiation characteristic of what would result from a 25% stratospheric ozone depletion under clear sky conditions in summer. They found that the elevated UV-B treatment significantly decreased microalgal dry weight and photosynthetic rate, while the elevated CO2 treatment enhanced dry weight and photosynthetic rate. They also report that elevated UV-B significantly increased the production of the toxic superoxide anion and hydrogen peroxide, as well as malonyldialdehyde, which is an end product of lipid peroxidation, whereas elevated CO2 did just the opposite. In addition, in the treatment consisting of both elevated UV-B and elevated CO2, the concentrations of these three substances were lower than those observed in the elevated UV-B and ambient CO2 treatment.
Yu et al. say their results suggest that "CO2 enrichment could reduce oxidative stress of reactive oxygen species to P. subcordiformis, and reduce the lipid peroxidation damage of UV-B to P. subcordiformis." They also say that "CO2 enrichment showed a protective effect against the oxidative damage of UV-B-induced stress," and, therefore, that elevated CO2 can enhance "the capacity of stress resistance." Put more simply, they say in their concluding paragraph that "algae grown under high CO2 would better overcome the adverse impact of environmental stress factors that act via generation of activated oxygen species."
It is difficult to state the implications of these studies in any clearer language, but we will try. Since, in the words of Smith et al. (2005), "thermal bleaching of many corals is ultimately the result of the destruction of photosynthetic pigments by ROS," and since, in the words of Oksanen et al. (2003), "CO2 enrichment appears to alleviate chloroplastic oxidative stress," it takes no imagination at all to reach the conclusion that some as-yet-undefined level of atmospheric CO2 enrichment should completely counter coral thermal bleaching. In addition, since the presence of hydrogen peroxide, in the words of Smith et al. (2005), "may be a signal that triggers a response in the host cell to eject the zooxanthellae or shed the host cell from the coral," and since, in the words of Yu et al. (2004), "CO2 enrichment could reduce ... lipid peroxidation damage," it readily follows that some degree of atmospheric CO2 enrichment should likewise cause host cells to not eject their zooxanthellae.
Clearly, it only remains for someone to do such experiments as those described above on coral itself. Who will do it first ... and become famous in the process?
Sherwood, Keith and Craig Idso
References
Brown, B.E. 1997. Coral bleaching: causes and consequences. Coral Reefs 16: S129-S138.
Fitt, W.K., Brown, B.E., Warner, M.E. et al. 2001. Coral bleaching: interpretation of thermal tolerance limits and thermal thresholds in tropical corals. Coral Reefs 20: 51-65.
Foyer, C., Lelandais, M. and Kunert, K. 1994. Photo-oxidative stress in plants. Physiologia Plantarum 92: 224-230.
Kangasjarvi, J., Talvinen, J., Utriainen, M. and Karjalainen, R. 1994. Plant defense systems induced by ozone. Plant, Cell and Environment 17: 783-794.
Lesser, M.P. 2004. Experimental biology of coral reef systems. Journal of Experimental Marine Biology and Ecology 300: 217-252.
Oksanen, E., Haikio, E., Sober, J. and Karnosky, D.F. 2003. Ozone-induced H2O2 accumulation in field-grown aspen and birch is linked to foliar ultrastructure and peroxisomal activity. New Phytologist 161: 791-799.
Podila, G.K., Paolacci, A.R. and Badiani, M. 2001. The impact of greenhouse gases on antioxidants and foliar defense compounds. In: Karnosky, D.F., Ceulemans, R., Scarascia-Mugnozza, G.E. and Innes, J.L. (Eds.). The Impact of Carbon Dioxide and Other Greenhouse Gases on Forest Ecosystems. CABI Publishing, Vienna, Austria, pp. 57-125.
Ren, H.X., Chen, X. and Wu, D.X. 2001. Effects of elevated CO2 on photosynthesis and antioxidative ability of broad bean plants grown under drought condition. Acta Agronomica Sinica 27: 729-736.
Schwanz, P. and Polle, A. 1998. Antioxidative systems, pigment and protein contents in leaves of adult mediterranean oak species (Quercus pubescens and Q. ilex) with lifetime exposure to elevated CO2. New Phytologist 140: 411-423.
Smith, D.J., Suggett, D.J. amd Baker, N.R. 2005. Is photoinhibition of zooxanthellae photosynthesis the primary cause of thermal bleaching in corals? Global Change Biology 11: 1-11.
Tchernov, D., Gorbunov, M.Y. de Vargas, C. et al. 2004. Membrane lipids of symbiotic algae are diagnostic of sensitivity to thermal bleaching in corals. Proceedings of the National Academy of Sciences USA 101: 13,531-13,535.
Wohlgemuth, H., Mittelstrass, K., Kschieschan, S., Bender, J., Weigel, H.-J., Overmyer, K., Kangasjarvi, J., Sandermann, H. and Langebartels, C. 2002. Activation of an oxidative burst is a general feature of sensitive plants exposed to the air pollutant ozone. Plant, Cell and Environment 25: 717-726.
Yu, J., Tang, X-X., Zhang, P-Y., Tian, J-Y. and Cai, H-J. 2004. Effects of CO2 enrichment on photosynthesis, lipid peroxidation and activities of antioxidative enzymes of Platymonas subcordiformis subjected to UV-B radiation stress. Acta Botanica Sinica 46: 682-690.