Volume 8, Number 45: 9 November 2005
Several researchers have suggested that many of earth's corals are destined to die, with some species even facing extinction, because of what they claim is a well-defined physical-chemical connection between the ongoing rise in the air's CO2 content and coral calcification rates, which are predicted to decline as atmospheric CO2 concentrations rise ( Buddemeier, 1994; Buddemeier and Fautin, 1996a,b; Gattuso et al., 1998; Kleypas et al., 1999; Langdon et al., 2000; Buddemeier 2001). Idso et al. (2000), however, contend that coral calcification is much more than a physical-chemical process that can be described by a set of well-defined equations, stating that it is a biologically-driven physical-chemical process that may not yet be amenable to explicit mathematical description. Enlarging on this statement, they note that the "photosynthetic activity of zooxanthellae is the chief source of energy for the energetically-expensive process of calcification," and that long-term reef calcification rates have generally been observed to rise in direct proportion to increases in rates of reef primary production, which they say may well be enhanced by increases in the air's CO2 concentration.
Muscatine et al. (2005) begin the report of their investigation of the subject by stating much the same thing, i.e., that endosymbiotic algae "release products of photosynthesis to animal cells ... and augment the rate of skeletal calcification." Then, noting that the "natural abundance of stable isotopes (δ13C and δ15N) has answered paleobiological and modern questions about the effect of photosymbiosis on sources of carbon and oxygen in coral skeletal calcium carbonate," they go on to investigate the natural abundance of these isotopes in another coral skeletal compartment - the skeletal organic matrix (OM) - in 17 species of modern scleractinian corals, after which they compare the results for symbiotic and nonsymbiotic forms to determine the role played by algae in OM development.
Why is this an important scientific undertaking? It is because, in the words of Muscatine et al., the scleractinian coral skeleton is a two-phase composite structure consisting of fiber-like crystals of aragonitic calcium carbonate intimately associated with an intrinsic OM," and although the OM generally comprises less than 0.1% of the total weight of the coral skeleton, it is, in their words, "believed to initiate nucleation of calcium carbonate and provide a framework for crystallographic orientation and species-specific architecture." In fact, they say that inhibition of OM synthesis "brings coral calcification to a halt."
So what did Muscatine et al. learn from their experiments? They say their "most striking observation is the significant difference in mean OM δ15N between symbiotic and nonsymbiotic corals," which makes OM δ15N an important proxy for photosymbiosis. As an example of its usefulness, they applied the technique to a fossil coral (Pachythecalis major) from the Triassic (which prevailed some 240 million years ago), finding that the ancient coral was indeed photosymbiotic. Even more importantly, however, they conclude in the final sentence of their paper that "it now seems that symbiotic algae may control calcification by both modification of physico-chemical parameters within the coral polyps (Gautret et al., 1997; Cuif et al., 1999) and augmenting the synthesis of OM (Allemand et al., 1998)."
Yes, in some respects it is true that life is at the mercy of the elements, but in other respects it is equally true that life rules.
Sherwood, Keith and Craig Idso
References
Allemand, D., Tambutte, E., Girard, J.-P. and Jaubert, J. 1998. Organic matrix synthesis in the scleractinian coral stylophora pistillata: role in biomineralization and potential target of the organotin tributyltin. Journal of Experimental Biology 201: 2001-2009.
Buddemeier, R.W. 1994. Symbiosis, calcification, and environmental interactions. Bulletin Institut Oceanographique, Monaco 13: 119-131.
Buddemeier, R.W. 2001. Is it time to give up? Bulletin of Marine Science 69: 317-326.
Buddemeier, R.W. and Fautin, D.G. 1996a. Saturation state and the evolution and biogeography of symbiotic calcification. Bulletin Institut Oceanographique, Monaco 14: 23-32.
Buddemeier, R.W. and Fautin, D.G. 1996b. Global CO2 and evolution among the Scleractinia. Bulletin Institut Oceanographique, Monaco 14: 33-38.
Cuif, J.-P., Dauphin, Y., Freiwald, A., Gautret, P. and Zibrowius, H. 1999. Biochemical markers of zooxanthellae symbiosis in soluble matrices of skeleton of 24 Scleractinia species. Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology 123: 269-278.
Gattuso, J.-P., Frankignoulle, M., Bourge, I., Romaine, S. and Buddemeier, R.W. 1998. Effect of calcium carbonate saturation of seawater on coral calcification. Global and Planetary Change 18: 37-46.
Gautret, P., Cuif, J.-P. and Freiwald, A. 1997. Composition of soluble mineralizing matrices in zooxanthellate and non-zooxanthellate scleractinian corals: Biochemical assessment of photosynthetic metabolism through the study of a skeletal feature. Facies 36: 189-194.
Idso, S.B., Idso, C.D. and Idso, K.E. 2000. CO2, global warming and coral reefs: Prospects for the future. Technology 7S: 71-94.
Kleypas, J.A., Buddemeier, R.W., Archer, D., Gattuso, J-P., Langdon, C. and Opdyke, B.N. 1999. Geochemical consequences of increased atmospheric carbon dioxide on coral reefs. Science 284: 118-120.
Langdon, C., Takahashi, T., Sweeney, C., Chipman, D., Goddard, J., Marubini, F., Aceves, H., Barnett, H. and Atkinson, M.J. 2000. Effect of calcium carbonate saturation state on the calcification rate of an experimental coral reef. Global Biogeochemical Cycles 14: 639-654.
Muscatine, L., Goiran, C., Land, L., Jaubert, J., Cuif, J.-P. and Allemand, D. 2005. Stable isotopes (δ13C and δ15N) of organic matrix from coral skeleton. Proceedings of the National Academy of Sciences USA 102: 1525-1530.