How does rising atmospheric CO2 affect marine organisms?

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Coral Reef Responses to Environmental Change
Kinzie, R.A., III.  1999.  Sex, symbiosis and coral reef communities.  American Zoologist 39: 80-91.

What was done
The author reviews our present state of knowledge of the interplay between changing environmental conditions and modern reef biology.  Specifically, he addresses the issues of coral reproductive and population biology at both the individual and population level, the symbiosis of coral zooxanthellae, and reef community ecology.

What was learned
In regard to coral reproduction, Kinzie states that knowledge on this front is very limited and that we are "hampered in our ability to predict how any species might respond to environmental change."  Nevertheless, he describes some features of reproductive biology that do address the potential for corals to confront such challenges.  For example, it has been predicted that the planet will experience a substantial melting of the polar ice caps as a result of CO2-induced global warming, which in turn is predicted to wreak havoc on the world's coral reefs as they succumb to rising sea levels.  Kinzie, however, suggests a fate other than death for the reefs, noting that "the ability of essentially all reef cnidarians, including framework builders, and the array of scleractinian and non-scleractinian reef organisms to produce free swimming planulae, spores, or dispersing larval stages, means that no matter how quickly sea level might rise, propagules of the species could keep pace and settle at suitable depths each generation."  Such "jump up" reefs, as he calls them, "might well contain most of the species present in the original community."  Additionally, he notes that reproductive mechanisms in corals on a population level may interact in such a way as to produce rapidly forming new coral hybrids "with the potential for increasing the diversity of responses to environmental change."  Such hybridizations may help explain the survival of coral populations throughout the varying environmental conditions of geologic time. 

Research conducted to examine the relationship exhibited between the symbiont algae and their coral host is proving to be "more complex than previously supposed."  Advances in understanding the role of zooxanthellae on a coral community level, for example, have led to the new perspective that coral bleaching "might not be simply a breakdown of a stable relationship that serves as a symptom of degenerating environmental conditions."  Instead, the loss of such algal symbionts "may be part of a mutualistic relationship on a larger temporal scale, wherein the identity of algal symbionts changes in response to a changing environment."

Understanding how coral reef communities respond to environmental change is severely lacking at the moment, as the author notes that "we have barely started teasing apart the community linkages responsible for reefs as we see them today, let alone how coral reef communities might respond to large- and small-scale environmental changes."  More research is needed on relevant temporal and spatial scales.

What it means
This review article clearly demonstrates our lack of knowledge on three important fronts of coral reef research.  In the words of the author, "we are probably not ready to hindcast, let alone forecast, the relative success of different reproductive modes because our current understanding is so incomplete."  In addition, he notes that "we currrently have neither the depth nor the breadth of basic knowledge to evaluate the most basic features regarding the ecological, biogeographical, or phylogenetic effects on the specificity (or lack of it) of the zooxanthellae-host relationship."  To his way of thinking, our knowledge of community reef ecology is still in its infancy.

Reviewed 1 August 1999