Alternative thinking about the past, present, and future of coral-algal symbioses

Many coral reef anthozoans (e.g. corals, anemones) form symbiotic associations with photosynthetic, dinoflagellate algae of the genus Symbiodinium (zooxanthellae). The association is mutualistic with the symbiont contributing energy, carbon and lipids directly to the nutrition of their invertebrate hosts, and the host providing nitrogen, CO₂, a habitat and protection to the zooxanthellae. Zooxanthellae also act indirectly to enhance calcification in the reef-building scleractinian corals that create the substrate supporting the entire coral reef ecosystem.; In consideration of the importance of the mutualistic symbiosis between corals and zooxanthellae to the existence of modern coral reefs, much research has been based on the assumption that reef-building corals evolved in the oceanic settings they primarily inhabit today; the symbiosis is universally beneficial; and the degradation of the symbiosis is catastrophic to coral reefs. However, there exist alternative points of view for each of these concepts which could influence how one thinks about the past, present, and future of coral-algal symbioses.; First, it is likely that reef-building corals and the coral-algal symbiosis evolved in nearshore, mesotrophic, turbid environments which then pre-adapted corals to the oceanic, oligotrophic, clear water settings they primarily inhabit today. This argument is based on the types of habitat geologically available during Scleractinian evolution and the advantages conferred by zooxanthella symbiosis in those habitats. Second, it is possible that zooxanthellae are costly to the host under particular environmental conditions. This hypothesis was tested using symbiotic and aposymbiotic anemones under two feeding and environmental regimes. It was determined that aposymbiotic anemones always had greater fitness than symbiotic anemones under the conditions tested, suggesting a nutritional cost to hosting zooxanthellae. Finally, zooxanthella exchange may provide a means of long-term evolutionary adaptation to the host-symbiont unit in the face of global climate change. This argument is based on the ability of hosts to associate with different zooxanthella types over time and on the potentially faster evolution of zooxanthellae to changing conditions.