The role of algal symbiosis in reefs through time

plays a dominant role in the energy budget, gross anatomy, biochemistry, skeletal structure, distribution, and reproductive strategy of most reef-buildingand many reef-dwelling organisms today. In our interpretations of ancient reef systems, the null hypothesis has usually been that organisms lacked algal symbionts. The burden ofproof lay with the investigator to produce evidence of symbiosis in ancient reef systems. Calcification powered by photosynthesis in shallow clear tropical waters is as old as the stromatolites that are the earliest evidence of life on Earth. Algal symbiosis has been evolved many times during the repeated evolution of different reef-dwelling organisms. Any uniformitarian hypothesis of reef ecosystems should anticipate a major role for symbiosis in any ancient reef community, with significant implications for paleoenvironments and paleoecology. Obviously, the ancient ecosystems whose current interpretations are most affected by reversing one's expectations in this way are the Paleozoic ones, including the reef-like structures built by stromatoporoids and archeocyathids. We are only beginning to understand the complexities of a full algal symbiosis. The relatively slow recovery of reef ecosystems from extinction events may reflect the time necessary for the re-establishment of algal symbiosis.