Photosymbiosis has been an important process in the evolution of ancient reef systems and in reef success today. Modern reefs and many of those in the geologic past inhabited nutrient-depleted settings. The complete collapse of some ancient reef ecosystems may be attributed to the breakdown of the ecologic and physiologic relationships between symbiont and host. Many algal groups developed symbioses with calcifying metazoans and protists and live with them, but the most common of these today are dinoflagellates in the genus Symbiodinium, sometimes called zooxanthellae. This photosymbiotic relationship conferred important metabolic advantages to both partners, allowing exploitation of tropical, shallow-water oligotrophic settings. In addition to improved metabolism, a by-product was rapid calcification which increased the growth of reefs and provided advantages to the hosts through larger and stronger skeletal support. Strong evolutionary pressures exerted by the symbiont-host relationship created bonds and favored longevity and adaptive novelty. Photosynthesis accounts for the remarkable reef growth and carbonate sedimentation in the tropics. Photosymbiosis gave reef organisms an adaptive edge to develop new life strategies that could not be developed by organisms which did not foster this relationship. Many living calcified organisms harbor many different photosymbionts and likely a variety of ancient calcified organisms did too (foraminifera, calcified sponges, corals, brachiopods and bivalve mollusks). Symbiodinium now a dominant symbiont apparently appeared in the Eocene and so was probably not utilized by earlier reef organisms, although the fossil record of dinoflagellates most closely related to Symbiodinium extends back to the Triassic. Today Symbiodinium inhabits a wide variety of unrelated host organisms from protists to mollusks. While the identity of more ancient photosymbionts is unclear, indirect evidence suggests photosymbiotic ecosystems existed as far back as the Proterozoic and possibly even earlier.
Assessment of photosymbiosis in ancient reef ecosystems requires recognition of specific characteristics possessed by the calcifying reef organisms. Since the symbionts do not fossilize, the presence of photosymbiosis in fossils is a working hypothesis based on modern symbioses and best confirmed by a set of specific morphologic adaptations and isotopic analyses. Important among these is the thin tissue syndrome—the modification to achieve the “solar panel” effect. Large size and unusual or complex morphology also may indicate photosymbiosis. In the case of colonial organisms such as corals, high levels of corallite integration, where corallites are modified for increasing cooperation, may assist because most colonial photosymbiotic organisms today, such as corals, are exclusively photosymbiotic.
Analysis of organisms and reefs through geologic time permits assessment of the strength of photosymbiosis as a driving force. Reef ecosystems revealing the strongest assessment for photosymbiosis are those of the mid-Paleozoic (Late Ordovician to Devonian), late Paleozoic, early Mesozoic and Neogene. The Early Cambrian archaeocyathan (sponge) reefs indicate photosymbiosis but perhaps with different ancient symbionts such as cyanobacteria, also contained in some modern sponges. Reef ecosystems of the late Paleozoic and early part of the Jurassic indicate the presence of some photosymbiosis. The extinction of many photosymbiotic reef ecosystems during critical intervals of mass extinctions may have been driven by the failure of the symbiosis or demise of the symbionts. Reef gaps in the geologic record reflect the absence of photosymbiosis. The present-day reef crisis involves disturbance of photosymbiosis, and study of future reef declines will benefit by application of data from the fossil record.