Light as a source of information and energy in zooxanthellate corals

Reef-building corals live in a mutualistic symbiosis with endocellular microalgae, the zooxanthellae that provide much of their animal host's metabolic energy needs through translocation of photosynthate. That dependence sets a limit on the depth distribution of zooxanthellate corals, commonly restricting it to depths where light exceeds 0.5% of its subsurface intensity. We review the various ways allowing corals to span over two orders of magnitude in the underwater light field over their bathymetric distribution range. These consist of adjustments by both components of the holobiont, the coral host, and its algal symbionts, and also affect the flows of energy and matter between them. Animal and algae optimize light harvesting and utilization on the molecular, biochemical, biophysical, metabolic, behavioral, and architectural levels. These photoacclimative responses result in corals exposed to the full blast of tropical sun being able to use such superabundant energy, while being spared from its destructive consequences. At the opposite extreme, corals under dim light make the most efficient and parsimonious use of it. We also bring up to date the ways in which moonlight triggers coral spawning cycles and also illustrate the synergy between visible light and UV in the photoacclimation process.