Ambient irradiance levels determine the rate of carbon influx into
zooxanthellae at any given time, and thereby the energy available for the whole
coral symbiotic association. Long-term photoacclimation of zooxanthellae to
the time-averaged light regime at which the host coral grows results in optimization
of light harvesting and utilization. Under high irradiance light harvesting
is reduced, thereby avoiding photodynamic damage, whereas under low light,
photon capture and quantum yield are maximized. Most of the photosynthate
produced by the algae is respired. However, the capability of the zooxanthellae
and the coral to retain carbon beyond that required to meet their respiratory
needs depends on the availability of the commonly limiting nutrients, nitrogen
and phosphorus. Therefore, the ratio of the flux of these nutrients into the
colony to that of the photosynthetically driven carbon flux will regulate the
growth of the zooxanthellae and of the animal. Nutrients acquired by predation
of the coral on zooplankton are available first to the animal, whereas those
absorbed by the zooxanthellae from seawater as inorganic compounds lead first
to growth of the algae
