Photo-acclimation dynamics of the coral Stylophora pistillata to low and extremely low light

Photo-acclimation dynamics of the symbiotic coral Stylophora pistillata to lowering light intensities in the range of 95% to 0.8% of incident surface photosynthetic active radiation (PAR(o)) was studied. Coral colonies were s ampled from 1- to 2-m depths in open and shaded habitats from the fringing reef of Sesoko Island (near the Tropical Biosphere Research Center, Univers ity of the Ryukyus) Okinawa, Japan. Photo-acclimation of exterior branches of coral colonies were examined in outdoor aquarium, where light regime and feeding were similar to field conditions. Two photo-acclimation reactions were studied (1) changes in chlorophyll concentrations in zooxanthellae; (2 ) changes in zooxanthellar population density in coral branches. In paralle l, we measured an average volume of zooxanthellae, proliferating zooxanthel lae frequency (PZF) and degrading zooxanthellae frequency (DZF). It was sho wn that the coral S. pistillata can survive and acclimate to a wide range o f light intensities from 95% to 0.8% PAR(o). Acclimation to low light (30% and 8% PAR(o)) involves maximizing the light harvesting capacity by increas ing photosynthetic pigment concentration in zooxanthellae and zooxanthellar population density in coral branches. Under extremely low light (0.8% PAR( o)), the coral lost zooxanthellae by degradation (perhaps digestion) and re tained zooxanthellae-accumulated high concentrations of chlorophyll. The ph otoacclimation process is dynamic and immediate. Changes in pigment concent rations in zooxanthellae occurred within 2-4 days and changes in zooxanthel lar population densities occurred within 40 days. Zooxanthellae population densities were regulated by changes in rates of division and degradation of symbiotic cells. (C) 2001 Published by Elsevier Science B.V.