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

Abstract

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 0) was studied. Coral colonies were sampled from 1- to 2-m depths in open and shaded habitats from the fringing reef of Sesoko Island (near the Tropical Biosphere Research Center, University 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 parallel, we measured an average volume of zooxanthellae, proliferating zooxanthellae frequency (PZF) and degrading zooxanthellae frequency (DZF). It was shown that the coral S. pistillata can survive and acclimate to a wide range of light intensities from 95% to 0.8% PAR 0. Acclimation to low light (30% and 8% PAR 0) involves maximizing the light harvesting capacity by increasing photosynthetic pigment concentration in zooxanthellae and zooxanthellar population density in coral branches. Under extremely low light (0.8% PAR 0), the coral lost zooxanthellae by degradation (perhaps digestion) and retained zooxanthellae-accumulated high concentrations of chlorophyll. The photo-acclimation process is dynamic and immediate. Changes in pigment concentrations in zooxanthellae occurred within 2–4 days and changes in zooxanthellar population densities occurred within 40 days. Zooxanthellae population densities were regulated by changes in rates of division and degradation of symbiotic cells.