The effect of irradiance on long-term skeletal growth and net photosynthesis in Galaxea fascicularis under four light conditions

The relation between irradiance, skeletal growth and net photosynthesis was studied for the scleractinian coral Galaxea fascicularis to provide experimental evidence for mediation of light-enhanced calcification through photosynthesis. The hypothesis was tested that skeletal growth and photosynthesis are linearly correlated.A long-term experiment was performed in a closed-circuit aquarium system, in which four series of nine nubbins (single polyp clones of a coral colony) of Galaxea fascicularis were exposed to four light treatments (10L:14D): 144 W T8 fluorescent lighting providing an irradiance of 68 µE/m²/s and 70, 250 and 400 W Metal Halide lighting providing an irradiance of 38 µE/m²/s, 166 µE/m²/s and 410 µE/m²/s, respectively. Growth of these nubbins was measured as buoyant weight at different time intervals in a 294 day experiment. A light-saturation curve for photosynthesis was measured in a respirometric flow cell using a 54 week Galaxea fascicularis colony grown at 60 µE/m²/s.No saturation of net photosynthesis of Galaxea fascicularis was found at the irradiances tested. The specific growth rate (µ, in day^- 1) of the coral nubbins increased with irradiance. Whereas irradiance varied 11-fold (38 to 410 µE/m²/s), buoyant weight (increase after 294 days) increased 5.7 times (2243 to 12374 mg), specific growth rate (1-294 days) increased 1.6 times (0.0103 to 0.0161 day^- 1), while net photosynthetic rate increased 8.9 times (0.009 µmol O₂/min/cm² to 0.077 µmol O₂/min/cm²). The increase of specific growth rate with irradiance was less than expected based on the increase in net photosynthetic rate with irradiance. This discrepancy between potential energy produced in photosynthesis and energy used for skeletal growth indicates that skeletal growth is not limited by photosynthetic potential at high irradiance levels.