The effect of irradiance on long-term skeletal growth and net photosynthesis in Galaxea fascicularis under four light conditions Miriam Schutter a,b, ⁎, Bas van Velthoven a , Max Janse c , Ronald Osinga a , Marcel Janssen b , Rene Wijffels b , Johan Verreth a a Aquaculture and Fisheries Group, Wageningen University, P.O. Box 338 6700 AH Wageningen, The Netherlands b Bioprocess Engineering Group, Wageningen University, PO Box 8129 , 6700 EV Wageningen, The Netherlands c Burgers Zoo, Antoon van Hooffplein 1, 6816 SH Arnhem, The Netherlands abstract article info Article history: Received 24 January 2008 Received in revised form 20 August 2008 Accepted 22 August 2008 Keywords: Buoyant weight Galaxea fascicularis Irradiance Photosynthesis Scleractinian coral Skeletal growth 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/m 2 /s and 70, 250 and 400 W Metal Halide lighting providing an irradiance of 38 μE/m 2 /s, 166 μE/m 2 /s and 410 μE/m 2 /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/m 2 /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/m 2 /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 O 2 /min/cm 2 to 0.077 μmol O 2 /min/cm 2 ). 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. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Light is one of the most important abiotic factors influencing the growth of scleractinian corals. Scleractinian corals live in symbiosis with unicellular algae, known as zooxanthellae, that reside in their endodermal tissue layers. In the light, zooxanthellae perform photosynthesis, during which process they produce oxygen and organic compounds. When their own respiratory needs are satisfied, zooxanthellae translocate the excess photosynthetic products to the coral host (Muscatine and Cernichiari 1969; Muscatine et al., 1981). Zooxanthellae can thus provide a con- siderable part of the energy needed for coral growth. Growth of scleractinian corals can be divided in two components: first, skeletal growth due to the deposition of an external skeleton of calcium carbonate aided by the synthesis of an organic matrix in a process called calcification, and second, tissue growth. According to the light-enhanced calcification theory (see Gattuso et al., 1999 and Allemand et al., 1998 for review), the symbiosis with zooxanthellae is aiding to the process of skeletal growth. According to this theory, calcification of the coral host is enhanced by photosynthesis of zooxanthellae (Goreau and Goreau 1959; Pearse and Muscatine 1971; Allemand et al., 2004). Indeed, on average, calcification in light is found to be around three times higher than calcification in darkness (review by Gattuso et al., 1999). Although photosynthesis and calcification are spatially separated processes (photo- synthesis occurs in the oral tissue layer and calcification in the aboral tissue layer), they do share a common pool of inorganic carbon inside the coelenteron of the coral host, accounting for the interaction between these two processes. The exact mechanisms of the enhancement of calcification by photosynthesis are still a matter of debate (Gattuso et al., 1999; Furla et al., 2000). Some of the proposed mechanisms include that: 1) photo- synthesis provides energy for the energy-demanding processes associated with calcification, such as calcium transport and organic matrix synthesis Journal of Experimental Marine Biology and Ecology 367 (2008) 75–80 ⁎ Corresponding author. Aquaculture and Fisheries Group, Wageningen University, P.O. Box 338 6700 AH Wageningen, The Netherlands. Tel.: +31 317 483307; fax: +31 317 483937. E-mail address: Miriam.schutter@wur.nl (M. Schutter). 0022-0981/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jembe.2008.08.014 Contents lists available at ScienceDirect Journal of Experimental Marine Biology and Ecology journal homepage: www.elsevier.com/locate/jembe