The influence of light and water flow on the growth and physiology of the scleractinian coral Galaxea fascicularis

Background Zooxanthellate scleractinian corals are sessile colonial animals that live in symbiosis with photosynthetic algae, the zooxanthellae. They can feed both phototrophically and heterotrophically and produce an external skeleton of calcium carbonate, which process is enhanced by light. They are the key organisms of tropical coral reefs and responsible for building the large carbonate structures. Tropical coral reefs are increasingly threatened by both natural and anthropogenic stresses. Concurrently with the gradual decline of coral reefs, a growing interest in keeping this delicate ecosystem in aquaria has emerged. To reduce harvest from the wild, increasing effort is put in developing cost-effective coral aquaculture culture. The objective of this thesis was to study the influence of light (irradiance and photoperiod) and flow on coral growth and physiology. Furthermore, the interaction between light and water flow was studied. Methods The effect of flow (Chapter 2), light (Chapter 3), photoperiod (Chapter 4) and the interaction between light and flow (Chapter 5) on coral growth were studied in long-term experiments monitoring several growth parameters such as buoyant weight (i.e. skeletal mass), surface area and polyp number. Physiological parameters such as photosynthesis and respiration were measured in respirometric flowcells to provide an explanation for the observed differences in growth. Moreover, an overview was given of different factors controlling coral growth and how such knowledge can be translated to aquaculture practice (Chapter 6). Results In the absence of water flow, coral growth was significantly lower and corals appeared unhealthy. In the presence of water flow (10, 20 and 25 cm s-1, at 90 µE m-2 s-1), growth rates were significantly increased. However, growth was not significantly different between 10 cm s-1 and 20 cm s-1, but again significantly increased at 25 cm s-1. Differences in growth could not be explained by net photosynthetic rate and Scope for Growth based on phototrophic carbon, since these parameters decreased with increasing water flow (Chapter 2). Increasing irradiance significantly increased the specific exponential growth rate of Galaxea fascicularis. The relation between skeletal growth and net photosynthesis was not directly proportional, but distorted at high irradiance levels (Chapter 3). Increasing photoperiod did not increase the specific exponential growth rate of Galaxea fascicularis. However, since growth neither increased with increasing irradiance, it is suggested that growth was limited by another factor and light was therefore saturating. The corals in the 24 hour light treatment were not able to adapt to prolonged light duration. However, the corals in the 16 hour light treatment probably photo-acclimated to prolonged photoperiod under light-saturating conditions by reducing their hourly photosynthetic rates (16 hour vs. 8 hour light) As a result, daily net photosynthetic was not significantly different, just as their growth rates (Chapter 4). The interaction between light and water flow for coral growth was significant. Water flow stimulated coral growth more at 600 µE m-2 s-1 than at 300 µE m-2 s-1 and the highest growth rates were attained at high irradiance (600 µE m-2 s-1) in combination with high flow rates (15-25 cm s-1). Nevertheless, enhancement of coral growth with either increasing irradiance or increasing water flow could not be explained by net photosynthetic rates (Chapter 5). There are many factors (both environmental and genetical) that can potentially limit or inhibit coral growth. Optimization of coral aquaculture therefore requires close fine-tuning of factors. Growth models can be used as a tool to determine the best culture strategy (Chapter 6). Conclusion Increasing water flow has a positive effect on coral growth at a wide range of irradiance levels (90, 300 and 600 µE/m2/s). Increasing irradiance also has a positive effect on coral growth. However, the positive relation between irradiance and coral growth is disturbed when other factors are limiting. Corals were able to retain their growth rates upon photoperiod extension under light-saturating conditions, presumably by means of reducing their hourly photosynthetic rate. At high irradiance levels, the enhancement of coral growth was not proportionally related to net photosynthesis, suggesting that other factors become limiting. Since the interaction between irradiance and water flow is significant, this indicates that water flow can remove some of those limitations at high irradiance levels. The mechanism of enhancement did not seem related to differences in net photosynthetic rate, but is possibly related to reduced energy allocation toward costly photo-protective mechanisms at high irradiance and high flow. The information in this thesis should not be used as a blueprint for coral aquaculture, however, its value lies in providing a blueprint for targeted optimization studies of coral aquaculture. Since the magnitude of effect of one factor often depends on the other, it is of importance for the future to perform multi-factorial experiments to provide insight in the interactions between factors. Under light-saturating conditions, further increases in irradiance or extension of photoperiod do not result in more growth. [ Increasing irradiance has a positive relation with both coral growth and net photosynthesis. At high irradiance levels, the enhancement of coral growth was not proportionally related to net photosynthesis, suggesting that other factors become limiting. Since the interaction between irradiance and water flow is significant, this indicates that water flow can remove some of those limitations at high irradiance levels. The mechanism did not seem related to differences in net photosynthetic rate, but is possibly related to reduced energy allocation toward costly photo-protective mechanisms at high irradiance and high flow. Under light-saturating conditions, further increases in irradiance or extension of photoperiod do not result in more growth. [ Both increasing irradiance and water flow enhance coral growth. Enhancement of coral growth by light was not proportionally related to net photosynthesis at higher irradiance levels, suggesting that other factors become limiting. Since the interaction between irradiance and water flow is significant, this indicates that water flow can remove some of those limitations at high irradiance levels. The mechanism did not seem related to differences in net photosynthetic rate, but is possibly related to reduced energy allocation toward costly photo-protective mechanisms at high irradiance and high flow. Under light-saturating conditions, further increases in irradiance or extension of photoperiod do not result in more growth. [ Increasing water flow has a positive effect on coral growth at a wide range of irradiance levels (90, 300 and 600 µE/m2/s). In our studies, differences in growth could not be explained with differences in net photosynthetic rate. Rather differences in growth could be related to algal competition and sedimentation or other physiological parameters such as (in)organic nutrient uptake, relief of photo-oxidative stress and dark respiration that are influenced by water flow. Increasing irradiance also has a positive effect on coral growth. Enhancement of coral growth by light was not proportionally related to net photosynthesis at higher irradiance levels, suggesting that other factors become limiting. The significant interaction between light and water flow indicates that water flow can remove some limitations and that the presence of water flow is very important for optimal light use for coral growth. The mechanism did not seem related to differences in net photosynthetic rate, but is possibly related to reduced energy allocation toward costly photo-protective mechanisms at high irradiance and high flow. The effect of increasing photoperiod under light-limiting conditions still needs to be established. Since enhancement of coral growth did not seem to be d… by photosynthesis at higher irradiance levels [WEL, maar niet proportional], it suggested limited.. One of such factors that can remove limitations is water flow. A significant interaction between light and water flow is detected, … but this effect is dependent on water flow. The interaction between light and water flow indicates that water flow is very important for optimal light use for coral growth. The mechanism is still unclear, but is possibly related to reduced energy allocation toward costly photo-protective mechanisms at high irradiance and high flow. Both increasing irradiance and water flow have a positive effect on coral growth, and a significant interaction is found between light and water flow. The positive effect of increasing water flow on coral growth was found to be significant both at an irradiance of 90 µE/m2/s (Chapter 2), 300 µE/m2/s and 600 µE/m2/s (Chapter 5). Neither of these differences in growth were supported by a significant increase in net photosynthetic rate, in contrast to our expectations. The positive effect of increasing water flow on coral growth is probably a consequence of both external (algal competition and sedimentation) and internal mechanisms( (in)organic nutrient uptake, relief of photo-oxidative stress, respiration). Different at different irradiance levels. External at 90, internal at 300 and 600 The positive effect of irradiance on skeletal growth was demonstrated in both Chapter 3 and 5. The relationship with photosynthesis irradiance curve not proportional. - mediation by P not clear. - The relation between skeletal growth and net photosynthesis was not proportional, In contrast, no positive effect of irradiance on skeletal growth was found in Chapter 4. Both specific growth rate and net photosynthesis increased with irradiance, however, this relationship was not proportional. It is suggested that .at high irradiance levels, skeletal growth (i.e. calcification and organic matrix synthesis) is not limited by light or photosynthesis. At high irradiance, other factor (e.g. availability of bicarbonate (i.e. aragonite saturation state), heterotrophic feeding and/or water flow) may become limiting. Water flow and light… Photoperiod… still needs to be established. .. offset limitations . optimize balance between factors At an irradiance of 90 µE m-2 s-1, water flow enhanced coral growth Increased water flow enhanced coral growth at a wide range of irradiances: 90 µE m-2 s-1 (Chapter 2), 300 and 600 µE m-2 s-1 (Chapter 5). AT 90 µE m-2 s-1, no The mechanism of enhancement is however not clear. Flow enhanced growth.. absence of flow detrimental.. did… however.. P, algae.. Light… relation growth and photosynthesis distorted.. light enhanced calcificcaiotn. Photoperiod… no effect .. light not limiting.. photoacclimation.. 24 not able to adapt, but 16 hours well, probably by reducing hourly P. Light x flow… Interaction between light and water flow (chapter 5).. … no diff chapter 4… Review factors (chapter 6) .. examined… chapter.. Furthermore… In addition.. different mechanisms at different irradiance levels. No relation with P. At 90… reduce disturbance of coral growth by competing algae. at 300 and 600 …. Reduce photo-protective mechanisms