Research by marine biologists from Wageningen University has shown that feeding on zooplankton by scleractinian corals has been greatly underestimated.
|A new perspective on aquarium lighting|
|Written by Tim Wijgerde|
The intensive use of powerful lighting above marine aquaria is common practice amongst aquarists nowadays. Many coral tanks are battered by 400W metal halides and T5 HO bulbs, mimicking natural light intensities experienced by corals in the wild. This is now regarded as a necessity for keeping healthy and fast growing corals. Recent studies have indicated that this philosophy is just part of the story…
The modern view of coral husbandry is that aquarium parameters should mimic natural conditions as closely as possible. For this purpose, a vast array of equipment is available: filtration systems, high-current pumps, wave makers, and of course powerful lighting.
Lighting has always been a major issue; many articles on light intensities, reflectors, spectral qualities and life expectancies of bulbs have been published in popular (on-line) journals. The main dogma has always been; the more light, the better. It is said that aquarists just cannot provide enough light, especially when dealing with stony corals such as Acropora sp. The main argument for this is that in nature, the sun illuminates coral reefs with a power of over 100,000 lux (which is a PAR value of about 2600 µE/m2/s)1. And when corals experience high light intensities between 0 and 10 m of depth, they must like it, and we should try to mimic it. But is this really true?
Figure 1: Coral reefs, such as this one at the Gulf of Aqaba, Red Sea, experience high irradiance levels on a daily basis (Photograph: Tim Wijgerde).
Not necessarily. When organisms thrive in certain habitats, this does not always imply that conditions are ideal. Many organisms actually survive despite those conditions. The reason for it is that the more difficult things get for you, the more difficult things get for your competitors out there. Think about orchids growing in tropical areas at places which receive little sunlight, or very little rain. These plants do not look well when found at such sites, but they do survive. Think of emperor penguins, surviving on the darkest, driest, coldest, and windiest continent on earth. Natural isn’t always ideal. It’s simply a matter of survival of the fittest; a cornerstone of evolution.
Coral reefs; Edens or ordeals?
Coral reefs are often called the Edens of the sea; seeming like a wondrous place where life thrives under ideal conditions. This bubble is somewhat burst when you think of stony corals and Tridacna clams being battered by UV radiation when exposed to air during low tides. Clams shut completely, and mucus produced by corals prevents them from dehydrating during their daily ordeal above water. Not to forget the continuous waves and storms, pounding on their fragile skeletons.
Figure 2: Corals such as this colorful Pocillopora verrucosa are faced with low tides, and the dry winds and battering UV-radiation that accompany it (photograph: Tim Wijgerde).
When focusing on light, it is known that it is both an ordeal and a blessing for reef-building corals. Studies have found that the sun’s energy provides up to 95% of a coral’s daily energy budget2,3,4, and it has also been found that this generated energy stimulates coral growth. The reason for it is that coral growth by means of calcification is energy-demanding, as calcium and carbonate ions have to be transported over the calicoblastic layer5. Furthermore, the synthesis of the organic matrix during calcification also uses up quite some energy6. Finally, photosynthesis raises intracellular pH and carbonate ion saturation, enhancing precipitation of calcium carbonate7,8,9. More light usually leads to more coral growth, which brings us back to the original dogma of mimicking the sun’s energy with powerful lighting. Recent studies have again found that more light does indeed stimulate growth, but not to the extent as previously thought.
Irradiance versus coral growth
Scientists from the CORALZOO project (M. Schutter et al.)10 found that irradiance is not directly proportional to either photosynthesis or coral growth (calcification). They used the stony coral Galaxea fascicularis to determine the relationships between irradiance, photosynthesis and calcification (growth). They subjected specimens of this species to various light intensities, ranging from 38 to 410 µE/m2/s, and took regular measurements over a 294 day period (frontpage picture shows setup). Figure 3 (left) shows the relationship between irradiance (in µE/m2/s) and specific growth rate. It immediately shows that growth rate leveled off at higher light intensities. More light indeed produced more growth, but not as much as expected.
Figure 3, left: Specific growth rate of Galaxea fascicularis colonies, grown under different light conditions, calculated over the total growth period (1-294 days) and plotted against irradiance. Values are mean ±S.D., N=9. Right: Effect of irradiance on net photosynthesis in μmol O2/min/cm2 (plotted on left axis) and specific growth rate per day, calculated from day 1 to 294 (plotted on right axis). Values of specific growth rate are mean ±S.D., N=9 (modified from M. Schutter et al, JEMBE, 2008)
When comparing growth rates to photosynthesis rates at different light intensities, it became clear that photosynthesis also leveled off a higher irradiance levels (figure 3, right). Although figure 3 seems to imply that calcification is limited by photosynthesis, this is not the case. When photosynthesis rates increased 8.9-fold, specific growth rates only increased 1.6-fold. This clearly indicates that light and photosynthesis are not the only factors involved in coral growth. It must be noted that the photosynthesis-irradiance curve was measured by using a coral which was grown under a light intensity of 60 µE/m2/s, which makes the comparison of the two graphs somewhat more difficult. When this coral is subjected to high light intensities, it will photosynthesize intensely as its zooxanthellae have adapted to low light levels. The corals which grew under the long-term setup would have adapted to higher irradiance levels, which would most likely have decreased their photosynthesis rates, and thereby their growth rates. A comparison with corals adapted to the light intensities used in the growth experiment would have possibly yielded different results. Even so, if this new experiment would have shown a Pi curve where photosynthesis would only have increased by a factor of 5, for example, there would still be a clear discrepancy with coral growth rate. Simply put, a 5-fold increase in photosynthesis compared to a 1.6-fold increase in coral growth would still imply other limiting factors besides light.
The scientists also made a direct comparison between photosynthesis and growth. Figure 4 shows the relation between photosynthesis and specific growth rate. At higher photosynthesis rates, the specific growth rate leveled off. This proved that at higher light intensities, light and photosynthesis were not the limiting factors for coral growth, as coral growth was not proportional to photosynthesis. As the scientists put it; “Thus it seems that enhancement of calcification is not entirely photosynthesis-driven: light enhanced calcification seems only to be mediated by photosynthesis at lower irradiances, while at higher irradiances the relation between calcification and photosynthesis is distorted”.
Figure 4: Specific growth rate plotted against net photosynthetic rate (modified from M. Schutter et al, JEMBE, 2008).
The scientists hypothesize that coral growth is not limited by irradiance and photosynthesis at high levels because other factors play a role as well. Photosynthates such as carbohydrates do provide energy, but are not sufficient alone for building new coral tissue. Other elements such as nitrogen, and possibly sulphur, phosphorous and trace elements are also required to generate biomass. They also stated that bicarbonate and plankton availability may play a role. Plankton is known to be rich in amino acids, which are important for building the organic matrix during calcification. Finally, light itself may become growth inhibiting because of the need to build extra light-blocking pigments (which creates the attractive coral coloration so loved by aquarists) and the induction of oxygen radicals.
These results have important implications for coral aquaculture, but for home aquarists as well. We now know that mimicking nature isn’t always required, or even ideal, in this case that very high light intensities (and UV radiation for that matter) are not a necessity for proper coral growth. It must be said, however, that even small daily differences in coral growth will eventually yield large differences in size. This fact was recognized by the scientists (figure 5), and they state that “a fair trade-off between lighting costs and coral growth has to be made” (pers. comm.).
Figure 5: Effect of irradiance on the calculated buoyant weight of Galaxea fascicularis colonies. Values are mean ±S.D., N=9. Error bars indicate standard deviations (modified from M. Schutter et al, JEMBE, 2008).
Combining the addition of extra plankton with extra light may yield significantly higher coral growth rates, by removing other limiting factors such as nitrogen-availability. In nature, it may be true that corals are able to effectively harness PAR levels of over 800 µE/m2/s due to the abundance of plankton (such as during summer periods). For home aquarists, the take home message is that without extra plankton, high intensity lighting may not be that useful...
Figure 6: A large Galaxea fascicularis colony in a public display at the Oceanario de Lisboa, Lisbon, Portugal. According to this new study, corals such as this species need more than just light to grow properly (photograph: Tim Wijgerde).
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