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You are here: Home Volume XIX April 2018 Study: Effects of light spectrum on growth and colouration of the corals Stylophora pistillata and Porites cylindrica

Study: Effects of light spectrum on growth and colouration of the corals Stylophora pistillata and Porites cylindrica

By Tim Wijgerde (1,2) Posted Apr 11, 2018 09:00 AM Pomacanthus Publications, Inc.
Dr. Tim Wijgerde investigates the effects of blue and red light on the health, colouration, growth and zooxanthellae density of two coral species, Stylophora pistillata and Porites cylindrica.
  1. Marine Animal Ecology, Department of Animal Sciences, Wageningen University, Wageningen University and Research Centre, Wageningen, The Netherlands
  2. CORALAB, Zeewolde, The Netherlands

lightspectrumcorals2.jpg

Abstract

Light spectrum plays a key role in the biology of reef-building corals. For Stylophora pistillata, it was recently shown that blue and red light regulate zooxanthellae density in opposing ways. It is, however, still unclear how these light sources affect coral growth. This study investigated the medium-term effects of blue and red light on the health, colouration, growth and zooxanthellae density of two coral species, S. pistillata and Porites cylindrica. Microcolonies of both species were exposed to white, blue and red LED light (N=5 per light treatment per species), at an equal photosynthetically active radiation (PAR) of 300 µmol photons m-2 s-1 for a period of 10 weeks. Coral survival was 100% and colonies exhibited good polyp expansion under all treatments. Blue light enhanced pink colouration of S. pistillata and stimulated pronounced horizontal outgrowths at the base of the colonies. Growth of S. pistillata was not significantly affected by light colour, in contrast to P. cylindrica, which grew faster than the former species under white light. Zooxanthellae density of S. pistillata cultured under red light was reduced by 40% as compared to blue light, which was reflected by paler corals. These results support the view that light sources lacking emission in the blue portion of the spectrum do not support normal zooxanthellae growth, which may ultimately lead to growth stagnation and death of the host coral. Finally, blue light may trigger morphological adaptations to low light in reef-building corals.

Introduction

Light plays a key role in the physiology, growth and reproduction of reef-building corals (Muscatine et al. 1981; Davies 1984; Levy et al. 2007; Osinga et al. 2011). Until now, most studies on the effects of light on zooxanthellate corals have focused on light quantity. Only relatively few studies have investigated the effects of light quality, namely the individual roles played by different colours within the visible light spectrum (Kinzie et al. 1984, 1987; D'Angelo et al. 2008; Mass et al. 2010; Wijgerde et al. 2014). This is surprising, as it is known that not all wavelengths are equally efficient at promoting photosynthesis in zooxanthellate corals, with orange (at ~600 nm) seeming the least potent, and violet (~440 nm), blue (~460 nm) and red light (~670 nm) the most (Halldal 1969; Kühl et al. 1995). As photosynthesis is a key driver of coral calcification (Osinga et al. 2011), exposure to different colours may result in variable coral growth rates. In addition, other biological processes than photosynthesis in corals may be affected by light spectrum. For example, blue light is essential to zooxanthellae growth, chlorophyll a synthesis and colourful pigment production in corals (Kinzie et al. 1984, 1987; D'Angelo et al. 2008; Mass et al. 2010; Wijgerde et al. 2014). Wang et al. (2008) studied the role of light colour in the growth and photobiology of ex hospite zooxanthellae of Euphyllia glabrescens (Symbiodinium sp., clade B), and found that blue light is key to maintaining normal cell division: exposure to red or infrared light disrupted mitosis of these zooxanthellae, while blue or white light (thus including blue light) resulted in normal cell division. Wijgerde et al. (2014) showed that increasing blue light intensity increases zooxanthellae density in Stylophora pistillata, whereas increasing red light intensity inhibits zooxanthellae growth and represses chlorophyll synthesis with a dominance over blue light. In addition, their corals showed considerable health loss when cultured under red light only. Thus, it seems 1) that blue light, by itself or combined with other colours, is required to maintain normal zooxanthellae growth and a healthy coral host, and 2) that red light acts as an "emergency brake" on zooxanthellae and chlorophyll a synthesis, possibly to prevent photodamage under high light conditions-i.e. within the first 10 meters (33 feet) of the water column where red light penetrates-sensu Kinzie et al. (1984). However, as previous studies used relatively broad bandwidth light filters, it is possible that the effects of various colours (i.e. violet, blue and green) on corals have been confounded. Although Wijgerde et al. (2014) used more narrow-bandwidth light LED fixtures rather than filters, it is still unclear how these light sources affect coral growth.

To increase our understanding of how light colour affects corals, this study investigated the effects of narrow-bandwidth blue and red light on the growth of Stylophora pistillata (Pocilloporidae Gray 1842) and its zooxanthellae, using white light as a reference. In addition, a species from another family, Porites cylindrica (Poritidae Gray 1842), was included for comparison. I also determined how white, blue and red light affected overall colouration of S. pistillata and P. cylindrica, as D'Angelo et al. (2008) found that the production of colourful coral pigments, such as pink chromoproteins (Pocilloporins) and green fluorescent proteins, is enhanced by blue light. These pigments likely protect the coral against energetic (UV, violet and blue) radiation which otherwise damages the photosynthetic machinery within the zooxanthellae (i.e. photosystem II), by blocking some of the light or by fluorescing it as lower energy green to red light (Salih et al. 2000; Gilmore et al. 2003). In this study, microcolonies of S. pistillata and P. cylindrica were exposed to white, blue and red light at equal photosynthetically active radiation levels (PAR, wavelength range of ~400-700) of 300 µmol photons m-2 s-1 for a period of 10 weeks. This light level was chosen as it lies within the range of blue and red irradiance found within the first 10 meters of the seawater column, based on a PAR of 2,000 µmol m-2 s-1 at sea level and seawater light attenuation (Lesser et al. 2010; Mass et al. 2010). Several larger S. pistillata colonies were also cultured under all light sources to allow further comparison. This study could provide new insights into coral ecology and may benefit sustainable coral aquaculture, which is reliant upon attractive colouration and reduced coral culture costs (Osinga et al. 2011; Leal et al. 2013; Rocha et al. 2013).

Materials and Methods

Coral Fragmentation and Husbandry

The Indo-Pacific scleractinian corals Stylophora pistillata (Esper 1797) and Porites cylindrica (Dana 1846) were used in this study. The S. pistillata parent colony was previously cultured for approximately 10 years at Wageningen University (Wageningen, The Netherlands) under similar conditions after being obtained from Burgers' Zoo (Arnhem, The Netherlands). The P. cylindrica colony was obtained via wholesale (De Jong Marinelife, Spijk, The Netherlands). For each species, coral microcolonies (N=20) were randomly cut from one parent colony and vertically mounted onto ceramic tiles (De Jong Marinelife, Spijk, The Netherlands) using two-component epoxy resin (Tunze Aquarientechnik GmbH, Penzberg, Germany). Only the growing tips were cut, resulting in uniform fragments about 1 cm in length.

All 40 microcolonies were allowed to recover for 4 weeks in a ~850 L holding aquarium (length x width x height: 260 x 110 x 30 cm) connected to a ~420 liter sump (140 x 75 x 40 cm water height) before the start of the 10-week experiment. The corals were first exposed to a full spectrum 190W LED (CoralCare, Philips Lighting, Eindhoven, The Netherlands) at 100% setting for both channels (Fig. 1), at a PAR of 300 µmol photons m-2 s-1 and a 10h:14h light:dark regime. Water flow was provided by four Turbelle nanostream 6085 circulation pumps (Tunze Aquarientechnik GmbH, Penzberg, Germany) providing a total output of 18,000 L h-1. Water flow rate around the corals was measured with a current velocity meter (Model 2100, Swoffer Instruments, Inc., Seattle, USA) in 10 cm intervals around the corals, and averaged between 7 and 8 cm s-1.

To maintain water quality and clarity, the aquarium was equipped with a model 700 DyMiCo filter (DyMiCo Filter BV, Utrecht, The Netherlands) and a 20W UVC-light (Aqua Holland, Dordrecht, The Netherlands) powered by a 1,000 L h-1 aquarium pump (Eheim GmbH & Co. KG, Deizisau, Germany). Constant salinity was ensured by a water top-off system (Tunze Aquarientechnik GmbH, Penzberg, Germany), which supplied RO/DI water from a 60 L holding tank. The aquarium was fed daily with five small cubes of frozen zooplankton (Aquadip, Oss, The Netherlands) and dried algae flakes (De Jong Marinelife, Spijk, The Netherlands). The aquarium was kept free of turf algae by various herbivores, which included 6 Zebrasoma flavescens, 1 Siganus vulpinus, 1 Siganus uspi, 1 Naso elegans and 15 Turbo sp. Water parameters were maintained at the following levels: salinity 34.5±0.3 g L-1, temperature 25.0±0.5°C, pH 8.2±0.3, nitrate 0.4±0.3 mg L-1, phosphate <0.03 mg L-1, calcium 477±18 mg L-1, alkalinity 2.83±0.26 mEq L-1 (N=15-20).

Light Treatments

After the 4-week recovery period, microcolonies were randomly assigned to three different light treatments (N=5 per treatment per species); white, blue or red light (Fig. 1). Several larger S. pistillata colonies were also included to provide additional information (N=2 per treatment). A 10h:14h light:dark regime was used for all treatments. The white light treatment consisted of one 190W CoralCare LED unit (Philips Lighting, Eindhoven, The Netherlands), whereas the blue and red treatments were achieved with two custom-built LED fixtures (Philips Lighting, Eindhoven, The Netherlands). The full spectrum white light was included as a control, the supplied light spectrum and PAR level being identical to those of the recovery period. Irradiance values (as photosynthetically active radiation or PAR, ~400-700 nm) was measured in situ around the corals in the experimental tank, at 10 cm space intervals for each group, using a LI-1400 data logger with LI-192 underwater quantum sensor (LI-COR, Lincoln, USA). PAR values were adjusted to 300 µmol photons m-2 s-1 for each treatment by using dimming software provided by the manufacturer. The light spectra provided by the LED fixtures were determined with a calibrated HR4000 spectrometer (Ocean Optics, Dunedin, USA). The blue LED fixture (168W at full power) emitted a light spectrum with a peak at 452 nm (73 nm total bandwidth, Fig. 1) and the red LED fixture (120W at full power) showed a peak at 665 nm (74 nm total bandwidth, Fig.1). The full spectrum white control light exhibited a continuous spectrum with a peak at 450 nm (Fig. 1), and a blue:red ratio of 4.2:1 (82:20 µmol m-2 s-1). To prevent spectral crossover effects, two dark-grey PVC plates were positioned between the different light sources, starting just above the light fixtures and penetrating into the aquarium to about 14 cm depth. This still allowed good flow water flow throughout the aquarium and around the corals (~10 cm s-1).

The corals were placed on the aquarium bottom, with no substrate other than the ceramic tiles, at a depth of 26 cm. To ensure equal light and flow regimes for all corals within each treatment, all colonies were rotated within their respective treatment once a week during the entire experimental period.

image1.jpg

Figure 1. Spectral analysis of the LED fixtures used for the experiment. Note that the corals were exposed to the same PAR regardless of spectrum, i.e. the integrals of the graphs were equal at the experimental positions in the tank (shown only approximately here).

Specific Growth Rate

To determine coral growth rates, drip-dry weights of individuals were measured at the start and end of the experiment using a JE120 emerald scale (Ohaus Corporation, Parsippany, USA). All weights were measured before and after mounting the corals on ceramic tiles, to obtain net coral weights and total weights. From these, combined epoxy resin and ceramic tile weights were calculated. At the end of the growth interval, total weights were corrected for the combined weights of the epoxy resin and ceramic tiles to obtain net coral weights. As ceramic tiles take up water, which affects total weight, all tiles were pre-incubated in 35 g L-1 artificial seawater for several weeks before the start of the experiment. In addition, all tiles were cleaned thoroughly with seawater and a brush before every measurement, to minimise the effects of biofouling on total weights. To calculate specific growth rates (SGR) for each individual, we used the following formula:

SGR = [(ln Wt) - (ln Wt -1)] * 100 / Δt

where Wt and Wt-1 are the final and initial net coral weights expressed in grams (g), respectively, and t is the growth interval in days. SGR is expressed as % growth day-1.

Zooxanthellae Density

To establish baseline zooxanthellae densities, coral microcolonies were analysed after the 4-week recovery period for both S. pistillata and P. cylindrica (N=5 per species, not shown). This left 15 fragments of both species for the experiment (see above). Five microcolonies of each species were randomly selected and ~5 mm tips were cut from their PVC plates and stored in seawater in 50 mL accordingly labelled Falcon tubes. These were subsequently transported in a Styrofoam box to the laboratory of Wageningen University and analysed on the same day. In the lab, all corals were first drip-dry weighed. The fragments were subsequently transferred to 15 mL Falcon tubes and their tissues removed by leading a jet stream of pressurized air through the tubes for 2 minutes. To prevent tissue from being ejected from the tubes, their openings were covered with a layer of parafilm through which the air nozzle was punched. Next, 10 ml of artificial seawater (ASW, 35 g L-1) was added and each tube was shaken vigorously for 3 minutes to remove all tissue from the wall of the tube and the skeleton. The coral skeleton was then removed with tweezers and the tubes centrifuged for 10 minutes at room temperature and 2,000 g. The supernatant containing the animal fraction was carefully discarded and the pellet, containing the heavier zooxanthellae was resuspended in 750 µL ASW. The total volume of each suspension was measured using a 1,000 µl pipette. Next, two homogenised subsamples of each suspension were transferred to a Fuchs-Rosenthal counting chamber with 0.2 mm depth (LO-Laboroptik Ltd, Lancing, UK) and zooxanthellae photographed under a microscope (Axioskop, Carl Zeiss BV, Breda, The Netherlands) at 100x magnification (excluding digital zoom by the camera). Subsequently, zooxanthellae concentrations in all samples were determined using ImageJ software (NIH, Bethesda, USA), based on an area of 4 x 4 large squares (equaling 1 mm2 or 0.2 µL volume at 0.2 mm depth). Finally, zooxanthellae densities for each sample were calculated by multiplying the zooxanthellae concentration by sample volume and dividing this value by the corresponding drip-dry weight of each coral fragment. Although most studies express zooxanthellae density as cells per cm2 coral, normalising zooxanthellae by mass is valid for branching corals as they have a highly constant surface/mass ratio (Tilstra et al. 2017). After the 10-week exposure to the various light treatments, zooxanthellae densities were measured again using the same protocol, but only for S. pistillata due to time constraints (N=3 per treatment, N=9 in total).

Photographic Analysis

At the end of the 10-week experiment, one coral of each treatment and species was randomly selected for close-up photography. Each coral was placed in a fixed position close to the aquarium's front panel and individually photographed with a D610 DSLR camera equipped with a Nikkor AF-D 60 mm macro lens (Nikon, Tokyo, Japan). All corals were photographed under the full spectrum white LED light used in the experiment, using the same manual camera settings, including white balance, aperture, exposure time and ISO sensitivity.

Data Analysis

Normality of coral growth and zooxanthellae data was tested by plotting residuals of each dataset versus predicted values, and by performing a Shapiro-Wilk test. Homogeneity of variances was determined using Levene's test. All data were found to be normally distributed and homoscedastic after a log10 transformation (P>0.050). We used a two-way factorial ANOVA to test the (interactive) effects of spectrum and species on specific growth rates, and a one-way ANOVA to test the effect of spectrum on zooxanthellae densities. Simple effects analysis was used to break down interactive effects. A P<0.050 value was considered statistically significant. Statistical analysis was performed with SPSS Statistics 20 (IBM, Somers, USA). Graphs were plotted with SigmaPlot 12 (Systat software, San Jose, USA).

Results

Coral Health and Colouration

During the experiment, all corals appeared healthy, with good polyp expansion. Only slight necrosis was observed at the base of one colony under the white light source (not shown). The survival rate at the end of the experiment was 100% for both species. After about four weeks, S. pistillata colonies showed conspicuous colour variations, with specimens growing under blue led appearing more purple compared to those under white light (Fig. 2). Colonies cultured under red light showed the opposite, and turned somewhat pale (Fig. 2). Blue and red light also affected the colouration of larger S. pistillata colonies, with increased and decreased purple colouration under blue and red light as compared to white light, respectively (Fig. 3).

image2.jpg

Figure 2. Representative photographs of S. pistillata and P. cylindrica microcolonies grown under white (left column), blue (middle column) and red LED light (right column) at a PAR of 300 µmol m-2 s-1 for 10 weeks, all photographed under full spectrum white LED light for comparison. Note that portions of the P. cylindrica colonies had withdrawn polyps at the moment of photographing, which should not be confused with a colour change.

image3.jpg

Figure 3. Representative photographs of S. pistillata colonies grown under blue (left) and red light (right) at a PAR of 300 µmol m-2 s-1 for 10 weeks, both photographed under full spectrum white LED light for comparison.

Specific Growth Rate

After several weeks, S. pistillata microcolonies started forming horizontal outgrowths around their base, in contrast to those growing under white or red light (Fig. 2). P. cylindrica microcolonies produced horizontal outgrowths under all light sources (Fig. 2).

After 10 weeks of culture, S. pistillata showed a specific growth rate of 1.00% to 1.27% (Fig. 4). For P. cylindrica, this range was 1.09 % to 1.56% (Fig. 4). No main effect of spectrum or species on coral growth rates was found. Thus, generally, the corals grew at comparable rates under the three light spectra provided. However, an interactive effect of spectrum and species was found (Table 1). This was reflected by a higher growth of P. cylindrica compared to S. pistillata under white light only (F1,23=9.38, P=0.006, simple effects contrast). Under blue or red light, no growth difference between the two species was found (F1,23=1.29, P=0.268 and F1,23=0.07, P=0.797, respectively).

<0.010)."><0.010).">image4.jpg

Figure 4. Specific growth rates of S. pistillata and P. cylindrica microcolonies grown under white, blue and red LED light at 300 µmol m-2 s-1 for 10 weeks. Values are means + s.d. (N=5). **Indicates significant difference (P<0.010).

Zooxanthellae Density

After 10 weeks of culture, zooxanthellae densities of S. pistillata were found to be in the range of 1.64 * 106 to 2.80 * 106 cells per gram coral (Fig. 5). A significant effect of spectrum on zooxanthellae densities of S. pistillata was found (Table 1). Specifically, corals cultured under red light had significantly lower zooxanthellae densities compared to corals grown under blue light, with a 40% loss (P=0.033, Bonferroni).

<0.050)."><0.050).">image5.jpg

Figure 5. Zooxanthellae densities of S. pistillata microcolonies grown under white, blue and red LED light at 300 µmol m-2 s-1 for 10 weeks. Values are means + s.d. (N=3). *Indicates significant difference (P<0.050).

Table 1. Two-way factorial ANOVA demonstrating main and interactive effects of spectrum and species on specific growth rate of S. pistillata and P. cylindrica (N=5), and one-way ANOVA showing effect of spectrum on zooxanthellae density of S. pistillata (N=3).
Factor Variable F df P
Specific Growth Rate
Spectrum 1.23 2 0.310
Species 1.55 1 0.225
Spectrum * Species 4.55 2 0.022*
Zooxanthellae Density
Spectrum 5.23 2 0.019*
*Indicates significant effect (P<0.050)

Discussion

This study was aimed at investigating the effects of blue and red light on coral growth, colouration and zooxanthellae density. Indeed, blue and red light exhibited profound effects on the coral and its symbiotic dinoflagellates, as reported by previous studies (Kinzie et al. 1984, 1987; D'Angelo et al. 2008; Wang et al. 2008; Mass et al. 2010; Wijgerde et al. 2014).

First of all, within the experimental period (10 weeks), all corals appeared healthy with good polyp expansion. Only minimal necrosis was observed at the base of one colony under the white light source. The survival rate at the end of the experiment was 100% for both species, which is in contrast to the findings of Wijgerde et al. (2014), who reported extensive mortality of S. pistillata microcolonies cultured under red light at a comparable irradiance level and photoperiod. This discrepancy could be attributed to the high zinc value (79.70 g L1) reported in the experimental culture system of Wijgerde et al. (2014), which is known to be in the toxic range for corals (Reichelt-Brushett and Harrison 2005; Tijssen et al. 2017). The zinc level measured in the culture system used for the current study was 1.28 µg L-1 and thus below the toxic range, which could explain why both coral species appeared healthy under red light.

After about four weeks, S. pistillata colonies showed conspicuous colour variations, with specimens cultured under blue light appearing more purple compared to those grown under white light. Colonies cultured under red light showed the opposite, and turned somewhat pale. These observations are in agreement with the study of D'Angelo et al. (2008), who found that blue light stimulates the production of both fluorescent proteins and non-fluorescent chromoproteins, in a dose-response relationship. Similarly, they also found that red light exposure results in the lowest production of certain fluorescent proteins and chromoproteins. More specifically, they showed that Seriatopora hystrix produces virtually no pink chromoprotein (known as shysCP562) when cultured under red light, which is in agreement with the much less pronounced purple colouration of S. pistillata colonies grown under red light in this study. Although he did not measure coral protein content, Riddle (2003) also found that Pocillopora damicornis, a relative of S. pistillata and S. hystrix, shows similar pigment changes after being illuminated with blue and red LED's for one month. It is interesting to note that at least three members of the Pocilloporidae seem to increase pink/purple chromoprotein synthesis as a response to blue light, which may in fact be a stress response; by reducing the amount of energetic blue light reaching the corals' zooxanthellae, chromoproteins may reduce physiological stress such as photosystem II damage and oxygen radical formation through photosynthesis. In the current study, the red, white and blue treatments can be regarded as having a gradient in terms of blue output (here defined as 450-495 nm), ranging from no output (red) to intermediate (82 µmol m-2 s-1, white) to high (149 µmol m-2 s-1, blue). Thus, it seems logical that this blue output gradient is reflected by a similar one in purple colouration exhibited by the S. pistillata colonies in this experiment. The purple colouration is likely due to one or more chromoproteins known as stylCP574 and spisCP560, which derive their pink to purple colour from absorbing yellow and green light and reflecting mostly blue and red light (Alieva et al. 2008). Blue light could trigger colourful pigment production in S. pistillata by causing tissue damage and subsequent induction of protective pigment synthesis, or via cryptochrome signalling (Levy et al. 2007; D'Angelo et al. 2008). Cryptochromes are blue-sensitive flavoproteins found in animals and plants, and regulate circadian rhythm, i.e. the 24-hour day night cycle, and other biological processes (Gressel 1979). In some corals, cryptochromes may also regulate annual spawning behaviour by responding to changes in moonlight intensity caused by the lunar cycle (Levy et al. 2007). Whatever the underlying functions of colourful coral pigments, the notion that their production can be controlled by blue light has practical implications for coral aquaculture as colouration is a key determinant for coral value. Aquarium hobbyists can increase coral colouration by using blue lights or full spectrum lights with high output in the blue range to enhance coral colouration, with immediate (excitement of fluorescent proteins) and long-term effects (enhanced production of fluorescent and non-fluorescent chromoproteins via blue-light signalling). Based on the work of D'Angelo et al. (2008), a PAR value of at least 700 µmol m-2 s-1 at 12 hours illumination is required to fully saturate colourful pigmentation in stony corals, although it is yet unclear whether shorter photoperiods have a similar effect. It is likely that when exclusively blue light sources are used, lower PAR levels will yield a comparable result, as it is blue light specifically which induces coral colouration. However, this would likely negatively impact coral growth rates, as calcification rates of branching corals show a positive relationship to PAR levels (Osinga et al. 2011; Wijgerde and Laterveer 2013). A strategy could be using exclusively blue light sources at high irradiance (e.g. 700 µmol m-2 s-1) to maximise growth and colouration of Acroporids and Pocilloporids. This will require sufficient water flow rates (i.e. above 10 cm s-1) to alleviate light stress by enhancing gas exchange through coral tissue (reviewed by Wijgerde 2013). At present, it remains unclear why P. cylindrica did not seem to exhibit altered pigmentation under blue or red light as compared to white light.

Under blue light, S. pistillata microcolonies developed clear horizontal outgrowths around the colony base, in contrast to those cultured under white and red light. It is unclear how blue light may promote horizontal growth, but the causal mechanism could again involve cryptochromes, although this remains pure speculation at present. Corals growing in deep waters, where blue light is dominant, are known to show morphological adaptations to low light, including horizontal plate-like growth and flattened branches (Lesser et al. 2010; Muir et al. 2015). Thus, corals could use a high relative abundance of blue light as an indicator for a low light environment, which could explain why predominantly blue light sources alter colony morphology. However, when compared to the white light source, the blue light fixture emitted more blue in absolute (149 versus 82 µmol m-2 s-1) as well as relative terms (50% versus 27%). Hence, it remains to be investigated whether lower blue irradiance levels, i.e. lower in absolute value but equally high in relative terms (50% or higher), also result in the morphological changes observed here. It is also unclear why P. cylindrica did not respond morphologically to various light spectra, as it produced horizontal outgrowths under all light conditions. Long-term experiments could reveal other morphological adaptations to light of varying spectral quality in this species.

Within the 10 weeks of the experiment, both S. pistillata and P. cylindrica displayed favourable growth rates which can be regarded as normal in aquaria (Wijgerde and Laterveer 2013). Interestingly, both species grew similarly under red as compared to white and blue light, despite a decreased zooxanthellae density measured for S. pistillata cultured under red light. This contradicts with the findings of Kinzie et al. (1984), who found reduced growth of Montipora verrucosa and Pocillopora damicornis under red light as compared to blue and white light in a similar time frame (i.e. 8 weeks). It is possible that the species in the current study did not exhibit negative effects within 10 weeks because their zooxanthellae populations were high enough to support normal calcification rates. The capture of plankton and/or detrital matter could also have compensated for a loss of autotrophic input (i.e. sugars and other organic compounds produced through photosynthesis) due to a decreased zooxanthellae population. Indeed, Anthony and Fabricius (2000) have reported that corals can temporarily shift from an autotrophic (i.e. photosynthesis) to a heterotrophic (i.e. particle capture) feeding mode. However, they also found that P. cylindrica is unable to compensate for reduced photosynthetic input by increasing its feeding rate, making this theory less likely for this species. S. pistillata, however, is a known voracious planktivore (Houlbrèque and Ferrier-Pagès 2009, pers. obs.), and may have increased its feeding rate as a response to a red light-driven decreased zooxanthellae population. Whatever the reasons for the sustained growth of S. pistillata and P. cylindrica in this experiment, it is possible that long-term exposure to red light would have further reduced zooxanthellae populations and subsequently induced lower growth rates or even mortality. A longer experimental duration, therefore, is warranted. Of note is the observation that P. cylindrica grew significantly faster under white light than S. pistillata. This could be explained by the chromatic adaptation by the former species to a shallow, full spectrum environment, in which all colours including red light are present. Indeed, Veron (2000) reports that P. cylindrica can be dominant in shallow lagoons.

The ~40% reduction in zooxanthellae density for S. pistillata microcolonies cultured under red light confirms previous studies (Kinzie et al. 1984, 1987; Wang et al. 2008; Wijgerde et al. 2014), and also helps explain why these colonies appeared somewhat pale: the absence of blue light did not only result in a loss of purple chromoprotein, but also in a loss of zooxanthellae and thus brownish photopigments.

In conclusion, these results support the view that light sources lacking emission in the blue portion of the solar spectrum do not support normal zooxanthellae growth, which may ultimately lead to growth stagnation and death of the host coral. In addition, blue light may trigger morphological adaptations to low light in some reef-building corals such as S. pistillata. A future study, which is now in an early stage of preparation, will focus on the effects of several major wavelength ranges within the solar spectrum on corals and their zooxanthellae, including UV, violet, blue, cyan, green, yellow, orange and red. Such a comprehensive study will significantly contribute to our fundamental understanding of the role these wavelengths play in regulating coral growth and (photo)physiology and could improve coral aquaculture and husbandry.

Acknowledgements

I am greatly indebted to all my private and corporate sponsors, without whom CORALAB and this study would not have been possible. Many thanks to Barry van 't Veer, Jan Harbers, Glenn Fong, Pieter Giraldo, Klaas Albert Houwaard, Shane Rijke, Alex Veldt, Johnny Veyt, Tanne Hoff, Luc Vogels, Corjan van der Kuil, Frances Wijnen, Jacqueline Koopman, Danny Geevers, Sven van den Berg, Dennis Spijkerbroer, Thijs Janzen, Rory Blauw, Twan Peeters, Oliver van Moerbeke, Arne Kruschat, Lennart Driehuis, Jon Henzen, Dinesh Hoogen Stoevenbeld, Marjan Hofmans, Armando Hofmans, Pim van Stratum, Ruud Hendriks, Robin Samyn, Wim Zwanenburg, Roel HG, Harald de Valck, Jacobus Leendert van Mourik, Michael Jie, Dietger Houben, John van Geene, Niels de Smit, Frank van Es, Mark Einerhand, Menno van de Burgt, John de Rooij, Jasper van den Berg, Nico de Roo, Remko van Es, Arjen Tilstra, Christiaan Blikman, Ron, Nancy Willekus, Wesley Malcorps, Wesly Bruins, Jack Ravensbergen, Edo van Bruggen, Willem Kruizinga, Coral Publications, BlueLinked, Wageningen UR, DyMiCo Filter BV, Philips Lighting, Easy Booster, DSR Technics, Seneye, Barry van 't Veer Timmer & Onderhoudsbedrijf, Antwerpse Zee Aquarium Club (AZAC), Diebo Huisdierwereld, and my wife and family.

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