The effect of heterotrophy on photosynthesis and tissue composition of two scleractinian corals under elevated temperature

Exogenous food can increase protein levels of coral host tissue, zooxanthellae densities, chlorophyll (chl) concentrations and rates of photosynthesis and is thought to play an important role in the resilience of bleached corals. There is however no information about the effect of heterotrophy on the bleaching susceptibility of corals under elevated temperature conditions. This study investigates potential interactions between food availability, basal metabolic functions (photosynthesis and respiration), energy status (lipid concentrations), total protein concentrations and the bleaching susceptibility (loss of chl and/or zooxanthellae) of the scleractinian corals Stylophora pistillata (Esper) and Galaxea fascicularis (Linnaeus) in response to elevated temperature (daily temperature rises of 3-4 °C) over 15 days. Feeding experiments were carried out in which the corals were either fed daily with zooplankton or starved. Compared to fed corals, starvation of both species resulted in a significant decrease in daily photosynthetic oxygen evolution over time. Gross (Pg) and net (Pn) photosynthetic production of starved corals of both species between 10:00-11:00 hrs had declined by ~50% at day 15 while there were no marked changes in Pg and Pn of fed corals. After 15 days, starved S. pistillata contained significantly lower zooxanthellae densities, lipid and protein concentrations than fed corals. Starved G. fascicularis also displayed a decrease in zooxantllae densities which was accompanied by a significant decline in algal chl concentrations. Contrary to S. pistillata, feeding treatment had no effect on the lipid concentrations of G. fascicularis. Total protein concentrations however were significantly lower in straved than in fed G. fascicularis. Furthermore, starvation resulted in a significant decrease in respiration of S. pistillata during the last four days of the experiment while treatment had no effect on the respiration rates of G. fascicularis. Overall the oxygen consumption of S. pistillata of both treatments was about 39-67% higher than the respiration of G. fascicularis indicating that low metabolic rates may have allowed starved G. fascicularis to conserve energy reserves over the course of the experiment. The combined results reveal a strong positive relationship between food availability, sustained photosynthetic activity and reduced loss in pigmentation of both species under elevated temperature conditions.     

Differential susceptibility to oxidative stress of two scleractinian corals: antioxidant functioning of mycosporine-glycine

This study examined the importance of mycosporine-glycine (Myc-Gly) as a functional antioxidant in the thermal-stress susceptibility of two scleractinian corals, Platygyra ryukyuensis and Stylophora pistillata. Photochemical efficiency of PSII (F_v/F_m), activity of antioxidant enzymes, superoxide dismutase (SOD) and catalase (CAT), and composition and abundance of mycosporine-like amino acids (MAAs) in the coral tissue and in symbiotic zooxanthellae were analyzed during 12-h exposure to high temperature (33 °C). After 6- and 12-h exposures at 33 °C, S. pistillata showed a significantly more pronounced decline in F_v/F_m compared to P. ryukyuensis. A 6-h exposure at 33 °C induced a significant increase in the activities of SOD and CAT in both host and zooxanthellae components of S. pistillata while in P. ryukyuensis a significant increase was observed only in the CAT activity of zooxanthellae. After 12-h exposure, the SOD activity of P. ryukyuensis was unaffected in the coral tissue but slightly increased in zooxanthellae, whereas the CAT activity in the coral tissue showed a 2.5-fold increase. The total activity of antioxidant enzymes was significantly higher in S. pistillata than in P. ryukyuensis, suggesting that P. ryukyuensis is less sensitive to oxidative stress than S. pistillata. This differential susceptibility of the corals is consistent with a 20-fold higher initial concentration of Myc-Gly in P. ryukyuensis compared to S. pistillata. In the coral tissue and zooxanthellae of both species investigated, the first 6 h of exposure to thermal stress induced a pronounced reduction in the abundance of Myc-Gly but not in other MAAs. When exposure was prolonged to 12 h, the Myc-Gly pool continued to decrease in P. ryukyuensis and was completely depleted in S. pistillata. The delay in the onset of oxidative stress in P. ryukyuensis and the dramatic increase in the activities of the antioxidant enzymes in S. pistillata, which contains low concentrations of Myc-Gly suggest that Myc-Gly provides rapid protection against oxidative stress before the antioxidant enzymes are induced. These findings strongly suggest that Myc-Gly is functioning as a biological antioxidant in the coral tissue and zooxanthellae and demonstrate its importance in the survival of reef-building corals under thermal stress.     

Engineering of coral reef larval supply through transplantation of nursery-farmed gravid colonies

The continuous worldwide degradation of coral reefs raises an urgent need for novel active restoration techniques as traditional conservation practices have failed to impede the incessant reefs' decline. While applying the “gardening coral reefs” methodology in Eilat (Red Sea, Israel), we examined reproductive outputs of naturally-grown and outplanted, nursery-farmed Stylophora pistillata colonies from three coral-transplantation trials (November 2005, May 2007, and September 2008), along three reproductive seasons. Surprisingly, transplanted colonies showed better reproductive capacities than the natal Stylophora colonies during > 4 post-transplantation years. A higher percentage of nursery-farmed colonies released planula larvae as compared to naturally-grown colonies. Gravid transplants also shed more planulae per colony, yielding significantly augmented numbers of total planulae over naturally developed S. pistillata colonies. Our results indicate that nursery-grown corals may be used to enhance reef resilience by contributing to the larval pool, forming an engineered larval dispersal instrument for reef rehabilitation.     

Urea uptake by the scleractinian coral Stylophora pistillata

Urea can be one of the major sources of nitrogen for phytoplankton, but little is known about its importance for corals. Experiments were therefore designed to assess the uptake rates of urea by the scleractinian coral Stylophora pistillata; ¹⁵N-urea was used to follow the incorporation of nitrogen into the zooxanthellae and animal tissue. The uptake kinetics of urea in the tissue of S. pistillata showed that there is a concentration-dependent uptake of urea. The transport of urea was composed of a linear component (diffusion) at concentrations higher than 6 μmol N-urea l^− 1 and an active carrier-mediated component, at lower concentrations. The value of the carrier affinity (K_m = 1.05 μmol urea l^− 1) indicates a good adaptation of the corals to low levels of urea in seawater. At the in situ concentration of ca. 0.2 μmol N-urea l^− 1, the uptake rate was equal to 0.1 nmol N h^− 1 cm^− 2. Urea uptake was at least four times higher in the animal than in the algal fraction, and five times higher when corals were incubated in the light than in the dark. These results could be explained by the involvement of urea in the calcification process, which is also enhanced by light. Comparison of urea uptake rates with nitrate or ammonium uptake rates for the same S. pistillata species, at in situ concentrations, showed that urea is preferred to nitrate and may therefore be an important source of nitrogen for scleractinian corals.     

Host-symbiont specificity during onset of symbiosis between the dinoflagellates Symbiodinium spp. and planula larvae of the scleractinian coral Fungia scutaria

Many corals which engage in symbioses with dinoflagellates from the genus Symbiodinium (zooxanthellae) produce offspring which initially lack zooxanthellae. These species must choose their symbionts from numerous genetically distinct strains of zooxanthellae co-occurring in the environment. In most cases, symbiosis onset results in an association between a specific host coral and a specific strain of algal symbiont. This is the first study to examine host-symbiont specificity during symbiosis onset in a larval cnidarian, and the first to examine such events in a scleractinian of any life stage. We infected planula larvae of the solitary Hawaiian scleractinian Fungia scutaria with both homologous zooxanthellae, freshly isolated from F. scutaria adults, and heterologous zooxanthellae, isolated from Montipora verrucosa, Porites compressa, and Pocillopora damicornis, three species of scleractinians which co-occur with F. scutaria. We found that homologous zooxanthellae were better able to establish symbioses with larval hosts than were heterologous isolates, by two separate measures: percent of a larval population infected, and densities of zooxanthellae per larva. We also measured algal densities in larvae over a 4-day period until the onset of settlement and metamorphosis. We found no changes in zooxanthella population densities, regardless of zooxanthella type or the light environment in which they were incubated. Strong infection of host larvae with homologous algae compared to heterologous algae suggests that there is a specificity process which occurs sometime during the early stages of infection between the partners, and which results in the establishment of a specific symbiosis.     

Inhibition of Calvin–Benson cycle suppresses the repair of photosystem II in Symbiodinium: implications for coral bleaching

Inhibition of Calvin–Benson cycle (CBC) activity by thermal stress has been hypothesized to cause photoinhibition of photosystem II (PSII) in zooxanthellae of reef-building corals and consequently lead to bleaching. This study tests whether the interruption of CBC by glycolaldehyde (GA) leads to photoinhibition and subsequent coral bleaching in Stylophora pistillata. When S. pistillata was incubated with GA, the O₂ evolution rate declined in a dose-dependent manner and the extent of photoinhibition, reflected by a decreased maximum quantum yield of PSII (F _v/F _m), was enhanced. The effect of GA on photoinhibition was similar to that of chloramphenicol (CAP), an inhibitor of protein synthesis in chloroplasts. When S. pistillata was incubated in weak light following a high-light-induced photoinhibitory treatment, the recovery of PSII from photoinhibition was suppressed in a similar manner to both GA- and CAP-treated samples. After incubation in moderate light at 26°C, S. pistillata showed a bleaching response only in presence of GA. These results suggest that coral bleaching-like responses are caused by interruption of the CBC activity in S. pistillata and are associated with accelerated photoinhibition through suppression of the protein synthesis-dependent repair of PSII but not to an increase in photodamage to PSII.     

Settlement induction of Acropora palmata planulae by a GLW-amide neuropeptide

Complex environmental cues dictate the settlement of coral planulae in situ; however, simple artificial cues may be all that is required to induce settlement of ex situ larval cultures for reef re-seeding and restoration projects. Neuropeptides that transmit settlement signals and initiate the metamorphic cascade have been isolated from hydrozoan taxa and shown to induce metamorphosis of reef-building Acropora spp. in the Indo-Pacific, providing a reliable and efficient settlement cue. Here, the metamorphic activity of six GLW-amide cnidarian neuropeptides was tested on larvae of the Caribbean corals Acropora palmata, Montastraea faveolata and Favia fragum. A. palmata planulae were induced to settle by the exogenous application of the neuropeptide Hym-248 (concentrations ≥1 × 10⁻⁶ M), achieving 40–80% attachment and 100% metamorphosis of competent planulae (≥6 days post-fertilization) during two spawning seasons; the remaining neuropeptides exhibited no activity. Hym-248 exposure rapidly altered larval swimming behavior (<1 h) and resulted in >96% metamorphosis after 6 h. In contrast, M. faveolata and F. fragum planulae did not respond to any GLW-amides tested, suggesting a high specificity of neuropeptide activators on lower taxonomic scales in corals. Subsequent experiments for A. palmata revealed that (1) the presence of a biofilm did not enhance attachment efficiency when coupled with Hym-248 treatment, (2) neuropeptide-induced settlement had no negative effects on early life-history developmental processes: zooxanthellae acquisition and skeletal secretion occurred within 12 days, colonial growth occurred within 36 days, and (3) Hym-248 solutions maintained metamorphic activity following storage at room temperature (10 days), indicating its utility in remote field settings. These results corroborate previous studies on Indo-Pacific Acropora spp. and extend the known metamorphic activity of Hym-248 to Caribbean acroporids. Hym-248 allows for directed and reliable settlement of larval cultures and has broad applications to the study and rehabilitation of threatened Acropora populations in the Caribbean.     

Cloning and Use of a Coral 36B4 Gene to Study the Differential Expression of Coral Genes Between Light and Dark Conditions

This paper aims to validate reference genes for gene expression studies between light and dark conditions in the scleractinian coral Stylophora pistillata for future gene expression studies of the “light-enhanced calcification” phenomenon. For this purpose, we cloned, sequenced, and characterized a candidate reference gene, the 36B4 gene from the coral S. pistillata, and validated 36B4 and β-actin as reference genes. To illustrate the future applications of these reference genes, we tested the dark and light expression of two photosynthetic genes (Rubisco and D1 protein of the photosystem II) and two genes encoding proteins involved in calcium transport for coral calcification (a calcium ATPase and a calcium channel). Results show that both photosynthetic genes are enhanced during the light when standardized against 36B4 and β-actin, whereas the two genes encoding proteins involved in calcium transport are not differentially expressed between light and dark conditions. The characterization of a coral 36B4 and the establishment of such valid reference genes will be useful for future gene expression studies between diverse conditions (aposymbiotic/symbiotic, stress/control, light/dark conditions) in scleractinian corals. Nucleotide sequence of the coral 36B4 gene cloned in this study is available in the Genbank database under the accession number EU069460.     

Effects of salinity and temperature on the recruitment of Aurelia coerulea planulae

The recruitment of scyphomedusae planulae to the benthic polyp stage is important for population size and may be affected under projected climate change scenarios. In a laboratory study, we determined the combined effects of elevated temperature and reduced salinity on the behaviour, survival and settlement of Aurelia coerulea planulae. Three temperature levels (21, 24 and 27°C) and two salinity levels (31 and 22) were used. Reduced salinity had a significant negative effect on the swimming behaviour and settlement of A. coerulea planulae. The planulae moved quickly and preferred to settle under ambient salinity conditions. The settlement rate of A. coerulea planulae was high during the current ambient summer temperature (24°C), and elevated temperature increased the mortality rate and reduced their settlement rate. A. coerulea planulae were significantly smaller under the combined conditions of elevated temperature and reduced salinity. Our study provides information on the response of A. coerulea planulae to temperature and salinity, which is helpful for understanding how environmental factors will influence the recruitment dynamics of A. coerulea.     

Experimental assessment of the feeding effort of three scleractinian coral species during a thermal stress: Effect on the rates of photosynthesis

This study aimed at investigating changes in feeding rates of three scleractinian coral species (Stylophora pistillata, Turbinaria reniformis and Galaxea fascicularis) between control (26 °C) and short-term stress conditions (31 °C), and to assess the effect of feeding on the photosynthetic efficiency of the corals. Feeding rates varied according to the feeding effort of the corals, itself depending on the environmental conditions. Indeed, S. pistillata significantly decreased its feeding rates at 31 °C, while rates of T. reniformis and G. fascicularis were increased between 26 and 31 °C. Independently of the feeding rates, food supply helped in preventing damage to the photosynthetic apparatus of the zooxanthellae. Indeed, starved corals from the three species showed significant decrease in both the electron transport rates and in the photosynthetic rates, following a loss in the amount of chlorophyll and experiencing photoinhibition of the photosystem II. However, no bleaching was observed in heated fed corals, with no decrease in their photosynthetic efficiency or performance.     

The rôle of zooxanthellae in the hermatypic coral Plesiastrea urvillei (Milne Edwards and Haime) From cold waters

The presence of zooxanthellae in tissues of the cold-temperate water coral Plesiastrea urvillei (Milne Edwards and Haime) has been confirmed histologically. Numbers of zooxanthellae per unit surface area and increases in submerged wet weight as a measure of calcification have been followed for 150 days under four different conditions: light-fed, light-starved, dark-fed, and dark-starved. No significant difference was found in density of zooxanthellae or calcification rates between light-fed and light-starved, and between dark-fed and dark-starved. After Day 48 the calcification rate in the dark dropped, however, by a factor of ≈4 to a constant lower rate and was correlated with a decrease in density of zooxanthellae. Zooxanthellae thus enhance calcification about 4 times during photosynthesis. Measurements of oxygen consumption and production indicated that even at the low light intensities experienced on a cloudy winter day by the coral in its natural environment more energy was fixed during photosynthesis than was required by the host. The retention of zooxanthellae and continued calcification in the dark for upwards of 48 days is considered to be an adaptation to the lower light levels experienced by P. urvillei compared with tropical corals.     

Settlement-competency period of planulae and genetic differentiation of the scleractinian coral Acropora digitifera

Broadcast-spawning corals expel eggs and sperm, and the fertilized eggs develop into planulae in the water column. As these sessile corals generally disperse during the planktonic larval stage, their larval characteristics (e.g., survival and settlement rates) are thought to be important for their dispersal. Although some studies of coral larval dispersal have focused on the maximum time that larvae can remain viable and settle, the relevance of this maximum settlement competency period for long distance dispersal remains unclear. To examine the relationship between competency periods and genetic differentiation, we performed laboratory experiments to investigate settlement rates of planulae and determine the degree of genetic differentiation in Acropora digitifera in the Ryukyu Archipelago, southern Japan. In addition, we compared our findings to published data for A. tenuis, which was studied using our methods. Our results indicated that the maximum settlement competency period was lower in A. digitifera planulae (54 days) than in A. tenuis (69 days) planulae. The mean survival rates at 45 days and 59 days after spawning were less than 10%. Furthermore, percentages of planulae that remained viable and settle at 30 days after spawning (survival rate x settlement rates at 30 days) were approximately 18% and 25% in A. digitifera and A. tenuis, respectively. By contrast, gene flow (N(e)m: number of migrations per generation) was significantly higher in A. digitifera (7.8 to 41.4) than in A. tenuis (3.1 to 22.5). These results indicate that the settlement competency period and survival rates are unlikely to be robust predictors of gene flow. Overall, we detected significant genetic differentiation between Kerama and Okinawa in A. digitifera. As direct observation of planula dispersal between Kerama and Okinawa has been reported, we concluded that genetic mixing is not complete, but that some localized planulae may disperse from Kerama to Okinawa via a specific current depending on reef or locality.     

Bioaccumulation of zinc in the scleractinian coral Stylophora pistillata

The uptake kinetics of zinc (Zn), an essential nutrient for both photosynthesis and calcification, in the tissue of S. pistillata showed that the transport of Zn is composed of a linear component (diffusion) at high concentrations and an active carrier-mediated component at low concentrations. The carrier affinity (K _m=28 pmol l⁻¹) was very low, indicating a good adaptation of the corals to low levels of Zn in seawater. Zn accumulation in the skeleton was linear; its level was dependent on the length of the incubation as well as on the external concentration of dissolved Zn. There was also a light-stimulation of Zn uptake, suggesting that zooxanthellae, through photosynthesis, are involved in this process. An enrichment of the incubation medium with 10 nM Zn significantly increased the photosynthetic efficiency of S. pistillata. This result suggests that corals living in oligotrophic waters might be limited in essential metals, such as zinc.     

Reef waters stimulate substratum exploration in planulae from brooding Caribbean corals

This study tested the hypothesis that waters surrounding reefs with healthy coral populations are more likely than degraded sites to induce planulae to navigate downward and begin benthic probing. In the laboratory, larvae from two brooding Caribbean coral species, Agaricia tenuifolia and Porites astreoides, were introduced to seawater collected at (1) 1 m above shallow, healthy reef with high-coral cover, (2) 1 m above shallow, degraded reef with high-macroalgal cover, and (3) ~400 m ocean-ward of the reef in deep, blue water. Counter to the hypothesis, water from both the healthy and degraded reef caused the larvae to swim downward and begin benthic probing. These results suggest that substances carried in reef waters may contribute to macro-scale habitat selection by planulae and that understanding how these waterborne cues mesh with other stimuli used by planulae to select a settlement site may be valuable for deciphering a site’s recruitment potential for corals.     

Long-term heightened larval production in nursery-bred coral transplants

The ‘gardening coral reefs’ method is part of the approaches proposed for counteracting the substantial impacts of global climate change on the survival of coral reefs. It incorporates ecosystem engineering strategies for coral nursery farming and coral colonies out-planting. This study explores the reproductive output of three sets of nursery-grown Stylophora pistillata colonies along eight reproductive seasons following transplantation, as compared to that of native corals. When native and transplanted corals grew side by side in a disturbed environment, the nursery-grown transplants showed enhanced larval release (2.6–22.5 times more planulae/colony; multiyear average: 11.6±1.8 planulae/transplant vs. 1.5±0.3 planulae/native colony) with higher percentages of gravid colonies (91±2.1% transplants vs. 34±7.6% native colonies). The inherently enhanced larval production of transplants, maintained for such a long period of time post-transplantation, reveals a possible enduring impact of the nursery conditions on future fitness and ecological traits of transplants. This is further supported by the emerging documentation regarding the enhanced growth of corals under nursery conditions, which continues to be detected even years after transplantation was conducted on the natural reef. The above enhancement of coral reproduction can be harnessed as a human intervention tool for countering global climate change impacts.     

Latitudinal variation in coral communities in eastern Australia: a qualitative biophysical model of factors regulating coral reefs

The processes underlying the distributional limits of both corals and coral reefs can be elucidated by examining coral communities at high latitudes. Coral-dominated communities in eastern Australia cover a latitudinal range of >2,500 km, from the northern Great Barrier Reef (11°S) to South West Rocks (31.5°S). Patterns of coral species richness from 11 locations showed a clear separation between the Great Barrier Reef and subtropical sites, with a further abrupt change at around 31°S. Differences in community structure between the Great Barrier Reef and more southern sites were mainly attributable to higher cover of massive corals, branching Acropora, dead coral and coralline algae on the Great Barrier Reef, and higher cover of macroalgae and bare rock at more southern sites. The absence of some major reef-building taxa (i.e., staghorn Acropora and massive Porites) from most subtropical sites coincided with the loss of reef accretion capacity. Despite high cover of hard corals in communities at up to 31°S, only Lord Howe Island contained areas of reef accretion south of the Great Barrier Reef. Factors that have been hypothesized to account for latitudinal changes in coral community structure include water temperature, aragonite saturation, light availability, currents and larval dispersal, competition between corals and other biota including macroalgae, reduced coral growth rates, and failure of coral reproduction or recruitment. These factors do not operate independently of each other, and they interact in complex ways.     

A corrosive concoction: The combined effects of ocean warming and acidification on the early growth of a stony coral are multiplicative

Survival of coral planulae, and the successful settlement and healthy growth of primary polyps are critical for the dispersal of scleractinian corals and hence the recovery of degraded coral reefs. It is therefore important to explore how the warmer and more acidic oceanic conditions predicted for the future could affect these processes. This study used controlled culture to investigate the effects of a 1 °C increase in temperature and a 0.2-0.25 unit decrease in pH on the settlement and survival of planulae and the growth of primary polyps in the Tropical Eastern Pacific coral Porites panamensis. We found that primary polyp growth was reduced only marginally by more acidic seawater but the combined effect of high temperature and lowered pH caused a significant reduction in growth of primary polyps by almost a third. Elevated temperature was found to significantly reduce the amount of zooxanthellae in primary polyps, and when combined with lowered pH resulted in a significant reduction in biomass of primary polyps. However, survival and settlement of planula larvae were unaffected by increased temperature, lowered acidity or the combination of both. These results indicate that in future scenarios of increased temperature and oceanic acidity coral planulae will be able to disperse and settle successfully but primary polyp growth may be hampered. The recovery of reefs may therefore be impeded by global change even if local stressors are curbed and sufficient sources of planulae are available.     

Effect of zooplankton availability on the rates of photosynthesis, and tissue and skeletal growth in the scleractinian coral Stylophora pistillata

This work investigated the effect of light and feeding on tissue composition as well as on rates of photosynthesis and calcification in the zooxanthellae (zoox) scleractinian coral, Stylophora pistillata. Microcolonies were maintained at three different light levels (80, 200, 300 μmol m⁻² s⁻¹) and subjected to two feeding regimes (starved and fed) over 9 weeks. Corals were fed both natural plankton and Artemia salina nauplii four times a weeks and samplings were made after 2, 5, and 9 weeks. Results confirmed that feeding enhances coral growth rate and increases both the dark and light calcification rates. These rates were 50-75% higher in fed corals (FC; 60±20 and 200±40 nmol Ca²⁺ cm⁻² h⁻¹ for dark and light calcification, respectively) compared to control corals (CC; 30±9 and 124±23 nmol Ca²⁺ cm⁻² h⁻¹). The dark calcification rates, however, were four times lower than the rates of light calcification (independent of trophic status). After 5 weeks, chlorophyll a (chl-a) concentrations were four to seven times higher in fed corals (7-21 μg cm⁻²) than in control corals (2-5 μg cm⁻²). The amount of protein was also significantly higher in fed corals (2.11-2.50 mg cm⁻²) than in control corals (1.08-1.52 mg cm⁻²). Rates of photosynthesis in fed corals were 2-10 times higher (1.24±0.75 μmol O₂ h⁻¹ cm⁻²) than those measured in control corals (0.20±0.08 μmol O₂ h⁻¹ cm⁻²).     

Survival and settlement success of coral planulae: independent and synergistic effects of macroalgae and microbes

Restoration of degraded coral reef communities is dependent on successful recruitment and survival of new coral planulae. Degraded reefs are often characterized by high cover of fleshy algae and high microbial densities, complemented by low abundance of coral and coral recruits. Here, we investigated how the presence and abundance of macroalgae and microbes affected recruitment success of a common Hawaiian coral. We found that the presence of algae reduced survivorship and settlement success of planulae. With the addition of the broad-spectrum antibiotic, ampicillin, these negative effects were reversed, suggesting that algae indirectly cause planular mortality by enhancing microbial concentrations or by weakening the coral’s resistance to microbial infections. Algae further reduced recruitment success of corals as planulae preferentially settled on algal surfaces, but later suffered 100% mortality. In contrast to survival, settlement was unsuccessful in treatments containing antibiotics, suggesting that benthic microbes may be necessary to induce settlement. These experiments highlight potential complex interactions that govern the relationships between microbes, algae and corals and emphasize the importance of microbial dynamics in coral reef ecology and restoration. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Contrasting Light Spectra Constrain the Macro and Microstructures of Scleractinian Corals

The morphological plasticity of scleractinian corals can be influenced by numerous factors in their natural environment. However, it is difficult to identify in situ the relative influence of a single biotic or abiotic factor, due to potential interactions between them. Light is considered as a major factor affecting coral skeleton morphology, due to their symbiotic relation with photosynthetic zooxanthellae. Nonetheless, most studies addressing the importance of light on coral morphological plasticity have focused on photosynthetically active radiation (PAR) intensity, with the effect of light spectra remaining largely unknown. The present study evaluated how different light spectra affect the skeleton macro- and microstructures in two coral species (Acropora formosa sensu Veron (2000) and Stylophora pistillata) maintained under controlled laboratory conditions. We tested the effect of three light treatments with the same PAR but with a distinct spectral emission: 1) T5 fluorescent lamps with blue emission; 2) Light Emitting Diodes (LED) with predominantly blue emission; and 3) Light Emitting Plasma (LEP) with full spectra emission. To exclude potential bias generated by genetic variability, the experiment was performed with clonal fragments for both species. After 6 months of experiment, it was possible to detect in coral fragments of both species exposed to different light spectra significant differences in morphometry (e.g., distance among corallites, corallite diameter, and theca thickness), as well as in the organization of their skeleton microstructure. The variability found in the skeleton macro- and microstructures of clonal organisms points to the potential pitfalls associated with the exclusive use of morphometry on coral taxonomy. Moreover, the identification of a single factor influencing the morphology of coral skeletons is relevant for coral aquaculture and can allow the optimization of reef restoration efforts.