Photobehavior of stony corals: responses to light spectra and intensity

Corals nitrogen and carbon isotopic signatures alters under Artificial Light at Night (ALAN)

This study examines the impact of Artificial Light at Night (ALAN) on two coral species, Acropora eurystoma and Pocillopora damicornis, in the Gulf of Aqaba/Eilat Red Sea, assessing their natural isotopic responses to highlight changes in energy and nutrient sourcing due to sensory light pollution. Our findings indicate significant disturbances in photosynthetic processes in Acropora eurystoma, as evidenced by shifts in δ13C values under ALAN, pointing to alterations in carbon distribution or utilization. In Pocillopora damicornis, similar trends were observed, with changes in δ13C and δ15N values suggesting a disruption in its nitrogen cycle and feeding strategies. The study also uncovers species-specific variations in heterotrophic feeding, a crucial factor in coral resilience under environmental stress, contributing to the corals' fixed carbon budget. Light measurements across the Gulf demonstrated a gradient of light pollution which possess the potential of affecting marine biology in the region. ALAN was found to disrupt natural diurnal tentacle behaviors in both coral species, crucial for prey capture and nutrient acquisition, thereby impacting their isotopic composition and health. Echoing previous research, our study underscores the need to consider each species' ecological and physiological contexts when assessing the impacts of anthropogenic changes. The findings offer important insights into the complexities of marine ecosystems under environmental stress and highlight the urgency of developing effective mitigation strategies.

Combined toxicity of microplastics and copper on Goniopora columns

As microplastics (MP) become ubiquitous, their interactions with heavy metals threatens the coral ecosystem. This study aimed to assess the combined toxicity of MP and copper (Cu) in the environment of coral. Goniopora columna was exposed to polyethylene microplastics (PE-MP) combined with Cu2+ at 10, 20, 50, 100, and 300 μg/L for 7 days. Polyp length and adaptability were recorded daily, and coral samples were collected at 1, 3, 5, and 7 days to analyse zooxanthellae density and antioxidant activity. Tissue observations and the analysis of MP and Cu2+ accumulation were conducted on the 7th day. After 1 day of exposure, PE-MP combined with different concentrations of Cu2+ significantly decreased polyp length and adaptability compared with PE-MP alone. Simultaneously, a significant increase in malondialdehyde (MDA) content, lead to coral oxidative stress, which was a combined effect with PE-MP. After 3 days of exposure, PE-MP combined with Cu2+ at >50 μg/L significantly reduced zooxanthellae density, damaging the coral's symbiotic relationship. In antioxidant enzyme activity, superoxide dismutase (SOD) activity decreased significantly after 1 day of exposure. After 3 days of exposure, glutathione peroxidase (GPx) activity significantly increased with Cu2+ at >20 μg/L. After 5 days of exposure, PE-MP combined with different concentrations of Cu2+ significantly reduced catalase (CAT), glutathione (GSH), and glutathione transferase (GST) activity, disrupting the antioxidant enzyme system, and acting antagonistically to PE-MP alone. Tissue observations revealed that the PE-MP combined with Cu2+ at >50 μg/L caused severe mesenteric atrophy, vacuolar, and Cu2+ accumulation in the coral mesenteric compared with PE-MP alone. The results suggest that combined exposure of PE-MP and copper leads to more severe oxidative stress, disruption antioxidant enzyme system, tissue damage, and Cu2+ accumulation, resulting in a significant maladaptation of corals to the environment.

Negative phototaxis in the photosymbiotic sea anemone Aiptasia as a potential strategy to protect symbionts from photodamage

Photosymbiotic cnidarians generally seek bright environments so that their symbionts can be photosynthetically active. However, excess light may result in a breakdown of symbiosis due to the accumulation of photodamage in symbionts causing symbiont loss (bleaching). It is currently unknown if photosymbiotic cnidarians sense light only to regulate spawning time and to facilitate predation, or whether they also use their light-sensing capacities to protect their symbionts from photodamage. In this study, we examined how the sea anemone Aiptasia changes its behaviour when exposed to excess light. We reveal that Aiptasia polyps, when carrying symbionts, contract their bodies when exposed to high light intensities and subsequently migrate away in a direction perpendicular to the light source. Interestingly, this negative phototaxis was only evident under blue light and absent upon UV, green and red light exposure. Non-symbiotic Aiptasia did not exhibit this light response. Our study demonstrates that photosymbiotic Aiptasia polyps display negative phototactic behaviour in response to blue light, and that they also can perceive its direction, despite lacking specialized eye structures. We postulate that Aiptasia uses blue light, which penetrates seawater efficiently, as a general proxy for sunlight exposure to protect its symbionts from photodamage.

Nematostella vectensis exemplifies the exceptional expansion and diversity of opsins in the eyeless Hexacorallia

BACKGROUND

Opsins are the primary proteins responsible for light detection in animals. Cnidarians (jellyfish, sea anemones, corals) have diverse visual systems that have evolved in parallel with bilaterians (squid, flies, fish) for hundreds of millions of years. Medusozoans (e.g., jellyfish, hydroids) have evolved eyes multiple times, each time independently incorporating distinct opsin orthologs. Anthozoans (e.g., corals, sea anemones,) have diverse light-mediated behaviors and, despite being eyeless, exhibit more extensive opsin duplications than medusozoans. To better understand the evolution of photosensitivity in animals without eyes, we increased anthozoan representation in the phylogeny of animal opsins and investigated the large but poorly characterized opsin family in the sea anemone Nematostella vectensis.

RESULTS

We analyzed genomic and transcriptomic data from 16 species of cnidarians to generate a large opsin phylogeny (708 sequences) with the largest sampling of anthozoan sequences to date. We identified 29 opsins from N. vectensis (NvOpsins) with high confidence, using transcriptomic and genomic datasets. We found that lineage-specific opsin duplications are common across Cnidaria, with anthozoan lineages exhibiting among the highest numbers of opsins in animals. To establish putative photosensory function of NvOpsins, we identified canonically conserved protein domains and amino acid sequences essential for opsin function in other animal species. We show high sequence diversity among NvOpsins at sites important for photoreception and transduction, suggesting potentially diverse functions. We further examined the spatiotemporal expression of NvOpsins and found both dynamic expression of opsins during embryonic development and sexually dimorphic opsin expression in adults.

CONCLUSIONS

These data show that lineage-specific duplication and divergence has led to expansive diversity of opsins in eyeless cnidarians, suggesting opsins from these animals may exhibit novel biochemical functions. The variable expression patterns of opsins in N. vectensis suggest opsin gene duplications allowed for a radiation of unique sensory cell types with tissue- and stage-specific functions. This diffuse network of distinct sensory cell types could be an adaptive solution for varied sensory tasks experienced in distinct life history stages in Anthozoans.

Artificial light at night (ALAN) disrupts behavioural patterns of reef corals

Increasing levels of Artificial Light At Night (ALAN) alter the natural diel cycles of organisms at global scale. ALAN constitutes a potential threat to the light-dependent functioning of symbiotic scleractinian corals, the habit-founders of warm, shallow water reefs. Here, we show that ALAN disrupts the natural diel tentacle expansion and contraction behaviour, a key mechanism for prey capture and nutrient acquisition in corals. We exposed four symbiotic scleractinian coral species to different ALAN treatments (0.4-2.5 μmol quanta m-2 s-1). Exposure to ALAN levels of 1.2 μmol quanta m-2 s-1 and above altered the normal tentacle expansion response in diurnal species (Stylophora pistillata and Duncanopsammia axifuga). The tentacle expansion pattern of nocturnal species (Montastraea cavernosa and Lobophyllia hemprichii) was less affected, which may indicate a greater capacity to tolerate ALAN exposure. The results of this work suggest that ALAN has the potential to affect nutrient acquisition mechanisms of symbiotic corals which may in turn result in changes in the coral community structure in shallow water reefs in ALAN-exposed areas.

Transfer of knowledge from model organisms to evolutionarily distant non-model organisms: The coral Pocillopora damicornis membrane signaling receptome

With the ease of gene sequencing and the technology available to study and manipulate non-model organisms, the extension of the methodological toolbox required to translate our understanding of model organisms to non-model organisms has become an urgent problem. For example, mining of large coral and their symbiont sequence data is a challenge, but also provides an opportunity for understanding functionality and evolution of these and other non-model organisms. Much more information than for any other eukaryotic species is available for humans, especially related to signal transduction and diseases. However, the coral cnidarian host and human have diverged over 700 million years ago and homologies between proteins in the two species are therefore often in the gray zone, or at least often undetectable with traditional BLAST searches. We introduce a two-stage approach to identifying putative coral homologues of human proteins. First, through remote homology detection using Hidden Markov Models, we identify candidate human homologues in the cnidarian genome. However, for many proteins, the human genome alone contains multiple family members with similar or even more divergence in sequence. In the second stage, therefore, we filter the remote homology results based on the functional and structural plausibility of each coral candidate, shortlisting the coral proteins likely to have conserved some of the functions of the human proteins. We demonstrate our approach with a pipeline for mapping membrane receptors in humans to membrane receptors in corals, with specific focus on the stony coral, P. damicornis. More than 1000 human membrane receptors mapped to 335 coral receptors, including 151 G protein coupled receptors (GPCRs). To validate specific sub-families, we chose opsin proteins, representative GPCRs that confer light sensitivity, and Toll-like receptors, representative non-GPCRs, which function in the immune response, and their ability to communicate with microorganisms. Through detailed structure-function analysis of their ligand-binding pockets and downstream signaling cascades, we selected those candidate remote homologues likely to carry out related functions in the corals. This pipeline may prove generally useful for other non-model organisms, such as to support the growing field of synthetic biology.

Green fluorescent protein-like pigments optimize the internal light environment in symbiotic reef building corals

Pigments homologous to the green fluorescent protein (GFP) have been proposed to fine-tune the internal light microclimate of corals, facilitating photoacclimation of photosynthetic coral symbionts (Symbiodiniaceae) to life in different reef habitats and environmental conditions. However, direct measurements of the in vivo light conditions inside the coral tissue supporting this conclusion are lacking. Here, we quantified the intra-tissue spectral light environment of corals expressing GFP-like proteins from widely different light regimes. We focus on: (1) photoconvertible red fluorescent proteins (pcRFPs), thought to enhance photosynthesis in mesophotic habitats via wavelength conversion, and (2) chromoproteins (CPs), which provide photoprotection to the symbionts in shallow water via light absorption. Optical microsensor measurements indicated that both pigment groups strongly alter the coral intra-tissue light environment. Estimates derived from light spectra measured in pcRFP-containing corals showed that fluorescence emission can contribute to >50% of orange-red light available to the photosynthetic symbionts at mesophotic depths. We further show that upregulation of pink CPs in shallow-water corals during bleaching leads to a reduction of orange light by 10–20% compared to low-CP tissue. Thus, screening by CPs has an important role in mitigating the light-enhancing effect of coral tissue scattering and skeletal reflection during bleaching. Our results provide the first experimental quantification of the importance of GFP-like proteins in fine-tuning the light microclimate of corals during photoacclimation.

Impact of polyethylene microplastics on coral Goniopora columna causing oxidative stress and histopathology damages

In recent years, the increase of microplastics in the sea exerted a negative impact on coral health. This study has been undertaken to analyze the impact of microplastics on corals. Here, Goniopora columna was exposed to different concentrations of polyethylene microplastics (PE-MP). The daily polyps length and adaptability were recorded. Analysis of the zooxanthellae density and antioxidant activity of coral was done after 1, 3, 5 and 7 days. Further tissue morphology and accumulation of PE-MP were analyzed. The results showed that PE-MP at different concentrations can be adsorbed on the surface of corals and enter inside corals after 7 days. PE-MP at different concentrations reduced polyps length, adaptability and cause the changes in the density of zooxanthellae to be the reason for unbalancing of corals. PE-MP at different concentrations reduced the superoxide dismutase (SOD) activity after exposure to 1 day. PE-MP increased the catalase (CAT) activity at 100 mg/L after exposure; even after reducing the concentration has the same effect. PE-MP at various concentrations increased the glutathione peroxidase (GPx) activity after exposure to 5 and 7 days. It also increased the glutathione transferase (GST) and glutathione (GSH) activity after exposure to 5 and 7 days. PE-MP at different concentrations increased the malondialdehyde (MDA) content after exposure from 1 to 7 days. Analysis of tissue morphology and tissue accumulation shows that different concentrations of PE-MP cause mesenteric atrophy, vacuole, and accumulation in the coral mesenteric. These results indicate that the PE-MP can impact the antioxidant system and hampers the function of enzymes responsible for detoxification of G. columna, increase lipid peroxide content and also cause tissue damage through accumulating in the coral mesenteric.

Coral fluorescence: a prey-lure in deep habitats

Fluorescence is highly prevalent in reef-building corals, nevertheless its biological role is still under ongoing debate. This feature of corals was previously suggested to primarily screen harmful radiation or facilitate coral photosynthesis. In mesophotic coral ecosystems (MCEs; 30-150 m depth) corals experience a limited, blue-shifted light environment. Consequently, in contrast to their shallow conspecifics, they might not be able to rely on photosynthates from their photosymbionts as their main energy source. Here, we experimentally test an alternative hypothesis for coral fluorescence: a prey-lure mechanism for plankton. We show that plankton exhibit preferential swimming towards green fluorescent cues and that compared to other morphs, higher predation rates are recorded in a green fluorescing morph of the mesophotic coral Euphyllia paradivisa. The evidence provided here - that plankton are actively attracted to fluorescent signals - indicates the significant role of fluorescence in amplifying the nutritional sink adjacent to coral reefs.

Study on the Development and Growth of Coral Larvae

Studies on the early development of corals are required for academic research on coral reefs and applied reef conservation, but the interval between observations is usually weeks or months. Thus, no study has comprehensively explored the development of coral larvae after settlement. This study observed Galaxea fascicularis, Mycedium elephantotus, Pocillopora verrucosa, and Seriatopora caliendrum larvae after settlement, including their growth process and the formation of tentacles, skeletons, and polyps. The G. fascicularis and M. elephantotus polyps exhibited the skeleton-over-polyp mechanism, whereas the P. verrucosa and S. caliendrum polyps exhibited the polyp-over-skeleton mechanism. During asexual reproduction, the Symbiodiniaceae species clustered on the coenosarc, resulting in polyp development and skeletal growth. M. Elephantotus was unique in that its tentacles were umbrella-shaped, and its polyp growth and Symbiodiniaceae species performance during asexual reproduction differed from those of the other three corals. Although both P. verrucosa and S. caliendrum have branching morphologies, their vertical development stages were dissimilar. S. caliendrum relied on the mutual pushing of individuals in the colony to extend upward, whereas P. verrucosa had a center individual that developed vertically. The findings of this study can serve as a reference for future research on coral breeding, growth, and health assessments.

Effects of Temperature and Salinity on Growth, Metabolism and Digestive Enzymes Synthesis of Goniopora columna

Climate change is causing dramatic changes in global ocean temperature and salinity, threatening coral survival. Coral growth and metabolism are greatly affected by the temperature, salinity and feeding time of the environment. In order to explore the threats to coral survival caused by climate change, this study will investigate the changes in body composition, digestive enzymes and metabolism of G. columna at different temperatures and salinities. A maximum G. columna growth rate was observed at 25 °C and 30−35 psu salinity. The G. columna could survive in a wide salinity range of 25−40 psu. However, the maximum number and weight of G. columna polyps was determined at 30−35 psu. Furthermore, 30−35 psu salinity at 25 °C led to the best G. columna growth and survival, mainly because of their enhanced nutrient absorption rate, polyp expansion rate, metabolic rate and adaptability. Comparing various salinity-temperature treatment groups, all obtained values for growth, behavior and metabolism were significantly higher (p < 0.05) for 30 psu at 25 °C than other treatment groups resulting in maximum G. columna yield. In addition, the optimal timing of G. columna feeding was assessed by studying changes in body composition and digestive enzymes within 24 h of feeding. The results showed that G. columna has higher protein and protease activity between 6:00 a.m. to 12:00 noon. Therefore, at 25 °C, 30−35 psu and feeding will enhance G. columna growth and survival.

The Effect of Feeding on Briareum violacea Growth, Survival and Larval Development under Temperature and Salinity Stress

In recent years, climate change has often caused fluctuations in seawater salinity and temperature, which threaten the survival and growth of corals. Effectively improving the stress response to temperature and salinity changes in corals to prevent bleaching is one of the important issues. This study initially explored the use of artificial polyunsaturated fatty acids to assess the ability of Briareum violacea to slow bleaching, enhance growth, stabilize larval development and reduce antistress factors (superoxide dismutase and catalase) when they were exposed to temperature and salinity stress. The salinities used in the experiment were 25, 30, 35 and 40 psu, and the temperatures were 20, 25 and 30 °C. It was divided into two parts: Experiment 1-Effects of temperature and salinity and feeding on digestive enzymes, reproduction and stress response of B. violacea; Experiment 2-Effects of temperature and salinity and feeding on the settlement and survival of larvae. The results showed that the feeding treatment group reduced the superoxide dismutase, catalase and mortality of corals under stress and significantly improved larval development and larval settlement.

Evaluation of Clove Extract for Drug Therapy of Ciliate Infection in Coral (Goniopora columna)

Simple Summary

In recent years, studies have found that coral infectious diseases are gradually spreading. Ciliate disease poses a serious threat to corals, and infected corals will fester and die within a short period of time. Clove is a traditional Chinese medicine. In this study, Clove extract was used to evaluate the treatment of ciliate diseases to achieve safety and to reduce the stress response of corals. Studies have shown that 1500 ppm clove extract can effectively treat ciliate parasitism, and does not affect coral zooxanthellae, chlorophyll a, or stress response. This extract has been successfully applied to a Taiwan coral king coral farm, which will have great significance for large-scale coral aquaculture.

Abstract

In recent years, ciliate infections have caused serious casualties to corals in the ocean. Infected corals die within a short period of time, which not only poses a threat to wild coral reefs, but also has a major impact on large scale aquaculture of coral. Clove is a kind of Chinese medicine with antifungal, antibacterial, antiviral, insecticidal, and other functions. Clove is a natural product. If it can be used in the treatment of coral ciliates, it will reduce this threat to the environment. The clove extract was diluted with sterile seawater to 500 ppm, 1500 ppm, 2500 ppm, 5000 ppm, 7500 ppm, and 10,000 ppm to carry out virulence test on ciliates. The results show that the LC₅₀ value is 1500 ppm, which can cause the death of ciliates in 10 min without causing significant changes in G. columna SOD, CAT, chlorophyll a, and zooxanthellae. In addition, observation of tissue slices revealed that no ciliates and vacuum were found in the G. columna tissue after 10 min of medicated bathing. In summary, 1500 ppm of clove extract can be used for the treatment of coral ciliates.

Macro- and micro-scale adaptations allow distinct Stylophora pistillata-symbiodiniaceae holobionts to optimize performance across a broad light habitat

In sessile organisms, phenotypic plasticity represents an important strategy for dealing with environmental variability. Here we test if phenotypic plasticity enables the common coral Stylophora pistillata to occupy a broad niche. We find clear differences in the photo-physiology of four putative species of photosynthetic dinoflagellate symbionts associated with the coral S. pistillata, namely, Cladocopium 'C35a', 'C79', 'C78a' and 'C8a'. Coral phenotypic responses were also tightly linked to symbiont identity. Corals with Cladocopium 'C8a' have more "open" macro-morphology compared to colonies associating with depth-restricted Cladocopium 'C35a' or 'C78a' in the same shallow water habitat. Corals with Cladocopium 'C8a' had 40 to 60% lower symbiont cell densities compared to other holobionts but were more efficient at acclimating over a range of light levels, with clear mechanisms to dissipate excess light energy. This holobiont contains host-based green fluorescent pigments, increased concentrations of symbiont-based mycosporine amino acids, and xanthophyll cycling in high light habitats. Photosynthetic efficiency was also adjusted over the light habitat. In contrast, limited micro-scale responses were observed between three depth-restricted symbionts: Cladocopium 'C79', 'C35a', and 'C78a'. To optimize light levels reaching the photosynthetic unit, these colonies rely on a more closed macro-morphology under high light levels, which reduces incident light levels by up to 43%, and higher symbiont densities . Our results show that distinct macro- and micro-scale adaptations lead to functional differences between four distinct S. pistillata holobionts, allowing them to co-exist by filling specific niches on a small, but environmentally diverse, spatial scale. Key index words: Light, Symbiodiniaceae, coral, pigments, Stylophora pistillata, ITS2, phenotypic plasticity, niche diversification.

Effects of LED Light Illumination on the Growth, Digestive Enzymes, and Photoacclimation of Goniopora Columna in Captivity

Simple Summary

Coral aquaculture is a new industry, which is of great importance to the sustainable development of coral reefs and meeting commercial needs. Light sources are crucial for the growth of corals because zooxanthellae provide them with essential nutrients through photosynthesis. Different corals and zooxanthellae have different photoacclimation characteristics; therefore, selecting a suitable light wavelength remains the critical inhibitor of coral maintenance in marine aquariums. Accordingly, this study investigated the effects of different light wavelengths and feeding of G. columna. The results showed that blue light (440–470 nm) and purple light (400–430 nm) increased the protease and body protein in corals, and the growth and survival rate also increased. In summary, G. columna’s efficient cultivation can use 400–470 nm wavelengths as the primary source of illumination.

Abstract

Goniopora columna is a stony coral valued for its reef-building potential and its unique appearance. Thus, identifying the optimal culture conditions for G. columna would enable efficient cultivation and prevent the illegal exploitation of marine resources. Light sources are crucial for the growth of corals because zooxanthellae provide them with basic nutrients through photosynthesis. Different corals and zooxanthellae have different photoacclimation characteristics; therefore, selecting a suitable light wavelength remains the key inhibitor of coral maintenance in marine aquariums. Accordingly, this study investigated the effects of different light wavelengths on G. columna. It was illuminated for 6 or 12 h a day under white light, yellow light, red light (LR), green light (LG), blue light (LB), or purple light (LP) for 8 weeks. During the experiment, R(R; i.e., a formula feed that combines sodium alginate, protein and probiotics) of 5% (w/v) of G. columna tissue and skeletal dry weight was fed every day. Coral polyps were counted, zooxanthellae density, chlorophyll a concentration, specific growth rates, and survival rates were calculated; polyp stretching and contractile behaviors were observed; and body composition and digestive enzyme activity were analyzed. LB or LP (but not LG or LR) illumination for at least 6 h per day significantly promoted the growth, survival, protein content, and protease activity of the G. columna specimens. Furthermore, coral polyp extension reached 100% after 30 min of LP and LB light irradiation. Although no significant differences in the zooxanthellae density or chlorophyll a concentration were noted under various light wavelengths, significant reductions were detected in the absence of light. To achieve energy-efficient coral aquaculture with regard to G. columna cultivation, 6 h of LB or LP illumination per day can improve the growth.

Effects of Ciliate Infection on the Activities of Two Antioxidant Enzymes (SOD and CAT) in Captive Coral (Goniopora columna) and Evaluation of Drug Therapy

Ciliate infection is a serious parasitic disease of coral. Infected coral rots and dies in a short time. In addition to killing corals by infecting them in the oceans, ciliate infection also poses a threat to corals farmed on a large scale. In this study, two antioxidant enzymes (SOD and CAT) were used to judge the stress response in Goniopora columna after infection, and KCl and H2O2 were used to evaluate the therapeutic effect. The results showed that SOD and CAT increased during the early stage of infection but decreased with the extension of infection time. In terms of drug therapy, it was found that the treatment of ciliate infection with 1.5% of KCl had no significant effect on SOD and CAT of G. columna. The morphological changes of zooxanthellae, chlorophyll a, and coral were not significant. H2O2 leads to a stress response and polyp contraction. In conclusion, 1.5% of KCl can be used in the selection of drugs to treat ciliate infection.

Feeding of a Scleractinian Coral, Goniopora columna, on Microalgae, Yeast, and Artificial Feed in Captivity

Simple Summary

Coral aquaculture is an innovative and sustainable aquaculture industry. Coral husbandry can address ecological environment conservation needs and industrial demand for corals. Many previous studies have confirmed that corals also belong to heterotrophic organisms. Heterotrophic feeding is essential for overcoming nutrient deficiency. The preliminary results of this study indicate that Goniopora columna have high levels of proteases, and artificial feeds containing high protein can be used for feeding during aquaculture, which can increase the growth rate. In conclusion, we have initially explored that Goniopora columna will have better growth by feeding artificial PUFA rich in animal protein. In addition, the best feeding time is 6:00–12:00 in the morning, when there is better digestion and absorption. It is hoped that this research will be helpful to the development of coral aquaculture in the future.

Abstract

Nutritional requirements are critical in the process of coral aquaculture. In addition to energy from symbiotic algae, corals obtain sufficient nutrition through heterotrophic feeding. Microalgae and yeast are commonly used as nutritional supplements for many aquaculture organisms. In addition, if artificial feed can match or improve upon the nutritional supplementation provided by microalgae and yeast in the case of G. columna, then feeding this coral would be markedly easier. Hence, this article preliminarily discusses feeds suitable for G. columna. In this study, artificial PUFA rich in animal protein (R), Saccharomyces cerevisiae, Isochrysis galbana tml, and Nannochloropsis oculate were fed to G. columna at quantities of 5% and 10% of body weight. Growth, survival, body composition, and digestive enzymes were assessed. Regarding body composition, the coral’s protein content is higher than that of carbohydrate or fat; thus, evaluating the heterotrophic nutrition of G. columna by using protein absorption is appropriate. The protease content is also high in digestive enzymes. Protein content, protease activity, and specific growth rate were significantly higher in the R group than in other groups. The number of polyps in the groups fed R at 5% and 10% of body weight increased by 40.00 ± 2.43 and 47.33 ± 0.89 number, respectively, significantly greater increases than those achieved in the other groups (p < 0.05). Changes in body composition and digestive enzymes over a 24-h period were compared to determine the optimal feeding time. Protein content and protease activity increased markedly between 6:00 and 12:00. The experimental results suggest that R can improve the activity of G. columna digestive enzymes and their protein and lipid content in body tissue, shorten the cultivation time, and enhance the profitability of coral aquaculture.

Multiphysics modelling of photon, mass and heat transfer in coral microenvironments

Coral reefs are constructed by calcifying coral animals that engage in a symbiosis with dinoflagellate microalgae harboured in their tissue. The symbiosis takes place in the presence of steep and dynamic gradients of light, temperature and chemical species that are affected by the structural and optical properties of the coral and their interaction with incident irradiance and water flow. Microenvironmental analyses have enabled quantification of such gradients and bulk coral tissue and skeleton optical properties, but the multi-layered nature of corals and its implications for the optical, thermal and chemical microenvironment remains to be studied in more detail. Here, we present a multiphysics modelling approach, where three-dimensional Monte Carlo simulations of the light field in a simple coral slab morphology with multiple tissue layers were used as input for modelling the heat dissipation and photosynthetic oxygen production driven by photon absorption. By coupling photon, heat and mass transfer, the model predicts light, temperature and O₂ gradients in the coral tissue and skeleton, under environmental conditions simulating, for example, tissue contraction/expansion, symbiont loss via coral bleaching or different distributions of coral host pigments. The model reveals basic structure-function mechanisms that shape the microenvironment and ecophysiology of the coral symbiosis in response to environmental change.

Experimental Techniques to Assess Coral Physiology in situ Under Global and Local Stressors: Current Approaches and Novel Insights

Coral reefs are declining worldwide due to global changes in the marine environment. The increasing frequency of massive bleaching events in the tropics is highlighting the need to better understand the stages of coral physiological responses to extreme conditions. Moreover, like many other coastal regions, coral reef ecosystems are facing additional localized anthropogenic stressors such as nutrient loading, increased turbidity, and coastal development. Different strategies have been developed to measure the health status of a damaged reef, ranging from the resolution of individual polyps to the entire coral community, but techniques for measuring coral physiology in situ are not yet widely implemented. For instance, while there are many studies of the coral holobiont response in single or limited-number multiple stressor experiments, they provide only partial insights into metabolic performance under more complex and temporally and spatially variable natural conditions. Here, we discuss the current status of coral reefs and their global and local stressors in the context of experimental techniques that measure core processes in coral metabolism (respiration, photosynthesis, and biocalcification) in situ, and their role in indicating the health status of colonies and communities. We highlight the need to improve the capability of in situ studies in order to better understand the resilience and stress response of corals under multiple global and local scale stressors.

Digital image processing to detect subtle motion in stony coral

Coral reef ecosystems support significant biological activities and harbor huge diversity, but they are facing a severe crisis driven by anthropogenic activities and climate change. An important behavioral trait of the coral holobiont is coral motion, which may play an essential role in feeding, competition, reproduction, and thus survival and fitness. Therefore, characterizing coral behavior through motion analysis will aid our understanding of basic biological and physical coral functions. However, tissue motion in the stony scleractinian corals that contribute most to coral reef construction are subtle and may be imperceptible to both the human eye and commonly used imaging techniques. Here we propose and apply a systematic approach to quantify and visualize subtle coral motion across a series of light and dark cycles in the scleractinian coral Montipora capricornis. We use digital image correlation and optical flow techniques to quantify and characterize minute coral motions under different light conditions. In addition, as a visualization tool, motion magnification algorithm magnifies coral motions in different frequencies, which explicitly displays the distinctive dynamic modes of coral movement. Specifically, our assessment of displacement, strain, optical flow, and mode shape quantify coral motion under different light conditions, and they all show that M. capricornis exhibits more active motions at night compared to day. Our approach provides an unprecedented insight into micro-scale coral movement and behavior through macro-scale digital imaging, thus offering a useful empirical toolset for the coral research community.

Long-term imaging of the photosensitive, reef-building coral Acropora muricata using light-sheet illumination

Coral reefs are in alarming decline due to climate emergency, pollution and other man-made disturbances. The numerous ecosystem services derived from coral reefs are underpinned by the growth and physical complexity of reef-forming corals. Our knowledge of their fundamental biology is limited by available technology. We need a better understanding of larval settlement and development, skeletogenesis, interactions with pathogens and symbionts, and how this biology interacts with environmental factors such as light exposure, temperature, and ocean acidification. We here focus on a fast-growing key coloniser, Acropora muricata (Linnaeus, 1758). To enable dynamic imaging of this photosensitive organism at different scales, we developed light-sheet illumination for fluorescence microscopy of small coral colonies. Our approach reveals live polyps in previously unseen detail. An imaging range for Acropora muricata with no measurable photodamage is defined based upon polyp expansion, coral tissue reaction, and photobleaching. We quantify polyp retraction as a photosensitive behavioural response and show coral tissue rupture at higher irradiance with blue light. The simple and flexible technique enables non-invasive continuous dynamic imaging of highly photosensitive organisms with sizes between 1 mm³ and 5 cm³, for eight hours, at high temporal resolution, on a scale from multiple polyps down to cellular resolution. This live imaging tool opens a new window into the dynamics of reef-building corals.

Decoupling behavioral and transcriptional responses to color in an eyeless cnidarian

Background

Animals have specific molecular, physiological, and behavioral responses to light that are influenced by wavelength and intensity. Predictable environmental changes – predominantly solar and lunar cycles – drive endogenous daily oscillations by setting internal pacemakers, otherwise known as the circadian clock. Cnidarians have been a focal group to discern the evolution of light responsiveness due to their phylogenetic position as a sister phylum to bilaterians and broad range of light-responsive behaviors and physiology. Marine species that occupy a range of depths will experience different ranges of wavelengths and light intensities, which may result in variable phenotypic responses. Here, we utilize the eyeless sea anemone Nematostella vectensis, an estuarine anemone that typically resides in shallow water habitats, to compare behavioral and molecular responses when exposed to different light conditions.

Results

Quantitative measures of locomotion clearly showed that this species responds to light in the blue and green spectral range with a circadian activity profile, in contrast to a circatidal activity profile in the red spectral range and in constant darkness. Differences in average day/night locomotion was significant in each condition, with overall peak activity during the dark period. Comparative analyses of 96 transcriptomes from individuals sampled every 4 h in each lighting treatment revealed complex differences in gene expression between colors, including in many of the genes likely involved in the cnidarian circadian clock. Transcriptional profiling showed the majority of genes are differentially expressed when comparing mid-day with mid-night, and mostly in red light. Gene expression profiles were largely unique in each color, although animals in blue and green were overall more similar to each other than to red light.

Conclusions

Together, these analyses support the hypothesis that cnidarians are sensitive to red light, and this perception results in a rich transcriptional and divergent behavioral response. Future work determining the specific molecular mechanisms driving the circadian and potential circatidal rhythms measured here would be impactful to connect gene expression variation with behavioral variation in this eyeless species.

Ciliary vortex flows and oxygen dynamics in the coral boundary layer

The exchange of metabolites between environment and coral tissue depends on the flux across the diffusive boundary layer (DBL) surrounding the tissue. Cilia covering the coral tissue have been shown to create vortices that enhance mixing in the DBL in stagnant water. To study the role of cilia under simulated ambient currents, we designed a new light-sheet microscopy based flow chamber setup. Microparticle velocimetry was combined with high-resolution oxygen profiling in the coral Porites lutea under varying current and light conditions with natural and arrested cilia beating. Cilia-generated vortices in the lower DBL mitigated extreme oxygen concentrations close to the tissue surface. Under light and arrested cilia, oxygen surplus at the tissue surface increased to 350 µM above ambient, in contrast to 25 µM under ciliary beating. Oxygen shortage in darkness decreased from 120 µM (cilia arrested) to 86 µM (cilia active) below ambient. Ciliary redistribution of oxygen had no effect on the photosynthetic efficiency of the photosymbionts and overall oxygen flux across the DBL indicating that oxygen production and consumption was not affected. We found that corals actively change their environment and suggest that ciliary flows serve predominantly as a homeostatic control mechanism which may play a crucial role in coral stress response and resilience.

Expression and Characterization of a Bright Far-red Fluorescent Protein from the Pink-Pigmented Tissues of Porites lobata

Members of the anthozoan green fluorescent protein (GFP) family display a diversity of photo-physical properties that can be associated with normal and damaged coral tissues. Poritid coral species often exhibit localized pink pigmentation in diseased or damaged tissues. Our spectral and histological analyses of pink-pigmented Porites lobata lesions show co-localization of bright red fluorescence with putative amoebocytes concentrating in the epidermis, suggesting an activated innate immune response. Here we report the cloning, expression, and characterization of a novel red fluorescent protein (plobRFP) from the pink-pigmented tissues associated with lesions on Porites lobata. In vitro, the recombinant plobRFP exhibits a distinct red emission signal of 614 nm (excitation maximum: 578 nm), making plobRFP the furthest red-shifted natural fluorescent protein isolated from a scleractinian coral. The recombinant protein has a high molar extinction coefficient (84,000 M-1 cm-1) and quantum yield (0.74), conferring a notable brightness to plobRFP. Sequence analysis suggests the distinct brightness and marked red shift may be inherent features of plobRFP's chromophore conformation. While plobRFP displays a tendency to aggregate, its high pH stability, photostability, and spectral properties make it a candidate for cell imaging applications and a potential template for engineering optimized RFPs. The association of plobRFP with a possible immune response furthers its potential use as a visual diagnostic and molecular biomarker for monitoring coral health.

A method to disentangle and quantify host anabolic turnover in photosymbiotic holobionts with subcellular resolution

A wide range of organisms host photosynthesizing symbionts. In these animals the metabolic exchange between host and symbionts has prevented in situ host anabolic turnover to be studied without the confounding effect of translocated photosynthates. Using the symbiotic coral Stylophora pistillata as a model organism and [1-¹³C]-pyruvate and [2,3-¹³C]-pyruvate in different incubation conditions (light, light + DCMU, and darkness), we employed NanoSIMS isotopic imaging to quantify host anabolism, with and without translocated metabolites from their photosynthesizing dinoflagellate symbionts. Under our experimental conditions, host de novo lipid synthesis accounted for ~40% of the total holobiont lipid reserve, and dinoflagellate recycling of metabolic ¹³CO₂ enhanced host tissue ¹³C-enrichment by 13–22% in the epidermis, 40–58% in the gastrodermis, and 135–169% in host lipid bodies. Furthermore, we show that host anabolic turnover in different tissue structures differs, in a manner consistent with the localisation, function and cellular composition of these structures.

Gibbin et al. use [1–13C]-pyruvate and [2,3–13C]-pyruvate in different incubation conditions (light, light+DCMU, and dark) to discern and quantify coral host anabolism, with and without translocated metabolites from their photosynthesizing symbionts.

Demystifying Circalunar and Diel Rhythmicity in Acropora digitifera under Constant Dim Light

Life on earth has evolved under constant environmental changes; in response to these changes, most organisms have developed an endogenous clock that allows them to anticipate daily and seasonal changes and adapt their biology accordingly. Light cycles synchronize biological rhythms and are controlled by an endogenous clock that is entrained by environmental cues. Light is known to play a key role in the biology of symbiotic corals as they exhibit many biological processes entrained by daily light patterns. In this study, we aimed at determining the effect of constant dim light on coral's perception of diel and monthly cycles. Our results show that under constant dim light corals display a loss of rhythmic processes and constant stimuli by light, which initiates signal transduction that results in an abnormal cell cycle, cell proliferation, and protein synthesis. The results emphasize how constant dim light can mask the biological clock of Acropora digitifera.

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Light entrains many biological processes governed by the endogenous clock

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Constant dim light overrides the biological clock of A. digitifera corals

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Artificial light impacts the processes that allow corals to thrive in our oceans

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The increase of artificial light in coastal areas is a growing threat to coral reefs

Sustainability of coral reefs are affected by ecological light pollution in the Gulf of Aqaba/Eilat

As human populations grow and lighting technologies improve, artificial light gradually alters natural cycles of light and dark that have been consistent over long periods of geological and evolutionary time. While considerable ecological implications of artificial light have been identified in both terrestrial and aquatic habitats, knowledge about the physiological and molecular effects of light pollution is vague. To determine if ecological light pollution (ELP) impacts coral biological processes, we characterized the transcriptome of the coral Acropora eurystoma under two different light regimes: control conditions and treatment with light at night. Here we show that corals exposed to ELP have approximately 25 times more differentially expressed genes that regulate cell cycle, cell proliferation, cell growth, protein synthesis and display changes in photo physiology. The finding of this work confirms that ELP acts as a chronic disturbance that may impact the future of coral reefs.

Electrophysiological evidence for light-activated cation transport in calcifying corals

Light has been demonstrated to enhance calcification rates in hermatypic coral species. To date, it remains unresolved whether calcifying epithelia change their ion transport activity during illumination, and whether such a process is mediated by the endosymbiotic algae or can be controlled by the coral host itself. Using a modified Ussing chamber in combination with H+ sensitive microelectrode measurements, the present work demonstrates that light triggers the generation of a skeleton positive potential of up to 0.9 mV in the hermatypic coral Stylophora pistillata. This potential is generated by a net flux of cations towards the skeleton and reaches its maximum at blue (450 nm) light. The effects of pharmacological inhibitors targeting photosynthesis 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) and anion transport 4,4'-diisothiocyano-2,2'-stilbenedisulfonic acid (DIDS) were investigated by pH microelectrode measurements in coral tissues demonstrating a rapid decrease in tissue pH under illumination. However, these inhibitors showed no effect on the electrophysiological light response of the coral host. By contrast, metabolic inhibition by cyanide and deoxyglucose reversibly inhibited the light-induced cation flux towards the skeleton. These results suggest that ion transport across coral epithelia is directly triggered by blue light, independent of photosynthetic activity of algal endosymbionts. Measurements of this very specific and quantifiable physiological response can provide parameters to identify photoreception mechanisms and will help to broaden our understanding of the mechanistic link between light stimulation and epithelial ion transport, potentially relevant for calcification in hermatypic corals.

Coral Food, Feeding, Nutrition, and Secretion: A Review

Tropical scleractinian corals are dependent to varying degrees on their photosymbiotic partners. Under normal levels of temperature and irradiance, they can provide most, but not all, of the host's nutritional requirements. Heterotrophy is required to adequately supply critical nutrients, especially nitrogen and phosphorus. Scleractinian corals are known as mesozooplankton predators, and most employ tentacle capture. The ability to trap nano- and picoplankton has been demonstrated by several coral species and appears to fulfill a substantial proportion of their daily metabolic requirements. The mechanism of capture likely involves mucociliary activity or extracoelenteric digestion, but the relative contribution of these avenues have not been evaluated. Many corals employ mesenterial filaments to procure food in various forms, but the functional morphology and chemical activities of these structures have been poorly documented. Corals are capable of acquiring nutrition from particulate and dissolved organic matter, although the degree of reliance on these sources generally has not been established. Corals, including tropical, deep- and cold-water species, are known as a major source of carbon and other nutrients for benthic communities through the secretion of mucus, despite wide variation in chemical composition. Mucus is cycled through the planktonic microbial loop, the benthos, and the microbial community within the sediments. The consensus indicates that the dissolved organic fraction of mucus usually exceeds the insoluble portion, and both serve as sources for the growth of nano- and picoplankton. As many corals employ mucus to trap food, a portion is taken back during feeding. The net gain or loss has not been evaluated, although production is generally thought to exceed consumption. The same is true for the net uptake and loss of dissolved organic matter by mucus secretion. Octocorals are thought not to employ mucus capture or mesenterial filaments during feeding and generally rely on tentacular filtration of weakly swimming mesozooplankton, particulates, dissolved organic matter, and picoplankton. Nonsymbiotic species in the tropics favor phytoplankton and weakly swimming zooplankton. Azooxanthellate soft corals are opportunistic feeders and shift their diet according to the season from phyto- and nanoplankton in summer to primarily particulate organic matter (POM) in winter. Cold-water species favor POM, phytodetritus, microplankton, and larger zooplankton when available. Antipatharians apparently feed on mesozooplankton but also use mucus nets, possibly for capture of POM. Feeding modes in this group are poorly known.

In vivo imaging of coral tissue and skeleton with optical coherence tomography

Application of optical coherence tomography (OCT) for in vivo imaging of tissue and skeleton structure of intact living corals enabled the non-invasive visualization of coral tissue layers (endoderm versus ectoderm), skeletal cavities and special structures such as mesenterial filaments and mucus release from intact living corals. Coral host chromatophores containing green fluorescent protein-like pigment granules appeared hyper-reflective to near-infrared radiation allowing for excellent optical contrast in OCT and a rapid characterization of chromatophore size, distribution and abundance. In vivo tissue plasticity could be quantified by the linear contraction velocity of coral tissues upon illumination resulting in dynamic changes in the live coral tissue surface area, which varied by a factor of 2 between the contracted and expanded state of a coral. Our study provides a novel view on the in vivo organization of coral tissue and skeleton and highlights the importance of microstructural dynamics for coral ecophysiology.

Assessing the impacts of sediments from dredging on corals

There is a need to develop water quality thresholds for dredging near coral reefs that can relate physical pressures to biological responses and define exposure conditions above which effects could occur. Water quality characteristics during dredging have, however, not been well described. Using information from several major dredging projects, we describe sediment particle sizes in the water column/seabed, suspended sediment concentrations at different temporal scales during natural and dredging-related turbidity events, and changes in light quantity/quality underneath plumes. These conditions differ considerably from those used in past laboratory studies of the effects of sediments on corals. The review also discusses other problems associated with using information from past studies for developing thresholds such as the existence of multiple different and inter-connected cause-effect pathways (which can confuse/confound interpretations), the use of sediment proxies, and the reliance on information from sediment traps to justify exposure regimes in sedimentation experiments.

The genetics of colony form and function in Caribbean Acropora corals

Background

Colonial reef-building corals have evolved a broad spectrum of colony morphologies based on coordinated asexual reproduction of polyps on a secreted calcium carbonate skeleton. Though cnidarians have been shown to possess and use similar developmental genes to bilaterians during larval development and polyp formation, little is known about genetic regulation of colony morphology in hard corals. We used RNA-seq to evaluate transcriptomic differences between functionally distinct regions of the coral (apical branch tips and branch bases) in two species of Caribbean Acropora, the staghorn coral, A. cervicornis, and the elkhorn coral, A. palmata.

Results

Transcriptome-wide gene profiles differed significantly between different parts of the coral colony as well as between species. Genes showing differential expression between branch tips and bases were involved in developmental signaling pathways, such as Wnt, Notch, and BMP, as well as pH regulation, ion transport, extracellular matrix production and other processes. Differences both within colonies and between species identify a relatively small number of genes that may contribute to the distinct “staghorn” versus “elkhorn” morphologies of these two sister species.

Conclusions

The large number of differentially expressed genes supports a strong division of labor between coral branch tips and branch bases. Genes involved in growth of mature Acropora colonies include the classical signaling pathways associated with development of cnidarian larvae and polyps as well as morphological determination in higher metazoans.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-1133) contains supplementary material, which is available to authorized users.

Coral photobiology: new light on old views

The relationship between reef-building corals and light-harvesting pigments of zooxanthellae (Symbiodinium sp.) has been acknowledged for decades. The photosynthetic activity of the algal endocellular symbionts may provide up to 90% of the energy needed for the coral holobiont. This relationship limits the bathymetric distribution of coral reefs to the upper 100 m of tropical shorelines. However, even corals growing under high light intensities have to supplement the photosynthates translocated from the algae by predation on nutrient-rich zooplankton. New information has revealed how the fate of carbon acquired through photosynthesis differs from that secured by predation, whose rates are controlled by light-induced tentacular extension. The Goreau paradigm of "light-enhanced calcification" is being reevaluated, based on evidence that blue light stimulates coral calcification independently from photosynthesis rates. Furthermore, under dim light, calcification rates were stoichiometrically uncoupled from photosynthesis. The rates of photosynthesis of the zooxanthellae exhibit a clear endogenous rhythmicity maintained by light patterns. This daily pattern is concomitant with a periodicity of all the antioxidant protective mechanisms that wax and wane to meet the concomitant fluctuation in oxygen evolution. The phases of the moon are involved in the triggering of coral reproduction and control the spectacular annual mass-spawning events taking place in several reefs. The intensity and directionality of the underwater light field affect the architecture of coral colonies, leading to an optimization of the exposure of the zooxanthellae to light. We present a summary of major gaps in our understanding of the relationship between light and corals as a roadmap for future research.

Effective light absorption and absolute electron transport rates in the coral Pocillopora damicornis

Pulse Amplitude Modulation (PAM) fluorometry has been widely used to estimate the relative photosynthetic efficiency of corals. However, both the optical properties of intact corals as well as past technical constrains to PAM fluorometers have prevented calculations of the electron turnover rate of PSII. We used a new Multi-colour PAM (MC-PAM) in parallel with light microsensors to determine for the first time the wavelength-specific effective absorption cross-section of PSII photochemistry, σII(λ), and thus PAM-based absolute electron transport rates of the coral photosymbiont Symbiodinium both in culture and in hospite in the coral Pocillopora damicornis. In both cases, σII of Symbiodinium was highest in the blue spectral region and showed a progressive decrease towards red wavelengths. Absolute values for σII at 440 nm were up to 1.5-times higher in culture than in hospite. Scalar irradiance within the living coral tissue was reduced by 20% in the blue when compared to the incident downwelling irradiance. Absolute electron transport rates of P. damicornis at 440 nm revealed a maximum PSII turnover rate of ca. 250 electrons PSII(-1) s(-1), consistent with one PSII turnover for every 4 photons absorbed by PSII; this likely reflects the limiting steps in electron transfer between PSII and PSI. Our results show that optical properties of the coral host strongly affect light use efficiency of Symbiodinium. Therefore, relative electron transport rates do not reflect the productivity rates (or indeed how the photosynthesis-light response is parameterised). Here we provide a non-invasive approach to estimate absolute electron transport rates in corals.

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.

The engine of the reef: photobiology of the coral-algal symbiosis

Coral reef ecosystems thrive in tropical oligotrophic oceans because of the relationship between corals and endosymbiotic dinoflagellate algae called Symbiodinium. Symbiodinium convert sunlight and carbon dioxide into organic carbon and oxygen to fuel coral growth and calcification, creating habitat for these diverse and productive ecosystems. Light is thus a key regulating factor shaping the productivity, physiology, and ecology of the coral holobiont. Similar to all oxygenic photoautotrophs, Symbiodinium must safely harvest sunlight for photosynthesis and dissipate excess energy to prevent oxidative stress. Oxidative stress is caused by environmental stressors such as those associated with global climate change, and ultimately leads to breakdown of the coral-algal symbiosis known as coral bleaching. Recently, large-scale coral bleaching events have become pervasive and frequent threatening and endangering coral reefs. Because the coral-algal symbiosis is the biological engine producing the reef, the future of coral reef ecosystems depends on the ecophysiology of the symbiosis. This review examines the photobiology of the coral-algal symbiosis with particular focus on the photophysiological responses and timescales of corals and Symbiodinium. Additionally, this review summarizes the light environment and its dynamics, the vulnerability of the symbiosis to oxidative stress, the abiotic and biotic factors influencing photosynthesis, the diversity of the coral-algal symbiosis, and recent advances in the field. Studies integrating physiology with the developing "omics" fields will provide new insights into the coral-algal symbiosis. Greater physiological and ecological understanding of the coral-algal symbiosis is needed for protection and conservation of coral reefs.

Lateral light transfer ensures efficient resource distribution in symbiont-bearing corals

Coral tissue optics has received very little attention in the past, although the interaction between tissue and light is central to our basic understanding of coral physiology. Here we used fibre-optic and electrochemical microsensors along with variable chlorophyll fluorescence imaging to directly measure lateral light propagation within living coral tissues. Our results show that corals can transfer light laterally within their tissues to a distance of ~2 cm. Such light transport stimulates O2 evolution and photosystem II operating efficiency in areas &gt;0.5–1 cm away from direct illumination. Light is scattered strongly in both coral tissue and skeleton, leading to photon trapping and lateral redistribution within the tissue. Lateral light transfer in coral tissue is a new mechanism by which light is redistributed over the coral colony and we argue that tissue optical properties are one of the key factors in explaining the high photosynthetic efficiency of corals.

Radiative energy budget reveals high photosynthetic efficiency in symbiont-bearing corals

The light field on coral reefs varies in intensity and spectral composition, and is the key regulating factor for phototrophic reef organisms, for example scleractinian corals harbouring microalgal symbionts. However, the actual efficiency of light utilization in corals and the mechanisms affecting the radiative energy budget of corals are underexplored. We present the first balanced light energy budget for a symbiont-bearing coral based on a fine-scale study of the microenvironmental photobiology of the massive coral Montastrea curta. The majority (more than 96%) of the absorbed light energy was dissipated as heat, whereas the proportion of the absorbed light energy used in photosynthesis was approximately 4.0% under an irradiance of 640 µmol photons m(-2) s(-1). With increasing irradiance, the proportion of heat dissipation increased at the expense of photosynthesis. Despite such low energy efficiency, we found a high photosynthetic efficiency of the microalgal symbionts showing high gross photosynthesis rates and quantum efficiencies (QEs) of approximately 0.1 O2 photon(-1) approaching theoretical limits under moderate irradiance levels. Corals thus appear as highly efficient light collectors with optical properties enabling light distribution over the corallite/tissue microstructural canopy that enables a high photosynthetic QE of their photosynthetic microalgae in hospite.

Autotrophic and heterotrophic responses of the coral Porites lutea to large amplitude internal waves

Large amplitude internal waves (LAIW) cause frequent and severe changes in the physico-chemical environment of Andaman Sea coral reefs and are a potentially important source of disturbance for corals. To explore the coral response to LAIW, prey capture disposition and photosynthesis were investigated in relation to changes in seawater temperature, pH, flow speed and food availability in LAIW simulation studies under controlled laboratory conditions, using Porites lutea as a model organism. Although food presence stimulated polyp expansion, we found an overriding effect of low temperature (19°C) causing retraction of the coral polyps into their calices, particularly when pH was altered concomitantly. Decreases in pH alone, however, caused the expansion of the polyps. The exposure history of the colonies played a crucial role in coral responses: prior field exposure to LAIW yielded lower retraction levels than in LAIW-inexperienced corals, suggesting acclimatization. Low temperature (19°C) exposure did not seem to influence the photosynthetic performance, but LAIW-experienced corals showed higher values of maximum dark-adapted quantum yield (Fv/Fm) of photosystem II than LAIW-inexperienced controls. Collectively, these data suggest that P. lutea, the dominant hermatypic coral in the Andaman Sea, can acclimatize to extreme changes in its abiotic environment by modulating its mixotrophic nutrition, through polyp expansion and potential feeding, as well as its photosynthetic efficiency.

Chronobiology by moonlight

Most studies in chronobiology focus on solar cycles (daily and annual). Moonlight and the lunar cycle received considerably less attention by chronobiologists. An exception are rhythms in intertidal species. Terrestrial ecologists long ago acknowledged the effects of moonlight on predation success, and consequently on predation risk, foraging behaviour and habitat use, while marine biologists have focused more on the behaviour and mainly on reproduction synchronization with relation to the Moon phase. Lately, several studies in different animal taxa addressed the role of moonlight in determining activity and studied the underlying mechanisms. In this paper, we review the ecological and behavioural evidence showing the effect of moonlight on activity, discuss the adaptive value of these changes, and describe possible mechanisms underlying this effect. We will also refer to other sources of night-time light ('light pollution') and highlight open questions that demand further studies.

A genome-wide survey of photoreceptor and circadian genes in the coral, Acropora digitifera

Corals exhibit circadian behaviors, but little is known about the molecular mechanisms underlying the regulation of these behaviors. We surveyed the recently decoded genome of the coral, Acropora digitifera, for photoreceptor and circadian genes, using molecular phylogenetic analyses. Our search for photoreceptor genes yielded seven opsin and three cryptochrome genes. Two genes from each family likely underwent tandem duplication in the coral lineage. We also found the following A. digitifera orthologs to Drosophila and mammalian circadian clock genes: four clock, one bmal/cycle, three pdp1-like, one creb/atf, one sgg/zw3, two ck2alpha, one dco (csnk1d/cnsk1e), one slim/BTRC, and one grinl. No vrille, rev-ervα/nr1d1, bhlh2, vpac2, adcyap1, or adcyaplr1 orthologs were found. Intriguingly, in spite of an extensive survey, we also failed to find homologs of period and timeless, although we did find one timeout gene. In addition, the coral genes were compared to orthologous genes in the sea anemone, Nematostella vectensis. Thus, the coral and sea anemone genomes share a similar repertoire of circadian clock genes, although A. digitifera contains more clock genes and fewer photoreceptor genes than N. vectensis. This suggests that the circadian clock system was established in a common ancestor of corals and sea anemones, and was diversified by tandem gene duplications and the loss of paralogous genes in each lineage. It will be interesting to determine how the coral circadian clock functions without period.

Characterization of circadian behavior in the starlet sea anemone, Nematostella vectensis

Background

Although much is known about how circadian systems control daily cycles in the physiology and behavior of Drosophila and several vertebrate models, marine invertebrates have often been overlooked in circadian rhythms research. This study focuses on the starlet sea anemone, Nematostella vectensis, a species that has received increasing attention within the scientific community for its potential as a model research organism. The recently sequenced genome of N. vectensis makes it an especially attractive model for exploring the molecular evolution of circadian behavior. Critical behavioral data needed to correlate gene expression patterns to specific behaviors are currently lacking in N. vectensis.

Methodology/Principal Findings

To detect the presence of behavioral oscillations in N. vectensis, locomotor activity was evaluated using an automated system in an environmentally controlled chamber. Animals exposed to a 24 hr photoperiod (12 hr light: 12 hr dark) exhibited locomotor behavior that was both rhythmic and predominantly nocturnal. The activity peak occurred in the early half of the night with a 2-fold increase in locomotion. Upon transfer to constant lighting conditions (constant light or constant dark), an approximately 24 hr rhythm persisted in most animals, suggesting that the rhythm is controlled by an endogenous circadian mechanism. Fourier analysis revealed the presence of multiple peaks in some animals suggesting additional rhythmic components could be present. In particular, an approximately 12 hr oscillation was often observed. The nocturnal increase in generalized locomotion corresponded to a 24 hr oscillation in animal elongation.

Conclusions/Significance

These data confirm the presence of a light-entrainable circadian clock in Nematostella vectensis. Additional components observed in some individuals indicate that an endogenous clock of approximately 12 hr frequency may also be present. By describing rhythmic locomotor behavior in N. vectensis, we have made important progress in developing the sea anemone as a model organism for circadian rhythm research.

Temporal and histological evaluation of melatonin patterns in a 'basal' metazoan

While recent advances suggest functional pleiotropy of melatonin in higher organisms, an understanding of the biological significance of this ancient molecule in early evolutionary groups is lacking. Here, endogenous melatonin production was identified for the first time in the sea anemone Actinia equina, a nonsymbiotic hexacorallian cnidarian. Day/night activity profiles of melatonin in this anemone indicated that melatonin levels oscillate with significant nocturnal peaks. However, dynamic changes in melatonin concentration did not persist under constant dark conditions and therefore were not circadian in nature. Thus, the oscillating pattern of melatonin in A. equina is presumed to be the result of alternative, simpler melatonin control mechanism that likely involves direct regulation by the daily photocycle. As nocturnal melatonin signals still potentially provide 'time-of-day' information and can illustrate the seasonally changing length of the biological night, we hypothesize that melatonin may be relevant to temporal coordination of timed processes also in anthozoans. Spatial patterns of melatonin distribution found in this study indicate abundant melatonin distribution in the endodermal filaments wrapped around gametes. This finding supports the possibility that one of the melatonin-responsive processes in this basal metazoan species may involve reproductive functions.