Growth, calcification, and photobiology of the threatened coral Acropora cervicornis in natural versus artificial light.

Land-based coral culture is of increasing interest for conservation and educational display. Shallow water corals generate most of their energy from photosynthesis, and light is a critical abiotic factor in their husbandry. We compared growth, calcification, and photobiology in the coral Acropora cervicornis between natural and artificial (light-emitting diode; LED) light to better understand the impact of light source on coral performance. One tank of a greenhouse recirculating system at The Florida Aquarium's Center for Conservation was used to culture replicate coral colonies. Half of the tank and corals were covered to block sunlight and illuminated with a commercial reef aquarium LED fixture, while the other half was exposed to natural sunlight. Treatments were matched in terms of maximum photosynthetically active radiation and spectral measurements characterized both light regimes. Coral growth and calcification were tracked over a period of 19 weeks by repeated measurements of total linear extension (TLE) and buoyant weight. For the first 5 weeks, photosynthetic yield was measured weekly using a pulse-amplitude-modulated fluorometer. Calcification was significantly higher under LED lighting relative to natural light, but TLE did not differ. Photobiology data suggest that corals in both treatments were acclimated to the same light level, but photosynthetic efficiency was ultimately greater in the natural light treatment. More consistent light delivery and different spectral composition under LED treatment conditions may explain the incongruity between calcification and photosynthetic efficiency. This experiment informs husbandry of shallow-water scleractinian corals maintained in both natural sunlight and enclosed structures.

Coral bleaching.

Tracy Ainsworth and Barbara Brown introduce the causes and consequences of coral bleaching.

Coral Gametogenesis Collapse under Artificial Light Pollution.

Artificial light at night (ALAN) can have negative impacts on the health of humans and ecosystems.1-4 Marine organisms, including coral reefs in particular, rely on the natural light cycles of sunlight and moonlight to regulate various physiological, biological, and behavioral processes.5-8 Here, we demonstrate that light pollution caused delayed gametogenesis and unsynchronized gamete release in two coral species, Acropora millepora and Acropora digitifera, from the Indo-Pacific Ocean. Given the urbanization along major coasts, light pollution could thus further threaten coral communities' populations, which are already under severe degradation. A worldwide-modeled light pollution impact assessment is provided, which can help incorporate an important variable in coral reef conservation planning.

Longevity strategies in response to light in the reef coral Stylophora pistillata.

Aging is a multifactorial process that results in progressive loss of regenerative capacity and tissue function while simultaneously favoring the development of a large array of age-related diseases. Evidence suggests that the accumulation of senescent cells in tissue promotes both normal and pathological aging. Oxic stress is a key driver of cellular senescence. Because symbiotic long-lived reef corals experience daily hyperoxic and hypoxic transitions, we hypothesized that these long-lived animals have developed specific longevity strategies in response to light. We analyzed transcriptome variation in the reef coral Stylophora pistillata during the day-night cycle and revealed a signature of the FoxO longevity pathway. We confirmed this pathway by immunofluorescence using antibodies against coral FoxO to demonstrate its nuclear translocation. Through qPCR analysis of nycthemeral variations of candidate genes under different light regimens, we found that, among genes that were specifically up- or downregulated upon exposure to light, human orthologs of two "light-up" genes (HEY1 and LONF3) exhibited anti-senescence properties in primary human fibroblasts. Therefore, these genes are interesting candidates for counteracting skin aging. We propose a large screen for other light-up genes and an investigation of the biological response of reef corals to light (e.g., metabolic switching) to elucidate these processes and identify effective interventions for promoting healthy aging in humans.

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.

Bionic 3D printed corals.

Corals have evolved as optimized photon augmentation systems, leading to space-efficient microalgal growth and outstanding photosynthetic quantum efficiencies. Light attenuation due to algal self-shading is a key limiting factor for the upscaling of microalgal cultivation. Coral-inspired light management systems could overcome this limitation and facilitate scalable bioenergy and bioproduct generation. Here, we develop 3D printed bionic corals capable of growing microalgae with high spatial cell densities of up to 109 cells mL-1. The hybrid photosynthetic biomaterials are produced with a 3D bioprinting platform which mimics morphological features of living coral tissue and the underlying skeleton with micron resolution, including their optical and mechanical properties. The programmable synthetic microenvironment thus allows for replicating both structural and functional traits of the coral-algal symbiosis. Our work defines a class of bionic materials that is capable of interacting with living organisms and can be exploited for applied coral reef research and photobioreactor design.

Ion transporter gene expression is linked to the thermal sensitivity of calcification in the reef coral Stylophora pistillata.

Coral calcification underpins biodiverse reef ecosystems, but the physiology underlying the thermal sensitivity of corals to changing seawater temperatures remains unclear. Furthermore, light is also a key factor in modulating calcification rates, but a mechanistic understanding of how light interacts with temperature to affect coral calcification is lacking. Here, we characterized the thermal performance curve (TPC) of calcification of the wide-spread, model coral species Stylophora pistillata, and used gene expression analysis to investigate the role of ion transport mechanisms in thermally-driven declines in day and nighttime calcification. Focusing on genes linked to transport of dissolved inorganic carbon (DIC), calcium and H+, our study reveals a high degree of coherence between physiological responses (e.g. calcification and respiration) with distinct gene expression patterns to the different temperatures in day and night conditions. At low temperatures, calcification and gene expression linked to DIC transport processes were downregulated, but showed little response to light. By contrast, at elevated temperature, light had a positive effect on calcification and stimulated a more functionally diverse gene expression response of ion transporters. Overall, our findings highlight the role of mechanisms linked to DIC, calcium and H+ transport in the thermal sensitivity of coral calcification and how this sensitivity is influenced by light.

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.

Response of fluorescence morphs of the mesophotic coral Euphyllia paradivisa to ultra-violet radiation.

Euphyllia paradivisa is a strictly mesophotic coral in the reefs of Eilat that displays a striking color polymorphism, attributed to fluorescent proteins (FPs). FPs, which are used as visual markers in biomedical research, have been suggested to serve as photoprotectors or as facilitators of photosynthesis in corals due to their ability to transform light. Solar radiation that penetrates the sea includes, among others, both vital photosynthetic active radiation (PAR) and ultra-violet radiation (UVR). Both types, at high intensities, are known to have negative effects on corals, ranging from cellular damage to changes in community structure. In the present study, fluorescence morphs of E. paradivisa were used to investigate UVR response in a mesophotic organism and to examine the phenomenon of fluorescence polymorphism. E. paradivisa, although able to survive in high-light environments, displayed several physiological and behavioral responses that indicated severe light and UVR stress. We suggest that high PAR and UVR are potential drivers behind the absence of this coral from shallow reefs. Moreover, we found no significant differences between the different fluorescence morphs' responses and no evidence of either photoprotection or photosynthesis enhancement. We therefore suggest that FPs in mesophotic corals might have a different biological role than that previously hypothesized for shallow corals.

Effects of ultraviolet radiation and nutrient level on the physiological response and organic matter release of the scleractinian coral Pocillopora damicornis following thermal stress.

Understanding which factors enhance or mitigate the impact of high temperatures on corals is crucial to predict the severity of coral bleaching worldwide. On the one hand, global warming is usually associated with high ultraviolet radiation levels (UVR), and surface water nutrient depletion due to stratification. On the other hand, eutrophication of coastal reefs increases levels of inorganic nutrients and decreases UVR, so that the effect of different combinations of these stressors on corals is unknown. In this study, we assessed the individual and crossed effects of high temperature, UVR and nutrient level on the key performance variables of the reef building coral Pocillopora damicornis. We found that seawater warming was the major stressor, which induced bleaching and impaired coral photosynthesis and calcification in all nutrient and UVR conditions. The strength of this effect however, was nutrient dependent. Corals maintained in nutrient-depleted conditions experienced the highest decrease in net photosynthesis under thermal stress, while nutrient enrichment (3 µM NO3- and 1 µM PO4+) slightly limited the negative impact of temperature through enhanced protein content, photosynthesis and respiration rates. UVR exposure had only an effect on total nitrogen release rates, which significantly decreased under normal growth conditions and tended to decrease also under thermal stress. This result suggests that increased level of UVR will lead to significant changes in the nutrient biogeochemistry of surface reef waters. Overall, our results show that environmental factors have different and interactive effects on each of the coral's physiological parameters, requiring multifactorial approaches to predict the future of coral reefs.

Before platelets: the production of platelet-activating factor during growth and stress in a basal marine organism.

Corals and humans represent two extremely disparate metazoan lineages and are therefore useful for comparative evolutionary studies. Two lipid-based molecules that are central to human immunity, platelet-activating factor (PAF) and Lyso-PAF were recently identified in scleractinian corals. To identify processes in corals that involve these molecules, PAF and Lyso-PAF biosynthesis was quantified in conditions known to stimulate PAF production in mammals (tissue growth and exposure to elevated levels of ultraviolet light) and in conditions unique to corals (competing with neighbouring colonies over benthic space). Similar to observations in mammals, PAF production was higher in regions of active tissue growth and increased when corals were exposed to elevated levels of ultraviolet light. PAF production also increased when corals were attacked by the stinging cells of a neighbouring colony, though only the attacked coral exhibited an increase in PAF. This reaction was observed in adjacent areas of the colony, indicating that this response is coordinated across multiple polyps including those not directly subject to the stress. PAF and Lyso-PAF are involved in coral stress responses that are both shared with mammals and unique to the ecology of cnidarians.

Phototoxic effects of two common marine fuels on the settlement success of the coral Acropora tenuis.

Coral reefs are at risk of exposure to petroleum hydrocarbons from shipping spills and uncontrolled discharges during extraction. The toxicity of petroleum hydrocarbons can substantially increase in the presence of ultraviolet radiation (UVR), therefore spills in shallow coral reef environments may be particularly hazardous to reef species. Here we investigated the sensitivity of coral larvae (Acropora tenuis) to dissolved hydrocarbons from heavy fuel oil (HFO) and diesel in the absence and presence of UVR. Larval settlement success decreased with increasing concentrations of dissolved HFO, and co-exposure to UVR doubled the toxicity: 50% effect concentrations (EC50) decreased from 96 (-UVR) to 51 (+UVR) total petroleum aromatic hydrocarbons (TPAH). Toxic thresholds for HFO were similar to concentrations reported during marine spills: EC10s of 24 (-UVR) and 15 (+UVR) µg l-1. While less toxic, diesel also reduced settlement and exhibited phototoxicity: EC10s of 122 (+UVR) and 302 (-UVR) µg l-1. This study demonstrates that the presence of UVR increases the hazard posed by oil pollution to tropical, shallow-water coral reefs. Further research on the effects of oils in the presence of UVR is needed to improve the environmental relevance of risk assessments and ensure appropriate protection for shallow reef environments against oil pollution.

Dynamics of coral-associated microbiomes during a thermal bleaching event.

Coral-associated microorganisms play an important role in their host fitness and survival. A number of studies have demonstrated connections between thermal tolerance in corals and the type/relative abundance of Symbiodinium they harbor. More recently, the shifts in coral-associated bacterial profiles were also shown to be linked to the patterns of coral heat tolerance. Here, we investigated the dynamics of Porites lutea-associated bacterial and algal communities throughout a natural bleaching event, using full-length 16S rRNA and internal transcribed spacer sequences (ITS) obtained from PacBio circular consensus sequencing. We provided evidence of significant changes in the structure and diversity of coral-associated microbiomes during thermal stress. The balance of the symbiosis shifted from a predominant association between corals and Gammaproteobacteria to a predominance of Alphaproteobacteria and to a lesser extent Betaproteobacteria following the bleaching event. On the contrary, the composition and diversity of Symbiodinium communities remained unaltered throughout the bleaching event. It appears that the switching and/or shuffling of Symbiodinium types may not be the primary mechanism used by P. lutea to cope with increasing seawater temperature. The shifts in the structure and diversity of associated bacterial communities may contribute more to the survival of the coral holobiont under heat stress.

Phototoxic effects of PAH and UVA exposure on molecular responses and developmental success in coral larvae.

Exposure to polycyclic aromatic carbons (PAHs) poses a growing risk to coral reefs due to increasing shipping and petroleum extraction in tropical waters. Damaging effects of specific PAHs can be further enhanced by the presence of ultraviolet radiation, known as phototoxicity. We tested phototoxic effects of the PAHs anthracene and phenanthrene on larvae of the scleractinian coral Acropora tenuis in the presence and absence of UVA (320-400 nm). Activity of superoxide dismutase (SOD) enzyme was reduced by anthracene while phenanthrene and UVA exposure did not have any effect. Gene expression of MnSod remained constant across all treatments. The genes Catalase, Hsp70 and Hsp90 showed increased expression levels in larvae exposed to anthracene, but not phenanthrene. Gene expression of p53 was upregulated in the presence of UVA, but downregulated when exposed to PAHs. The influence on stress-related biochemical pathways and gene expresson in A. tenuis larvae was considerably greater for anthracene than phenanthrene, and UVA-induced phototoxicity was only evident for anthracene. The combined effects of UVA and PAH exposure on larval survival and metamorphosis paralleled the sub-lethal stress responses, clearly highlighting the interaction of UVA on anthracene toxicity and ultimately the coral's development.

Impacts of light limitation on corals and crustose coralline algae.

Turbidity associated with elevated suspended sediment concentrations can significantly reduce underwater light availability. Understanding the consequences for sensitive organisms such as corals and crustose coralline algae (CCA), requires an understanding of tolerance levels and the time course of effects. Adult colonies of Acropora millepora and Pocillopora acuta, juvenile P. acuta, and the CCA Porolithon onkodes were exposed to six light treatments of ~0, 0.02, 0.1, 0.4, 1.1 and 4.3 mol photons m-2 d-1, and their physiological responses were monitored over 30 d. Exposure to very low light (<0.1 mol photons m-2 d-1) caused tissue discoloration (bleaching) in the corals, and discolouration (and partial mortality) of the CCA, yielding 30 d EI10 thresholds (irradiance which results in a 10% change in colour) of 1.2-1.9 mol photons m-2 d-1. Recent monitoring studies during dredging campaigns on a shallow tropical reef, have shown that underwater light levels very close (~500 m away) from a working dredge routinely fall below this value over 30 d periods, but rarely during the pre-dredging baseline phase. Light reduction alone, therefore, constitutes a clear risk to coral reefs from dredging, although at such close proximity other cause-effect pathways, such as sediment deposition and smothering, are likely to also co-occur.

Using light-dependent scleractinia to define the upper boundary of mesophotic coral ecosystems on the reefs of Utila, Honduras.

Shallow water zooxanthellate coral reefs grade into ecologically distinct mesophotic coral ecosystems (MCEs) deeper in the euphotic zone. MCEs are widely considered to start at an absolute depth limit of 30m deep, possibly failing to recognise that these are distinct ecological communities that may shift shallower or deeper depending on local environmental conditions. This study aimed to explore whether MCEs represent distinct biological communities, the upper boundary of which can be defined and whether the depth at which they occur may vary above or below 30m. Mixed-gas diving and closed-circuit rebreathers were used to quantitatively survey benthic communities across shallow to mesophotic reef gradients around the island of Utila, Honduras. Depths of up to 85m were sampled, covering the vertical range of the zooxanthellate corals around Utila. We investigate vertical reef zonation using a variety of ecological metrics to identify community shifts with depth, and the appropriateness of different metrics to define the upper MCE boundary. Patterns observed in scleractinian community composition varied between ordination analyses and approaches utilising biodiversity indices. Indices and richness approaches revealed vertical community transition was a gradation. Ordination approaches suggest the possibility of recognising two scleractinian assemblages. We could detect a mesophotic and shallow community while illustrating that belief in a static depth limit is biologically unjustified. The switch between these two communities occurred across bathymetric gradients as small as 10m and as large as 50m in depth. The difference between communities appears to be a loss of shallow specialists and increase in depth-generalist taxa. Therefore, it may be possible to define MCEs by a loss of shallow specialist species. To support a biological definition of mesophotic reefs, we advocate this analytical framework should be applied around the Caribbean and extended into other ocean basins where MCEs are present.

Acclimatization of symbiotic corals to mesophotic light environments through wavelength transformation by fluorescent protein pigments.

The depth distribution of reef-building corals exposes their photosynthetic symbionts of the genus Symbiodinium to extreme gradients in the intensity and spectral quality of the ambient light environment. Characterizing the mechanisms used by the coral holobiont to respond to the low intensity and reduced spectral composition of the light environment in deeper reefs (greater than 20 m) is fundamental to our understanding of the functioning and structure of reefs across depth gradients. Here, we demonstrate that host pigments, specifically photoconvertible red fluorescent proteins (pcRFPs), can promote coral adaptation/acclimatization to deeper-water light environments by transforming the prevalent blue light into orange-red light, which can penetrate deeper within zooxanthellae-containing tissues; this facilitates a more homogeneous distribution of photons across symbiont communities. The ecological importance of pcRFPs in deeper reefs is supported by the increasing proportion of red fluorescent corals with depth (measured down to 45 m) and increased survival of colour morphs with strong expression of pcRFPs in long-term light manipulation experiments. In addition to screening by host pigments from high light intensities in shallow water, the spectral transformation observed in deeper-water corals highlights the importance of GFP-like protein expression as an ecological mechanism to support the functioning of the coral-Symbiodinium association across steep environmental gradients.

Physiological plasticity related to zonation affects hsp70 expression in the reef-building coral Pocillopora verrucosa.

This study investigates for the first time the transcriptional regulation of a stress-inducible 70-kDa heat shock protein (hsp70) in the scleractinian coral Pocillopora verrucosa sampled at three locations and two depths (3 m and 12 m) in Bangka Island waters (North Sulawesi, Indonesia). Percentage of coral cover indicated reduced habitat suitability with depth and at the Tanjung Husi (TA) site, which also displayed relatively higher seawater temperatures. Expression of the P. verrucosa hsp70 transcript evaluated under field conditions followed a depth-related profile, with relatively higher expression levels in 3-m collected nubbins compared to the 12-m ones. Expression levels of metabolism-related transcripts ATP synthase and NADH dehydrogenase indicated metabolic activation of nubbins to cope with habitat conditions of the TA site at 3 m. After a 14-day acclimatization to common and fixed temperature conditions in the laboratory, corals were subjected for 7 days to an altered thermal regime, where temperature was elevated at 31°C during the light phase and returned to 28°C during the dark phase. Nubbins collected at 12 m were relatively more sensitive to thermal stress, as they significantly over-expressed the selected transcripts. Corals collected at 3 m appeared more resilient, as they showed unaffected mRNA expressions. The results indicated that local habitat conditions may influence transcription of stress-related genes in P. verrucosa. Corals exhibiting higher basal hsp70 levels may display enhanced tolerance towards environmental stressors.

UV-A induced delayed development in the larvae of coral Seriatopora caliendrum.

Coral reefs are vulnerable to ultraviolet radiation (UVR, 280-400nm). Not only do the fluxes of UVR fluctuate daily, they are also increasing due to global ocean and atmospheric changes. The deleterious effects of UVR on scleractinian corals have been intensively studied, but much less is known about the response of corals in the early pre-settlement phase. In this study, we tested how UVR exposure affects survival and development of Seriatopora caliendrum larvae and examined the photophysiological changes induced in the symbiotic dinoflagellate Symbiodinium. Results showed that the contents of chl c and carotenoids normalized to the number of algae cells in the larvae decreased significantly when larvae were exposed to UVR compared to those protected from UVR, while the cell density of Symbiodinium was higher in UVR-exposed larvae. The effective photochemical efficiency of the symbiotic algae increased when cultured under PAR plus UV-A (here taken as 320-395nm). We further present the novel finding that during the development experiment, presence of UV-A induced a decline in the rates of metamorphosis and settlement, which disappeared when the larvae were also exposed to UV-B (here defined as 295-320nm). However, UVR had no distinguishable effect on the numbers of larvae that either survived, metamorphosed or settled by the end of the culture period. Therefore, it is concluded from this study that UV-A radiation may extend the planktonic duration of coral larvae, but not have an overall inhibitory effect on developmental outcomes.