The impact of coral bleaching on the pigment profile of the symbiotic alga, Symbiodinium

Phosphorus limitation intensifies heat-stress effects on the potential mutualistic capacity in the coral-derived Symbiodinium

Coral reef ecosystems have been severely ravaged by global warming and eutrophication. Eutrophication often originates from nitrogen (N) overloading that creates stoichiometric phosphorus (P) limitation, which can be aggravated by sea surface temperature rises that enhances stratification. However, how P-limitation interacts with thermal stress to impact coral-Symbiodiniaceae mutualism is poorly understood and underexplored. Here, we investigated the effect of P-limitation (P-depleted vs. P-replete) superimposed on heat stress (31 °C vs. 25 °C) on a Symbiodinium strain newly isolated from the coral host by a 14-day incubation experiment. The heat and P-limitation co-stress induced an increase in alkaline phosphatase activity and reppressed cell division, photosynthetic efficiency, and expression of N uptake and assimilation genes. Moreover, P limitation intensified downregulation of carbon fixation (light and dark reaction) and metabolism (glycolysis) pathways in heat stressed Symbiodinium. Notably, co-stress elicited a marked transcriptional downregulation of genes encoding photosynthates transporters and microbe-associated molecular patterns, potentially undermining the mutualism potential. This work sheds light on the interactive effects of P-limitation and heat stress on coral symbionts, indicating that nutrient imbalance in the coral reef ecosystem can intensify heat-stress effects on the mutualistic capacity of Symbiodiniaceae.

Architecture of symbiotic dinoflagellate photosystem I-light-harvesting supercomplex in Symbiodinium

Symbiodinium are the photosynthetic endosymbionts for corals and play a vital role in supplying their coral hosts with photosynthetic products, forming the nutritional foundation for high-yield coral reef ecosystems. Here, we determine the cryo-electron microscopy structure of Symbiodinium photosystem I (PSI) supercomplex with a PSI core composed of 13 subunits including 2 previously unidentified subunits, PsaT and PsaU, as well as 13 peridinin-Chl a/c-binding light-harvesting antenna proteins (AcpPCIs). The PSI-AcpPCI supercomplex exhibits distinctive structural features compared to their red lineage counterparts, including extended termini of PsaD/E/I/J/L/M/R and AcpPCI-1/3/5/7/8/11 subunits, conformational changes in the surface loops of PsaA and PsaB subunits, facilitating the association between the PSI core and peripheral antennae. Structural analysis and computational calculation of excitation energy transfer rates unravel specific pigment networks in Symbiodinium PSI-AcpPCI for efficient excitation energy transfer. Overall, this study provides a structural basis for deciphering the mechanisms governing light harvesting and energy transfer in Symbiodinium PSI-AcpPCI supercomplexes adapted to their symbiotic ecosystem, as well as insights into the evolutionary diversity of PSI-LHCI among various photosynthetic organisms.

Rapid, high-throughput phenotypic profiling of endosymbiotic dinoflagellates (Symbiodiniaceae) using benchtop flow cytometry

Endosymbiotic dinoflagellates (Family Symbiodiniaceae) are the primary producer of energy for many cnidarians, including corals. The intricate coral-dinoflagellate symbiotic relationship is becoming increasingly important under climate change, as its breakdown leads to mass coral bleaching and often mortality. Despite methodological progress, assessing the phenotypic traits of Symbiodiniaceae in-hospite remains a complex task. Bio-optics, biochemistry, or "-omics" techniques are expensive, often inaccessible to investigators, or lack the resolution required to understand single-cell phenotypic states within endosymbiotic dinoflagellate assemblages. To help address this issue, we developed a protocol that collects information on cell autofluorescence, shape, and size to simultaneously generate phenotypic profiles for thousands of Symbiodiniaceae cells, thus revealing phenotypic variance of the Symbiodiniaceae assemblage to the resolution of single cells. As flow cytometry is adopted as a robust and efficient method for cell counting, integration of our protocol into existing workflows allows researchers to acquire a new level of resolution for studies examining the acclimation and adaptation strategies of Symbiodiniaceae assemblages.

The Zooxanthellate Jellyfish Holobiont Cassiopea andromeda, a Source of Soluble Bioactive Compounds

Cassiopea andromeda (Forsskål, 1775), commonly found across the Indo-Pacific Ocean, the Red Sea, and now also in the warmest areas of the Mediterranean Sea, is a scyphozoan jellyfish that hosts autotrophic dinoflagellate symbionts (family Symbiodiniaceae). Besides supplying photosynthates to their host, these microalgae are known to produce bioactive compounds as long-chain unsaturated fatty acids, polyphenols, and pigments, including carotenoids, with antioxidant properties and other beneficial biological activities. By the present study, a fractionation method was applied on the hydroalcoholic extract from two main body parts (oral arms and umbrella) of the jellyfish holobiont to obtain an improved biochemical characterization of the obtained fractions from the two body parts. The composition of each fraction (i.e., proteins, phenols, fatty acids, and pigments) as well as the associated antioxidant activity were analyzed. The oral arms proved richer in zooxanthellae and pigments than the umbrella. The applied fractionation method was effective in separating pigments and fatty acids into a lipophilic fraction from proteins and pigment-protein complexes. Therefore, the C. andromeda-dinoflagellate holobiont might be considered as a promising natural source of multiple bioactive compounds produced through mixotrophic metabolism, which are of interest for a wide range of biotechnological applications.

The pulsating soft coral Xenia umbellata shows high resistance to warming when nitrate concentrations are low

The resistance of hard corals to warming can be negatively affected by nitrate eutrophication, but related knowledge for soft corals is scarce. We thus investigated the ecophysiological response of the pulsating soft coral Xenia umbellata to different levels of nitrate eutrophication (control = 0.6, medium = 6, high = 37 μM nitrate) in a laboratory experiment, with additional warming (27.7 to 32.8 °C) from days 17 to 37. High nitrate eutrophication enhanced cellular chlorophyll a content of Symbiodiniaceae by 168%, while it reduced gross photosynthesis by 56%. After additional warming, polyp pulsation rate was reduced by 100% in both nitrate eutrophication treatments, and additional polyp loss of 7% d⁻¹ and total fragment mortality of 26% was observed in the high nitrate eutrophication treatment. Warming alone did not affect any of the investigated response parameters. These results suggest that X. umbellata exhibits resistance to warming, which may facilitate ecological dominance over some hard corals as ocean temperatures warm, though a clear negative physiological response occurs when combined with nitrate eutrophication. This study thus confirms the importance of investigating combinations of global and local factors to understand and manage changing coral reefs.

The diversity and ecology of Symbiodiniaceae: A traits-based review

Among the most successful microeukaryotes to form mutualisms with animals are dinoflagellates in the family Symbiodiniaceae. These photosynthetic symbioses drive significant primary production and are responsible for the formation of coral reef ecosystems but are particularly sensitive when environmental conditions become extreme. Annual episodes of widespread coral bleaching (disassociation of the mutualistic partnership) and mortality are forecasted from the year 2060 under current trends of ocean warming. However, host cnidarians and dinoflagellate symbionts display exceptional genetic and functional diversity, and meaningful predictions of the future that embrace this biological complexity are difficult to make. A recent move to trait-based biology (and an understanding of how traits are shaped by the environment) has been adopted to move past this problem. The aim of this review is to: (1) provide an overview of the major cnidarian lineages that are symbiotic with Symbiodiniaceae; (2) summarise the symbiodiniacean genera associated with cnidarians with reference to recent changes in taxonomy and systematics; (3) examine the knowledge gaps in Symbiodiniaceae life history from a trait-based perspective; (4) review Symbiodiniaceae trait variation along three abiotic gradients (light, nutrients, and temperature); and (5) provide recommendations for future research of Symbiodiniaceae traits. We anticipate that a detailed understanding of traits will further reveal basic knowledge of the evolution and functional diversity of these mutualisms, as well as enhance future efforts to model stability and change in ecosystems dependent on cnidarian-dinoflagellate organisms.

Effects of nanostructure antifouling biocides towards a coral species in the context of global changes

Biofouling prevention is one of the biggest challenges faced by the maritime industry, but antifouling agents commonly impact marine ecosystems. Advances in antifouling technology include the use of nanomaterials. Herein we test an antifouling nano-additive based on the encapsulation of the biocide 4,5-dichloro-2-octyl-4-isothiazolin-3-one (DCOIT) in engineered silica nanocontainers (SiNC). The work aims to assess the biochemical and physiological effects on the symbiotic coral Sarcophyton cf. glaucum caused by (1) thermal stress and (2) DCOIT exposure (free or nanoencapsulated forms), in a climate change scenario. Accordingly, the following hypotheses were addressed: (H1) ocean warming can cause toxicity on S. cf. glaucum; (H2) the nanoencapsulation process decreases DCOIT toxicity towards this species; (H3) the biocide toxicity, free or encapsulated forms, can be affected by ocean warming. Coral fragments were exposed for seven days to DCOIT in both free and encapsulated forms, SiNC and negative controls, under two water temperature regimes (26 °C and 30.5 °C). Coral polyp behavior and photosynthetic efficiency were determined in the holobiont, while biochemical markers were assessed individually in the endosymbiont and coral host. Results showed transient coral polyp retraction and diminished photosynthetic efficiency in the presence of heat stress or free DCOIT, with effects being magnified in the presence of both stressors. The activity of catalase and glutathione-S-transferase were modulated by temperature in each partner of the symbiosis. The shifts in enzymatic activity were more pronounced in the presence of free DCOIT, but to a lower extent for encapsulated DCOIT. Increased levels of oxidative damage were detected under heat conditions. The findings highlight the physiological constrains elicited by the increase of seawater temperature to symbiotic corals and demonstrate that DCOIT toxicity can be minimized through encapsulation in SiNC. The presence of both stressors magnifies toxicity and confirm that ocean warming enhances the vulnerability of tropical photosynthetic corals to local stressors.

Brooded coral offspring physiology depends on the combined effects of parental press and pulse thermal history

Reef-building corals respond to the temporal integration of both pulse events (i.e., heat waves) and press thermal history (i.e., local environment) via physiological changes, with ecological consequences. We used a "press-pulse-press" experimental framework to expose the brooding coral Porites astreoides to various thermal histories to understand the physiological response of temporal dynamics within and across generations. We collected adult colonies from two reefs (outer Rim reef and inner Patch reef) in Bermuda with naturally contrasting thermal regimes as our initial "press" scenario, followed by a 21-day ex situ "pulse" thermal stress of 30.4°C during larval brooding, and a "press" year-long adult reciprocal transplant between the original sites. Higher endosymbiont density and holobiont protein was found in corals originating from the lower thermal variability site (Rim) compared to the higher thermal variability site (Patch). The thermal pulse event drove significant declines in photosynthesis, endosymbiont density, and chlorophyll a, with bleaching phenotype convergence for adults from both histories. Following the reciprocal transplant, photosynthesis was higher in previously heated corals, indicating recovery from the thermal pulse. The effect of origin (initial press) modulated the response to transplant site for endosymbiont density and chlorophyll a, suggesting contrasting acclimation strategies. Higher respiration and photosynthetic rates were found in corals originating from the Rim site, indicating greater energy available for reproduction, supported by larger larvae released from Rim corals post-transplantation. Notably, parental exposure to the pulse thermal event resulted in increased offspring plasticity when parents were transplanted to foreign sites, highlighting the legacy of the pulse event and the importance of the environment during recovery in contributing to cross-generational or developmental plasticity. Together, these findings provide novel insight into the role of historical disturbance events in driving differential outcomes within and across generations, which is of critical importance in forecasting reef futures.

Metabolomic signatures of coral bleaching history

Coral bleaching has a profound impact on the health and function of reef ecosystems, but the metabolomic effects of coral bleaching are largely uncharacterized. Here, untargeted metabolomics was used to analyse pairs of adjacent Montipora capitata corals that had contrasting bleaching phenotypes during a severe bleaching event in 2015. When these same corals were sampled four years later while visually healthy, there was a strong metabolomic signature of bleaching history. This was primarily driven by betaine lipids from the symbiont, where corals that did not bleach were enriched in saturated lyso-betaine lipids. Immune modulator molecules were also altered by bleaching history in both the coral host and the algal symbiont, suggesting a shared role in partner choice and bleaching response. Metabolomics from a separate set of validation corals was able to predict the bleaching phenotype with 100% accuracy. Experimental temperature stress induced phenotype-specific responses, which magnified differences between historical bleaching phenotypes. These findings indicate that natural bleaching susceptibility is manifested in the biochemistry of both the coral animal and its algal symbiont. This metabolome difference is stable through time and results in different physiological responses to temperature stress. This work provides insight into the biochemical mechanisms of coral bleaching and presents a valuable new tool for resilience-based reef restoration.

Impact of Menthol on Growth and Photosynthetic Function of Breviolum Minutum (Dinoflagellata, Dinophyceae, Symbiodiniaceae) and Interactions with its Aiptasia Host

Environmental change, including global warming and chemical pollution, can compromise cnidarian-(e.g., coral-) dinoflagellate symbioses and cause coral bleaching. Understanding the mechanisms that regulate these symbioses will inform strategies for sustaining healthy coral-reef communities. A model system for corals is the symbiosis between the sea anemone Exaiptasia pallida (common name Aiptasia) and its dinoflagellate partners (family Symbiodiniaceae). To complement existing studies of the interactions between these organisms, we examined the impact of menthol, a reagent often used to render cnidarians aposymbiotic, on the dinoflagellate Breviolum minutum, both in culture and in hospite. In both environments, the growth and photosynthesis of this alga were compromised at either 100 or 300 µM menthol. We observed reduction in PSII and PSI functions, the abundances of reaction-center proteins, and, at 300 µM menthol, of total cellular proteins. Interestingly, for free-living algae exposed to 100 µM menthol, an initial decline in growth, photosynthetic activities, pigmentation, and protein abundances reversed after 5-15 d, eventually approaching control levels. This behavior was observed in cells maintained in continuous light, but not in cells experiencing a light-dark regimen, suggesting that B. minutum can detoxify menthol or acclimate and repair damaged photosynthetic complexes in a light- and/or energy-dependent manner. Extended exposures of cultured algae to 300 µM menthol ultimately resulted in algal death. Most symbiotic anemones were also unable to survive this menthol concentration for 30 d. Additionally, cells impaired for photosynthesis by pre-treatment with 300 µM menthol exhibited reduced efficiency in re-populating the anemone host.

Gene Expression and Photophysiological Changes in Pocillopora acuta Coral Holobiont Following Heat Stress and Recovery

The ability of corals to withstand changes in their surroundings is a critical survival mechanism for coping with environmental stress. While many studies have examined responses of the coral holobiont to stressful conditions, its capacity to reverse responses and recover when the stressor is removed is not well-understood. In this study, we investigated among-colony responses of Pocillopora acuta from two sites with differing distance to the mainland (Kusu (closer to the mainland) and Raffles Lighthouse (further from the mainland)) to heat stress through differential expression analysis of target genes and quantification of photophysiological metrics. We then examined how these attributes were regulated after the stressor was removed to assess the recovery potential of P. acuta. The fragments that were subjected to heat stress (2 °C above ambient levels) generally exhibited significant reduction in their endosymbiont densities, but the extent of recovery following stress removal varied depending on natal site and colony. There were minimal changes in chl a concentration and maximum quantum yield (Fv/Fm, the proportion of variable fluorescence (Fv) to maximum fluorescence (Fm)) in heat-stressed corals, suggesting that the algal endosymbionts’ Photosystem II was not severely compromised. Significant changes in gene expression levels of selected genes of interest (GOI) were observed following heat exposure and stress removal among sites and colonies, including Actin, calcium/calmodulin-dependent protein kinase type IV (Camk4), kinesin-like protein (KIF9), and small heat shock protein 16.1 (Hsp16.1). The most responsive GOIs were Actin, a major component of the cytoskeleton, and the adaptive immune-related Camk4 which both showed significant reduction following heat exposure and subsequent upregulation during the recovery phase. Our findings clearly demonstrate specific responses of P. acuta in both photophysiological attributes and gene expression levels, suggesting differential capacity of P. acuta corals to tolerate heat stress depending on the colony, so that certain colonies may be more resilient than others.

Measuring light scattering and absorption in corals with Inverse Spectroscopic Optical Coherence Tomography (ISOCT): a new tool for non-invasive monitoring

The success of reef-building corals for >200 million years has been dependent on the mutualistic interaction between the coral host and its photosynthetic endosymbiont dinoflagellates (family Symbiodiniaceae) that supply the coral host with nutrients and energy for growth and calcification. While multiple light scattering in coral tissue and skeleton significantly enhance the light microenvironment for Symbiodiniaceae, the mechanisms of light propagation in tissue and skeleton remain largely unknown due to a lack of technologies to measure the intrinsic optical properties of both compartments in live corals. Here we introduce ISOCT (inverse spectroscopic optical coherence tomography), a non-invasive approach to measure optical properties and three-dimensional morphology of living corals at micron- and nano-length scales, respectively, which are involved in the control of light propagation. ISOCT enables measurements of optical properties in the visible range and thus allows for characterization of the density of light harvesting pigments in coral. We used ISOCT to characterize the optical scattering coefficient (μ_s) of the coral skeleton and chlorophyll a concentration of live coral tissue. ISOCT further characterized the overall micro- and nano-morphology of live tissue by measuring differences in the sub-micron spatial mass density distribution (D) that vary throughout the tissue and skeleton and give rise to light scattering, and this enabled estimates of the spatial directionality of light scattering, i.e., the anisotropy coefficient, g. Thus, ISOCT enables imaging of coral nanoscale structures and allows for quantifying light scattering and pigment absorption in live corals. ISOCT could thus be developed into an important tool for rapid, non-invasive monitoring of coral health, growth and photophysiology with unprecedented spatial resolution.

Family matters: Variation in the physiology of brooded Porites astreoides larvae is driven by parent colony effects

The planktonic larval phase of scleractinian coral life-history represents a crucial stage when dispersal takes place and genetic diversity among populations is maintained. Understanding the dynamics influencing larval survival is especially relevant in the context of climate change, as larvae may be more vulnerable to environmental disturbances than adults. Several physiological parameters of coral larvae have been shown to vary by release time and past environmental history. However, the contribution of parental or genetic effects is largely unknown. To investigate these potential familial effects, we collected adult Porites astreoides colonies in April 2018 from two reef zones in the lower Florida Keys and quantified physiological traits and thermal tolerance of the newly released larvae. Family accounted for more variation than day of release and reef origin, with >60% of the variation in chlorophyll a and protein content explained by family. The survivorship of larvae under 36 °C acute temperature stress was also tightly linked to what parent colony they were released from. During a 32 °C moderate temperature stress experiment, inshore larvae tended to bleach less than offshore larvae, mirroring the enhanced bleaching resistance previously observed in inshore adult coral populations. The significant familial effects identified in the present study suggest that researchers should be cautious when interpreting results of studies which pool larvae among families, and that future studies should take care to account for this variation.

Proteomic Basis of Symbiosis: A Heterologous Partner Fails to Duplicate Homologous Colonization in a Novel Cnidarian- Symbiodiniaceae Mutualism

Reef corals and sea anemones form symbioses with unicellular symbiotic dinoflagellates. The molecular circumventions that underlie the successful intracellular colonization of hosts by symbionts are still largely unknown. We conducted proteomic analyses to determine molecular differences of Exaiptasia pallida anemones colonized by physiologically different symbiont species, in comparison with symbiont-free (aposymbiotic) anemones. We compared one homologous species, Symbiodinium linucheae, that is natively associated with the clonal Exaiptasia strain (CC7) to another heterologous species, Durusdinium trenchii, a thermally tolerant species that colonizes numerous coral species. This approach allowed the discovery of a core set of host genes that are differentially regulated as a function of symbiosis regardless of symbiont species. The findings revealed that symbiont colonization at higher densities requires circumvention of the host cellular immunological response, enhancement of ammonium regulation, and suppression of phagocytosis after a host cell in colonized. Furthermore, the heterologous symbionts failed to duplicate the same level of homologous colonization within the host, evidenced by substantially lower symbiont densities. This reduced colonization of D. trenchii correlated with its inability to circumvent key host systems including autophagy-suppressing modulators, cytoskeletal alteration, and isomerase activity. The larger capability of host molecular circumvention by homologous symbionts could be the result of a longer evolutionary history of host/symbiont interactions, which translates into a more finely tuned symbiosis. These findings are of great importance within the context of the response of reef corals to climate change since it has been suggested that coral may acclimatize to ocean warming by changing their dominant symbiont species.

Experimental Approaches for the Evaluation of Allelopathic Interactions Between Hermatypic Corals and Marine Benthic Cyanobacteria in the Colombian Caribbean

Blooms of marine benthic cyanobacteria are recurrent in several locations at the Colombian Caribbean. In these events, cyanobacteria grow over the substrate and benthic organisms although their effect has not been fully assessed. This study evaluated interactions between cyanobacteria and hermatypic corals, in order to identify any deleterious effects that could be related to allelopathic mechanisms. Organic extracts from cyanobacteria collected in San Andres, Old Providence and Rosario islands were tested against embryos of the reef-building coral Orbicella annularis. The indirect effect of cyanobacterial extracts was also assessed by resuspending the extracts in seawater and monitoring polyp retraction and recovery of the coral Madracis mirabilis (=auretenra). Additionally, the effect of direct contact between cyanobacterial extracts and the coral Porites porites was assessed by incorporating cyanobacterial extracts into PhytagelTM gels and placed in direct contact with the coral. After 24, 48 and 72 h of exposure, chromatographic profiles of associated zooxanthellae was evaluated by HPLC. A deleterious effect on the zooxanthellae was evidenced by an increase in pheophytin, a degradation product from chlorophyll. The competitive abilities of algae and cyanobacteria should be considered as a constraint to reef restoration initiatives. Cyanobacteria have the ability to compete with corals due to their growth rates, defenses against herbivory and potentially allelopathic mechanisms.

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.

Carotenoids from Marine Organisms: Biological Functions and Industrial Applications

As is the case for terrestrial organisms, carotenoids represent the most common group of pigments in marine environments. They are generally biosynthesized by all autotrophic marine organisms, such as bacteria and archaea, algae and fungi. Some heterotrophic organisms also contain carotenoids probably accumulated from food or partly modified through metabolic reactions. These natural pigments are divided into two chemical classes: carotenes (such as lycopene and α- and β-carotene) that are composed of hydrogen and carbon; xanthophylls (such as astaxanthin, fucoxanthin and lutein), which are constituted by hydrogen, carbon and oxygen. Carotenoids, as antioxidant compounds, assume a key role in the protection of cells. In fact, quenching of singlet oxygen, light capture and photosynthesis protection are the most relevant biological functions of carotenoids. The present review aims at describing (i) the biological functions of carotenoids and their benefits for human health, (ii) the most common carotenoids from marine organisms and (iii) carotenoids having large success in pharmaceutical, nutraceutical and cosmeceutical industries, highlighting the scientific progress in marine species cultivation for natural pigments production.

Natural Variation in Responses to Acute Heat and Cold Stress in a Sea Anemone Model System for Coral Bleaching

Rising ocean temperatures disrupt the symbiosis between corals and their microalgae, accelerating global decline of coral reef ecosystems. Because of the difficulty of performing laboratory experiments with corals, the sea anemone Aiptasia has emerged as an important model system for molecular studies of coral bleaching and symbiosis. Here, we investigate natural variation in bleaching responses among different genetic lineages of Aiptasia. Both heat- and cold-induced paths to symbiosis breakdown were analyzed. Significant genetic variation in response to acute heat stress was observed, with severe bleaching of two Aiptasia strains from Hawaii but minimal bleaching of strains from the U.S. South Atlantic, including the strain used to generate the Aiptasia reference genome. Both strains from Hawaii hosted Symbiodinium type B1, whereas strains from the U.S. South Atlantic hosted type A4 or B2. In contrast to the results from exposures to acute heat stress, negligible variation was observed in response to a pulsed cold shock despite moderate bleaching across all strains. These results support our hypothesis that bleaching responses to distinct stressors are independent. Our findings emphasize the role of stress regime when predicting adaptive responses of symbiotic cnidarians to changing climates, because genetic variation may exist for some forms of stress-induced bleaching but not others.

Symbiont dynamics during thermal acclimation using cnidarian-dinoflagellate model holobionts

Warming oceans menace reef ecosystems by disrupting symbiosis between cnidarians and Symbiodinium zooxanthellae, thus triggering bleach episodes. Temperature fluctuations promote adjustments in physiological variables and symbiont composition, which can cause stress responses, but can also yield adaptation if fitter host-symbiont homeostasis are achieved. To understand such processes manipulative studies are required, but many reef-building cnidarians pose limitations to experimental prospects. We exposed Exaiptasia anemones to Gradual Thermal Stress (GTS) and Heat Shock (HS) exposures and monitored chlorophyll and symbiont dynamics to test the phenotypic plasticity of these photosynthetic holobionts. GTS enhanced chlorophyll concentrations and decreased Symbiodinium proliferation. A recovery period after GTS returned chlorophyll to lower concentrations and symbiont divisions to higher rates. HS triggered a stress response characterized by intense symbiont declines through degradation and expulsion, algal compensatory proliferation, and chlorophyll accumulation. Anemones pre-exposed to GTS displayed more acute signs of symbiont paucity after HS, demonstrating that recurrent stress does not always induce bleaching-resistance. Our study is the first documenting Symbiodinium C and D, along with the predominant Clade B1 in Exaiptasia anemones. C subclades found in outdoor specimens faded under laboratory exposures. Clade D emerged after HS treatments, and especially after GTS pre-exposure. This highlights the thermotolerance of D subclades found in E. pallida and shows that bleaching-recovery can involve shifts of background symbiont phylotypes. This study enlightens the capability of Exaiptasia anemones to acclimate to gradually increased temperatures, and explores into how thermal history influences in subsequent stress tolerance in symbiotic cnidarians.

Effects of Temperature and pCO2 on Population Regulation of Symbiodinium spp. in a Tropical Reef Coral

This study tested the bleaching response of the Pacific coral Seriatopora caliendrum to short-term exposure to high temperature and elevated partial pressure of carbon dioxide (pCO₂). Juvenile colonies collected from Nanwan Bay, Taiwan, were used in a factorial experimental design in which 2 temperatures (∼27.6 °C and ∼30.4 °C) and 2 pCO₂ values (∼47.2 Pa and ∼90.7 Pa) were crossed to evaluate, over 12 days, the effects on the densities and physiology of the symbiotic dinoflagellates (Symbiodinium) in the corals. Thermal bleaching, as defined by a reduction of Symbiodinium densities at high temperature, was unaffected by high pCO₂. The division, or mitotic index (MI), of Symbiodinium remaining in thermally bleached corals was about 35% lower than in control colonies, but they contained about 53% more chlorophyll. Bleaching was highly variable among colonies, but the differences were unrelated to MI or pigment content of Symbiodinium remaining in the coral host. At the end of the study, all of the corals contained clade C Symbiodinium (either C1d or C15), and the genetic variation of symbionts did not account for among-colony bleaching differences. These results showed that high temperature causes coral bleaching independent of pCO₂, and underscores the potential role of the coral host in driving intraspecific variation in coral bleaching.

Photoacclimatory and photoprotective responses to cold versus heat stress in high latitude reef corals

Corals at the world's southernmost coral reef of Lord Howe Island (LHI) experience large temperature and light fluctuations and need to deal with periods of cold temperature (<18°C), but few studies have investigated how corals are able to cope with these conditions. Our study characterized the response of key photophysiological parameters, as well as photoacclimatory and photoprotective pigments (chlorophylls, xanthophylls, and β-carotene), to short-term (5-d) cold stress (~15°C; 7°C below control) in three LHI coral species hosting distinct Symbiodinium ITS2 types, and compared the coral-symbiont response to that under elevated temperature (~29°C; 7°C above control). Under cold stress, Stylophora sp. hosting Symbiodinium C118 showed the strongest effects with regard to losses of photochemical performance and symbionts. Pocillopora damicornis hosting Symbiodinium C100/C118 showed less severe bleaching responses to reduced temperature than to elevated temperature, while Porites heronensis hosting Symbiodinium C111* withstood both reduced and elevated temperature. Under cold stress, photoprotection in the form of xanthophyll de-epoxidation increased in unbleached P. heronensis (by 178%) and bleached Stylophora sp. (by 225%), while under heat stress this parameter increased in unbleached P. heronensis (by 182%) and in bleached P. damicornis (by 286%). The xanthophyll pool size was stable in all species at all temperatures. Our comparative study demonstrates high variability in the bleaching vulnerability of these coral species to low and high thermal extremes and shows that this variability is not solely determined by the ability to activate xanthophyll de-epoxidation.

Do the shuffle: Changes in Symbiodinium consortia throughout juvenile coral development

Previous studies of symbiotic associations between scleractinians corals and Symbiodinium have demonstrated that the consortium of symbionts can change in response to environmental conditions. However, less is known about symbiont shuffling during early coral development, particularly in brooding species. This study examined whether Symbiodinium consortia (1) varied in Porites astreoides on shallow (10m) and upper mesophotic (30m) reefs, (2) changed during coral development, and (3) influenced growth of juveniles in different environments. Symbiodinium ITS2 sequences were amplified using universal primers and analyzed using phylotype-specific primers designed for phylotypes A, B, and C. Adults from both depths were found to host only phylotype A, phylotypes A and B, or phylotypes A, B, and C and the frequency of the phylotype composition did not vary with depth. However, phylotype A was the dominant symbiont that was vertically transmitted to the planulae. The presence of phylotypes B and C was detected in the majority of juveniles when transplanted onto the shallow and upper mesophotic reefs whereas only phylotype A was detected in the majority of juveniles reared in outdoor aquaria. In addition, growth of juvenile P. astreoides harboring different combinations of Symbiodinium phylotypes did not vary when transplanted to different reef zones. However, juveniles reared in in situ reef environments grew faster than those reared in ex situ outdoor aquaria. These results show that Symbiodinium consortia change during development of P. astreoides and are influenced by environmental conditions.

The Coral Trait Database, a curated database of trait information for coral species from the global oceans

Trait-based approaches advance ecological and evolutionary research because traits provide a strong link to an organism's function and fitness. Trait-based research might lead to a deeper understanding of the functions of, and services provided by, ecosystems, thereby improving management, which is vital in the current era of rapid environmental change. Coral reef scientists have long collected trait data for corals; however, these are difficult to access and often under-utilized in addressing large-scale questions. We present the Coral Trait Database initiative that aims to bring together physiological, morphological, ecological, phylogenetic and biogeographic trait information into a single repository. The database houses species- and individual-level data from published field and experimental studies alongside contextual data that provide important framing for analyses. In this data descriptor, we release data for 56 traits for 1547 species, and present a collaborative platform on which other trait data are being actively federated. Our overall goal is for the Coral Trait Database to become an open-source, community-led data clearinghouse that accelerates coral reef research.

Relative Pigment Composition and Remote Sensing Reflectance of Caribbean Shallow-Water Corals

Reef corals typically contain a number of pigments, mostly due to their symbiotic relationship with photosynthetic dinoflagellates. These pigments usually vary in presence and concentration and influence the spectral characteristics of corals. We studied the variations in pigment composition among seven Caribbean shallow-water Scleractinian corals by means of High Performance Liquid Chromatography (HPLC) analysis to further resolve the discrimination of corals. We found a total of 27 different pigments among the coral species, including some alteration products of the main pigments. Additionally, pigments typically found in endolithic algae were also identified. A Principal Components Analysis and a Hierarchical Cluster Analysis showed the separation of coral species based on pigment composition. All the corals were collected under the same physical environmental conditions. This suggests that pigment in the coral’s symbionts might be more genetically-determined than influenced by prevailing physical conditions of the reef. We further investigated the use of remote sensing reflectance (Rrs) as a tool for estimating the total pigment concentration of reef corals. Depending on the coral species, the Rrs and the total symbiont pigment concentration per coral tissue area correlation showed 79.5–98.5% confidence levels demonstrating its use as a non-invasive robust technique to estimate pigment concentration in studies of coral reef biodiversity and health.

Rapid, Precise, and Accurate Counts of Symbiodinium Cells Using the Guava Flow Cytometer, and a Comparison to Other Methods

In studies of both the establishment and breakdown of cnidarian-dinoflagellate symbiosis, it is often necessary to determine the number of Symbiodinium cells relative to the quantity of host tissue. Ideally, the methods used should be rapid, precise, and accurate. In this study, we systematically evaluated methods for sample preparation and storage and the counting of algal cells using the hemocytometer, a custom image-analysis program for automated counting of the fluorescent algal cells, the Coulter Counter, or the Millipore Guava flow-cytometer. We found that although other methods may have value in particular applications, for most purposes, the Guava flow cytometer provided by far the best combination of precision, accuracy, and efficient use of investigator time (due to the instrument's automated sample handling), while also allowing counts of algal numbers over a wide range and in small volumes of tissue homogenate. We also found that either of two assays of total homogenate protein provided a precise and seemingly accurate basis for normalization of algal counts to the total amount of holobiont tissue.

Evidence for multiple stressor interactions and effects on coral reefs

Concern is growing about the potential effects of interacting multiple stressors, especially as the global climate changes. We provide a comprehensive review of multiple stressor interactions in coral reef ecosystems, which are widely considered to be one of the most sensitive ecosystems to global change. First, we synthesized coral reef studies that examined interactions of two or more stressors, highlighting stressor interactions (where one stressor directly influences another) and potentially synergistic effects on response variables (where two stressors interact to produce an effect that is greater than purely additive). For stressor-stressor interactions, we found 176 studies that examined at least 2 of the 13 stressors of interest. Applying network analysis to analyze relationships between stressors, we found that pathogens were exacerbated by more costressors than any other stressor, with ca. 78% of studies reporting an enhancing effect by another stressor. Sedimentation, storms, and water temperature directly affected the largest number of other stressors. Pathogens, nutrients, and crown-of-thorns starfish were the most-influenced stressors. We found 187 studies that examined the effects of two or more stressors on a third dependent variable. The interaction of irradiance and temperature on corals has been the subject of more research (62 studies, 33% of the total) than any other combination of stressors, with many studies reporting a synergistic effect on coral symbiont photosynthetic performance (n = 19). Second, we performed a quantitative meta-analysis of existing literature on this most-studied interaction (irradiance and temperature). We found that the mean effect size of combined treatments was statistically indistinguishable from a purely additive interaction, although it should be noted that the sample size was relatively small (n = 26). Overall, although in aggregate a large body of literature examines stressor effects on coral reefs and coral organisms, considerable gaps remain for numerous stressor interactions and effects, and insufficient quantitative evidence exists to suggest that the prevailing type of stressor interaction is synergistic.

Simultaneous effect of temperature and irradiance on growth and okadaic acid production from the marine dinoflagellate Prorocentrum belizeanum

Benthic marine dioflagellate microalgae belonging to the genus Prorocentrum are a major source of okadaic acid (OA), OA analogues and polyketides. However, dinoflagellates produce these valuable toxins and bioactives in tiny quantities, and they grow slowly compared to other commercially used microalgae. This hinders evaluation in possible large-scale applications. The careful selection of producer species is therefore crucial for success in a hypothetical scale-up of culture, as are appropriate environmental conditions for optimal growth. A clone of the marine toxic dinoflagellate P. belizeanum was studied in vitro to evaluate its capacities to grow and produce OA as an indicator of general polyketide toxin production under the simultaneous influence of temperature (T) and irradiance (I0). Three temperatures and four irradiance levels were tested (18, 25 and 28 °C; 20, 40, 80 and 120 µE·(m-2)·s(-1)), and the response variables measured were concentration of cells, maximum photochemical yield of photosystem II (PSII), pigments and OA. Experiments were conducted in T-flasks, since their parallelepipedal geometry proved ideal to ensure optically thin cultures, which are essential for reliable modeling of growth-irradiance curves. The net maximum specific growth rate (µ(m)) was 0.204 day(-1) at 25 °C and 40 µE·(m-2)·s(-1). Photo-inhibition was observed at I0 > 40 μEm(-2)s(-1), leading to culture death at 120 µE·m(-2)·s(-1) and 28 °C. Cells at I0 ≥ 80 µE·m(-2)·s(-1) were photoinhibited irrespective of the temperature assayed. A mechanistic model for µ(m)-I0 curves and another empirical model for relating µ(m)-T satisfactorily interpreted the growth kinetics obtained. ANOVA for responses of PSII maximum photochemical yield and pigment profile has demonstrated that P. belizeanum is extremely light sensitive. The pool of photoprotective pigments (diadinoxanthin and dinoxanthin) and peridinin was not able to regulate the excessive light-absorption at high I0-T. OA synthesis in cells was decoupled from optimal growth conditions, as OA overproduction was observed at high temperatures and when both temperature and irradiance were low. T-flask culture observations were consistent with preliminary assays outdoors.

Hyperspectral sensing of disease stress in the Caribbean reef-building coral, Orbicella faveolata - perspectives for the field of coral disease monitoring

The effectiveness of management plans developed for responding to coral disease outbreaks is limited due to the lack of rapid methods of disease diagnosis. In order to fulfill current management guidelines for responding to coral disease outbreaks, alternative methods that significantly reduce response time must be developed. Hyperspectral sensing has been used by various groups to characterize the spectral signatures unique to asymptomatic and bleached corals. The 2010 combined bleaching and Caribbean yellow band disease outbreak in Puerto Rico provided a unique opportunity to investigate the spectral signatures associated with bleached and Caribbean yellow band-diseased colonies of Orbicella faveolata for the first time. Using derivative and cluster analyses of hyperspectral reflectance data, the present study demonstrates the proof of concept that spectral signatures can be used to differentiate between coral disease states. This method enhanced predominant visual methods of diagnosis by distinguishing between different asymptomatic conditions that are identical in field observations and photographic records. The ability to identify disease-affected tissue before lesions become visible could greatly reduce response times to coral disease outbreaks in monitoring efforts. Finally, spectral signatures associated with the poorly understood Caribbean yellow band disease are presented to guide future research on the role of pigments in the etiology.

Extract from the zooxanthellate jellyfish Cotylorhiza tuberculata modulates gap junction intercellular communication in human cell cultures

On a global scale, jellyfish populations in coastal marine ecosystems exhibit increasing trends of abundance. High-density outbreaks may directly or indirectly affect human economical and recreational activities, as well as public health. As the interest in biology of marine jellyfish grows, a number of jellyfish metabolites with healthy potential, such as anticancer or antioxidant activities, is increasingly reported. In this study, the Mediterranean “fried egg jellyfish” Cotylorhiza tuberculata (Macri, 1778) has been targeted in the search forputative valuable bioactive compounds. A medusa extract was obtained, fractionated, characterized by HPLC, GC-MS and SDS-PAGE and assayed for its biological activity on breast cancer cells (MCF-7) and human epidermal keratinocytes (HEKa). The composition of the jellyfish extract included photosynthetic pigments, valuable ω-3 and ω-6 fatty acids, and polypeptides derived either from jellyfish tissues and their algal symbionts. Extract fractions showed antioxidant activity and the ability to affect cell viability and intercellular communication mediated by gap junctions (GJIC) differentially in MCF-7and HEKa cells. A significantly higher cytotoxicity and GJIC enhancement in MCF-7 compared to HEKa cells was recorded. A putative action mechanism for the anticancer bioactivity through the modulation of GJIC has been hypothesized and its nutraceutical and pharmaceutical potential was discussed.

Thermal acclimation of the symbiotic alga Symbiodinium spp. alleviates photobleaching under heat stress

A moderate increase in seawater temperature causes coral bleaching, at least partially through photobleaching of the symbiotic algae Symbiodinium spp. Photobleaching of Symbiodinium spp. is primarily associated with the loss of light-harvesting proteins of photosystem II (PSII) and follows the inactivation of PSII under heat stress. Here, we examined the effect of increased growth temperature on the change in sensitivity of Symbiodinium spp. PSII inactivation and photobleaching under heat stress. When Symbiodinium spp. cells were grown at 25°C and 30°C, the thermal tolerance of PSII, measured by the thermal stability of the maximum quantum yield of PSII in darkness, was commonly enhanced in all six Symbiodinium spp. tested. In Symbiodinium sp. CCMP827, it took 6 h to acquire the maximum PSII thermal tolerance after transfer from 25°C to 30°C. The effect of increased growth temperature on the thermal tolerance of PSII was completely abolished by chloramphenicol, indicating that the acclimation mechanism of PSII is associated with the de novo synthesis of proteins. When CCMP827 cells were exposed to light at temperature ranging from 25°C to 35°C, the sensitivity of cells to both high temperature-induced photoinhibition and photobleaching was ameliorated by increased growth temperatures. These results demonstrate that thermal acclimation of Symbiodinium spp. helps to improve the thermal tolerance of PSII, resulting in reduced inactivation of PSII and algal photobleaching. These results suggest that whole-organism coral bleaching associated with algal photobleaching can be at least partially suppressed by the thermal acclimation of Symbiodinium spp. at higher growth temperatures.

Structural Confirmation of a Unique Carotenoid Lactoside, P457, in Symbiodinium sp. Strain nbrc 104787 Isolated from a Sea Anemone and its Distribution in Dinoflagellates and Various Marine Organisms

The molecular structure of the carotenoid lactoside P457, (3S,5R,6R,3'S,5'R,6'S)-13'-cis-5,6-epoxy-3',5'-dihydroxy-3-(β-d-galactosyl-(1→4)-β-d-glucosyl)oxy-6',7'-didehydro-5,6,7,8,5',6'-hexahydro-β,β-caroten-20-al, was confirmed by spectroscopic methods using Symbiodinium sp. strain NBRC 104787 cells isolated from a sea anemone. Among various algae, cyanobacteria, land plants, and marine invertebrates, the distribution of this unique diglycosyl carotenoid was restricted to free-living peridinin-containing dinoflagellates and marine invertebrates that harbor peridinin-containing zooxanthellae. Neoxanthin appeared to be a common precursor for biosynthesis of peridinin and P457, although neoxanthin was not found in peridinin-containing dinoflagellates. Fucoxanthin-containing dinoflagellates did not possess peridinin or P457; green dinoflagellates, which contain chlorophyll a and b, did not contain peridinin, fucoxanthin, or P457; and no unicellular algae containing both peridinin and P457, other than peridinin-containing dinoflagellates, have been observed. Therefore, the biosynthetic pathways for peridinin and P457 may have been coestablished during the evolution of dinoflagellates after the host heterotrophic eukaryotic microorganism formed a symbiotic association with red alga that does not contain peridinin or P457.

Bleaching response of Symbiodinium (zooxanthellae): determination by flow cytometry

Coral bleaching is of increasing concern to reef management and stakeholders. Thus far, quantification of coral bleaching tends to be heavily reliant on the enumeration of zooxanthellae, with less emphasis on assessment of photosynthetic or physiological condition, these being often assessed separately by techniques such as liquid chromatography. Traditional methods of enumeration using microscopy are time consuming, subjected to low precision and great observer error. In this study, we presented a method for the distinction of physoiological condition and rapid enumeration of zooxanthellae using flow cytometry (FCM). Microscopy verified that healthy looking/live versus damaged/dead zooxanthellae could be reliably and objectively distinguished and counted by FCM on the basis of red and green fluorescence and light scatter. Excellent correlations were also determined between FCM and microscopy estimates of cell concentrations of fresh zooxanthellae isolates from Pocillopora damicornis. The relative intensities of chlorophyll and β-carotene fluorescences were shown to be important in understanding the results of increased cell counts in freshly isolated zooxanthellae experimentally exposed to high temperatures (34, 36, and 38°C) over 24 h, with ambient temperature (29°C) used as controls. The ability to simultaneously identify and enumerate subpopulations of different physiological states in the same sample provides an enormous advantage in not just determining bleaching responses, but elucidating adaptive response and mechanisms for tolerance. Therefore, this approach might provide a rapid, convenient, and reproducible methodology for climate change studies and reef management programs.

Cold induces acute stress but heat is ultimately more deleterious for the reef-building coral Acropora yongei

Climate change driven increases in intensity and frequency of both hot and cold extreme events contribute to coral reef decline by causing widespread coral bleaching and mortality. Here, we show that hot and cold temperature changes cause distinct physiological responses on different time scales in reef-building corals. We exposed the branching coral Acropora yongei in individual aquaria to a ± 5°C temperature change. Compared to heat-treated corals, cold-treated corals initially show greater declines in growth and increases in photosynthetic pressure. However, after 2–3 weeks, cold-treated corals acclimate and show improvements in physiological state. In contrast, heat did not initially harm photochemical efficiency, but after a delay, photosynthetic pressure increased rapidly and corals experienced severe bleaching and cessation of growth. These results suggest that short-term cold temperature is more damaging for branching corals than short-term warm temperature, whereas long-term elevated temperature is more harmful than long-term depressed temperature.

Importance of time and place: patterns in abundance of Symbiodinium clades A and B in the tropical sea anemone Condylactis gigantea

The capacity of some corals and other cnidarians to form symbioses with multiple algae (Symbiodinium) is a candidate route by which these symbioses tolerate variable environmental conditions. On Bermuda, the coral reef dwelling anemone Condylactis gigantea bears Symbiodinium of clades A and B. At thermally variable inshore and nearshore sites, clade A predominates (as sole symbiont or in mixed infection with clade B), whereas animals at offshore sites with more uniform temperatures bear only clade B or mixed infections. Individual animals at one nearshore site monitored over a year by sampling tentacles showed increased prevalence of clade A in March-November, when sea waters were warm (average 26 degrees C), and increased clade B in November-March when cool waters prevailed (average 18.5 degrees C). In laboratory analyses of excised tentacles, the symbiosis with clade B, but not clade A, bleached at elevated temperature (32 degrees C), suggesting that thermal tolerance may contribute to the higher prevalence of clade A at inshore/nearshore sites and in the summer. The temporal changes in the algal complement were not accompanied by bleaching, and Symbiodinium density fluctuated in hosts with stable Symbiodinium composition but not in hosts with variable composition. This suggests that changes in the relative abundance of Symbiodinium clades do not require bleaching and may even protect the symbiosis from large fluctuations in algal density.

Enhanced photoprotection pathways in symbiotic dinoflagellates of shallow-water corals and other cnidarians

Photoinhibition, exacerbated by elevated temperatures, underlies coral bleaching, but sensitivity to photosynthetic loss differs among various phylotypes of Symbiodinium, their dinoflagellate symbionts. Symbiodinium is a common symbiont in many cnidarian species including corals, jellyfish, anemones, and giant clams. Here, we provide evidence that most members of clade A Symbiodinium, but not clades B-D or F, exhibit enhanced capabilities for alternative photosynthetic electron-transport pathways including cyclic electron transport (CET). Unlike other clades, clade A Symbiodinium also undergo pronounced light-induced dissociation of antenna complexes from photosystem II (PSII) reaction centers. We propose these attributes promote survival of most cnidarians with clade A symbionts at high light intensities and confer resistance to bleaching conditions that conspicuously impact deeper dwelling corals that harbor non-clade A Symbiodinium.

Heat stress causes inhibition of the de novo synthesis of antenna proteins and photobleaching in cultured Symbiodinium

Coral bleaching, caused by heat stress, is accompanied by the light-induced loss of photosynthetic pigments in in situ symbiotic dinoflagellate algae (Symbiodinium spp.). However, the molecular mechanisms responsible for pigment loss are poorly understood. Here, we show that moderate heat stress causes photobleaching through inhibition of the de novo synthesis of intrinsic light-harvesting antennae [chlorophyll a-chlorophyll c(2)-peridinin-protein complexes (acpPC)] in cultured Symbiodinium algae and that two Clade A Symbiodinium species showing different thermal sensitivities of photobleaching also show differential sensitivity of this key protein synthesis process. Photoinhibition of photosystem II (PSII) and subsequent photobleaching were observed at temperatures of >31 degrees C in cultured Symbiodinium CS-73 cells grown at 25-34 degrees C, but not in cultures of the more thermally tolerant control Symbiodinium species OTcH-1. We found that bleaching in CS-73 is associated with loss of acpPC, which is a major antennae protein in Symbiodinium. In addition, the thermally induced loss of this protein is light-dependent, but does not coincide directly with PSII photoinhibition and is not caused by stimulated degradation of acpPC. In cells treated at 34 degrees C over 24 h, the steady-state acpPC mRNA pool was modestly reduced, by approximately 30%, whereas the corresponding synthesis rate of acpPC was diminished by >80%. Our results suggest that photobleaching in Symbiodinium is consequentially linked to the relative susceptibility of PSII to photoinhibition during thermal stress and occurs, at least partially, because of the loss of acpPC via undefined mechanism(s) that hamper the de novo synthesis of acpPC primarily at the translational processing step.