The adaptive bleaching hypothesis: experimental tests of critical assumptions

Triggers, cascades, and endpoints: connecting the dots of coral bleaching mechanisms

The intracellular coral-dinoflagellate symbiosis is the engine that underpins the success of coral reefs, one of the most diverse ecosystems on the planet. However, the breakdown of the symbiosis and the loss of the microalgal symbiont (i.e. coral bleaching) due to environmental changes are resulting in the rapid degradation of coral reefs globally. There is an urgent need to understand the cellular physiology of coral bleaching at the mechanistic level to help develop solutions to mitigate the coral reef crisis. Here, at an unprecedented scope, we present novel models that integrate putative mechanisms of coral bleaching within a common framework according to the triggers (initiators of bleaching, e.g. heat, cold, light stress, hypoxia, hyposalinity), cascades (cellular pathways, e.g. photoinhibition, unfolded protein response, nitric oxide), and endpoints (mechanisms of symbiont loss, e.g. apoptosis, necrosis, exocytosis/vomocytosis). The models are supported by direct evidence from cnidarian systems, and indirectly through comparative evolutionary analyses from non-cnidarian systems. With this approach, new putative mechanisms have been established within and between cascades initiated by different bleaching triggers. In particular, the models provide new insights into the poorly understood connections between bleaching cascades and endpoints and highlight the role of a new mechanism of symbiont loss, i.e. 'symbiolysosomal digestion', which is different from symbiophagy. This review also increases the approachability of bleaching physiology for specialists and non-specialists by mapping the vast landscape of bleaching mechanisms in an atlas of comprehensible and detailed mechanistic models. We then discuss major knowledge gaps and how future research may improve the understanding of the connections between the diverse cascade of cellular pathways and the mechanisms of symbiont loss (endpoints).

A gauge of coral physiology: re-examining temporal changes in Endozoicomonas abundance correlated with natural coral bleaching

Bacteria contribute to many physiological functions of coral holobionts, including responses to bleaching. The bacterial genus, Endozoicomonas, dominates the microbial flora of many coral species and its abundance appears to be correlated with coral bleaching. However, evidences for decoupling of bleaching and Endozoicomonas abundance changes have also been reported. In 2020, a severe bleaching event was recorded at reefs in Taiwan, providing a unique opportunity to re-examine bleaching-Endozoicomonas association using multiple stony corals in natural environments. In this study, we monitored tissue color and microbiome changes in three coral species (Montipora sp., Porites sp., and Stylophora pistillata) in Kenting National Park, following the bleaching event. All tagged Montipora sp. and Porites sp. recovered from bleaching within 1 year, while high mortality occurred in S. pistillata. Microbiome analysis found no correlation of Endozoicomonas relative abundance and bleaching severity during the sampling period, but found a stronger correlation when the month in which bleaching occurred was excluded. Moreover, Endozoicomonas abundance increased during recovery months in Montipora sp. and Porites sp., whereas in S. pistillata it was nearly depleted. These results suggest that Endozoicomonas abundance may represent a gauge of coral health and reflect recovery of some corals from stress. Interestingly, even though different Endozoicomonas strains predominated in the three corals, these Endozoicomonas strains were also shared among coral taxa. Meanwhile, several Endozoicomonas strains showed secondary emergence during coral recovery, suggesting possible symbiont switching in Endozoicomonas. These findings indicate that it may be possible to introduce Endozoicomonas to non-native coral hosts as a coral probiotic.

Marine Heatwave Caused Differentiated Dysbiosis in Photosymbiont Assemblages of Corals and Hydrocorals During El Niño 2015/2016

Reef corals have been threatened by climate change, with more frequent and intense bleaching events leading to extensive coral mortality and loss of coral cover worldwide. In the face of this, the corals' photosymbiont assemblages have received special attention as a key to better understand the bleaching process and its recovery. To assess the effects of thermal anomalies, the coral Mussismilia harttii and the hydrocoral Millepora alcicornis were monitored through the El Niño 2015/2016 at a Southwestern Atlantic (SWA) coral reef. A severe bleaching event (57% of colonies bleached) was documented, triggered by a < 3 °C-week heatwave, but no mortality was detected. The hydrocoral was more susceptible than the scleractinian, displaying bleaching symptoms earlier and experiencing a longer and more intense bleaching event. The composition of photosymbionts in the M. alcicornis population was affected only at the rare biosphere level (< 5% relative abundance), with the emergence of new symbionts after bleaching. Conversely, a temporary dysbiosis was observed in the M. harttii population, with one of the dominant symbiodiniaceans decreasing in relative abundance at the peak of the bleaching, which negatively affected the total β-diversity. After colonies' complete recovery, symbiodiniaceans' dominances returned to normal levels in both hosts. These results highlight critical differences in how the two coral species cope with bleaching and contribute to the understanding of the role of photosymbionts throughout the bleaching-recovery process.

Defining Coral Bleaching as a Microbial Dysbiosis within the Coral Holobiont

Coral microbiomes are critical to holobiont health and functioning, but the stability of host–microbial interactions is fragile, easily shifting from eubiosis to dysbiosis. The heat-induced breakdown of the symbiosis between the host and its dinoflagellate algae (that is, “bleaching”), is one of the most devastating outcomes for reef ecosystems. Yet, bleaching tolerance has been observed in some coral species. This review provides an overview of the holobiont’s diversity, explores coral thermal tolerance in relation to their associated microorganisms, discusses the hypothesis of adaptive dysbiosis as a mechanism of environmental adaptation, mentions potential solutions to mitigate bleaching, and suggests new research avenues. More specifically, we define coral bleaching as the succession of three holobiont stages, where the microbiota can (i) maintain essential functions for holobiont homeostasis during stress and/or (ii) act as a buffer to mitigate bleaching by favoring the recruitment of thermally tolerant Symbiodiniaceae species (adaptive dysbiosis), and where (iii) environmental stressors exceed the buffering capacity of both microbial and dinoflagellate partners leading to coral death.

Uncovering the role of Symbiodiniaceae assemblage composition and abundance in coral bleaching response by minimizing sampling and evolutionary biases

BACKGROUND

Biodiversity and productivity of coral-reef ecosystems depend upon reef-building corals and their associations with endosymbiotic Symbiodiniaceae, which offer diverse functional capabilities to their hosts. The number of unique symbiotic partners (richness) and relative abundances (evenness) have been hypothesized to affect host response to climate change induced thermal stress. Symbiodiniaceae assemblages with many unique phylotypes may provide greater physiological flexibility or form less stable symbioses; assemblages with low abundance phylotypes may allow corals to retain thermotolerant symbionts or represent associations with less-suitable symbionts.

RESULTS

Here we demonstrate that true richness of Symbiodiniaceae phylotype assemblages is generally not discoverable from direct enumeration of unique phylotypes in association records and that cross host-species comparisons are biased by sampling and evolutionary patterns among species. These biases can be minimized through rarefaction of richness (rarefied-richness) and evenness (Probability of Interspecific Encounter, PIE), and analyses that account for phylogenetic patterns. These standardized metrics were calculated for individual Symbiodiniaceae assemblages composed of 377 unique ITS2 phylotypes associated with 123 coral species. Rarefied-richness minimized correlations with sampling effort, while maintaining important underlying characteristics across host bathymetry and geography. Phylogenetic comparative methods reveal significant increases in coral bleaching and mortality associated with increasing Symbiodiniaceae assemblage richness and evenness at the level of host species.

CONCLUSIONS

These results indicate that the potential flexibility afforded by assemblages characterized by many phylotypes present at similar relative abundances does not result in decreased bleaching risk and point to the need to characterize the overall functional and genetic diversity of Symbiodiniaceae assemblages to quantify their effect on host fitness under climate change.

Adaptation to Bleaching: Are Thermotolerant Symbiodiniaceae Strains More Successful Than Other Strains Under Elevated Temperatures in a Model Symbiotic Cnidarian?

The ability of some symbiotic cnidarians to resist and better withstand stress factors that cause bleaching is a trait that is receiving increased attention. The adaptive bleaching hypothesis postulates that cnidarians that can form a stable symbiosis with thermotolerant Symbiodiniaceae strains may cope better with increasing seawater temperatures. We used polyps of the scyphozoan, Cassiopea xamachana, as a model system to test symbiosis success under heat stress. We sought to determine: (1) if aposymbiotic C. xamachana polyps could establish and maintain a symbiosis with both native and non-native strains of Symbiodiniaceae that all exhibit different tolerances to heat, (2) whether polyps with these newly acquired Symbiodiniaceae strains would strobilate (produce ephyra), and (3) if thermally tolerant Symbiodiniaceae strains that established and maintained a symbiosis exhibited greater success in response to heat stress (even if they are not naturally occurring in Cassiopea). Following recolonization of aposymbiotic C. xamachana polyps with different strains, we found that: (1) strains Smic, Stri, Slin, and Spil all established a stable symbiosis that promoted strobilation and (2) strains Bmin1 and Bmin2 did not establish a stable symbiosis and strobilation did not occur. Strains Smic, Stri, Slin, and Spil were used in a subsequent bleaching experiment; each of the strains was introduced to a subset of aposymbiotic polyps and once polyp tissues were saturated with symbionts they were subjected to elevated temperatures - 32°C and 34°C - for 2 weeks. Our findings indicate that, in general, pairings of polyps with Symbiodiniaceae strains that are native to Cassiopea (Stri and Smic) performed better than a non-native strain (Slin) even though this strain has a high thermotolerance. This suggests a degree of partner specificity that may limit the adaptive potential of certain cnidarians to increased ocean warming. We also observed that the free-living, non-native thermotolerant strain Spil was relatively successful in resisting bleaching during experimental trials. This suggests that free-living Symbiodiniaceae may provide a supply of potentially "new" thermotolerant strains to cnidarians following a bleaching event.

Comparison of Independent Evolutionary Origins Reveals Both Convergence and Divergence in the Metabolic Mechanisms of Symbiosis

Through the merger of previously independent lineages, symbiosis promotes the acquisition of new traits and exploitation of inaccessible ecological niches [1, 2], driving evolutionary innovation and important ecosystem functions [3-6]. The transient nature of establishment makes study of symbiotic origins difficult, but experimental comparison of independent origins could reveal the degree of convergence in the underpinning mechanisms [7, 8]. We compared the metabolic mechanisms of two independent origins of Paramecium bursaria-Chlorella photosymbiosis [9-11] using a reciprocal metabolomic pulse-chase method. This showed convergent patterns of nutrient exchange and utilization for host-derived nitrogen in the Chlorella genotypes [12, 13] and symbiont-derived carbon in the P. bursaria genotypes [14, 15]. Consistent with a convergent primary nutrient exchange, partner-switched host-symbiont pairings were functional. Direct competition of hosts containing native or recombined symbionts against isogenic symbiont-free hosts showed that the fitness benefits of symbiosis for hosts increased with irradiance but varied by genotype. Global metabolism varied more between the Chlorella than the P. bursaria genotypes and suggested divergent mechanisms of light management. Specifically, the algal symbiont genotypes either produced photo-protective carotenoid pigments at high irradiance or more chlorophyll, resulting in corresponding differences in photosynthetic efficiency and non-photochemical quenching among host-symbiont pairings. These data suggest that the multiple origins of P. bursaria-Chlorella symbiosis use a convergent nutrient exchange, whereas other photosynthetic traits linked to functioning of photosymbiosis have diverged. Although convergence enables partner switching among diverse strains, phenotypic mismatches resulting from divergence of secondary symbiotic traits could mediate host-symbiont specificity in nature.

Endosymbiont-Mediated Adaptive Responses to Stress in Holobionts

Endosymbiosis is found in all types of ecosystems and it can be sensitive to environmental changes due to the intimate interaction between the endosymbiont and the host. Indeed, global climate change disturbs the local ambient environment and threatens endosymbiotic species, and in some cases leads to local ecosystem collapse. Recent studies have revealed that the endosymbiont can affect holobiont (endosymbiont and host together) stress tolerance as much as the host does, and manipulation of the microbial partners in holobionts may mitigate the impacts of the environmental stress. Here, we first show how the endosymbiont presence affects holobiont stress tolerance by discussing three well-studied endosymbiotic systems, which include plant-fungi, aquatic organism-algae, and insect-bacteria systems. We then review how holobionts are able to alter their stress tolerance via associated endosymbionts by changing their endosymbiont composition, by adaptation of their endosymbionts, or by acclimation of their endosymbionts. Finally, we discuss how different transmission modes (vertical or horizontal transmission) might affect the adaptability of holobionts. We propose that the endosymbiont is a good target for modifying holobiont stress tolerance, which makes it critical to more fully investigate the role of endosymbionts in the adaptive responses of holobionts to stress.

Genome-Wide Analysis of Cell Cycle-Regulating Genes in the Symbiotic Dinoflagellate Breviolum minutum

A delicate relationship exists between reef-building corals and their photosynthetic endosymbionts. Unfortunately, this relationship can be disrupted, with corals expelling these algae when temperatures rise even marginally above the average summer maximum. Interestingly, several studies indicate that failure of corals to regulate symbiont cell divisions at high temperatures may underlie this disruption; increased proliferation of symbionts may stress host cells by over-production of reactive oxygen species or by disrupting the flow of nutrients. This needs to be further investigated, so to begin deciphering the molecular mechanisms controlling the cell cycle in these organisms, we used a computational approach to identify putative cell cycle-regulating genes in the genome of the dinoflagellate Breviolum minutum. This species is important as an endosymbiont of Aiptasia pallida—an anemone that is used as a model for studying coral biology. We then correlated expression of these putative cell cycle genes with cell cycle phase in diurnally growing B. minutum in culture. This approach allowed us to identify a cyclin/cyclin-dependent kinase pair that may function in the G1/S transition—a likely point for coral cells to exert control over algal cell divisions.

Long-term exposure to increasing temperatures on scleractinian coral fragments reveals oxidative stress

Global warming is leading to increases in tropical storms' frequency and intensity, allowing fragmentation of reef-forming coral species, but also to coral bleaching and mortality. The first level of organism's response to an environmental perturbation occurs at the cellular level. This study investigated the long-term oxidative stress on fragments of nine Indo-Pacific reef-forming coral species exposed for 60 days to increasing temperatures (30 °C and 32 °C) and compared results with control temperature (26 °C). Coral overall condition (appearance), lipid peroxidation (LPO), catalase activity (CAT), and glutathione S-transferase (GST) were assessed. The species Turbinaria reniformis, Galaxea fascicularis, and Psammocora contigua were the most resistant to heat stress, presenting no oxidative damage at 30 °C. Unlike G. fasciularis, both T. reniformis and P. contigua showed no evidence of oxidative damage at 32 °C. All remaining species' fragments died at 32 °C. Stylophora pistillata and Pocillopora damicornis were the most susceptible species to heat stress, not resisting at 30 °C.

Study on expelled but viable zooxanthellae from giant clams, with an emphasis on their potential as subsequent symbiont sources

Unlike most bivalve shellfishes, giant clams (tridacnines) harbor symbiotic microalgae (zooxanthellae) in their fleshy bodies. Zooxanthellae are not maternally inherited by tridacnine offspring, hence, the larvae must acquire zooxanthellae from external sources, although such algal populations or sources in the environment are currently unknown. It is well known that giant clams expel fecal pellets that contain viable zooxanthellae cells, but whether these cells are infectious or just an expelled overpopulation from the giant clams has not been investigated. In this study, we observed the ultrastructural and photosynthetic competencies of zooxanthellae in the fecal pellets of Tridacna crocea and further tested the ability of these cells to infect T. squamosa juveniles. The ultrastructure of the zooxanthellae cells showed that the cells were intact and had not undergone digestion. Additionally, these zooxanthellae cells showed a maximum quantum yield of photosystem II (Fv/Fm) as high as those retained in the mantle of the giant clam. Under the assumption that feces might provide symbionts to the larvae of other giant clams, fecal pellets from Tridacna squamosa and T. crocea were given to artificially hatched 1-day-old T. squamosa larvae. On the 9th day, 15-34% of the larvae provided with the fecal pellets took up zooxanthellae in their stomach, and on the 14th day, zooxanthellae cells reached the larval margin, indicating the establishment of symbiosis. The rate reaching this stage was highest, ca. 5.3%, in the larvae given whole (nonhomogenized) pellets from T. crocea. The composition of zooxanthellae genera contained in the larvae were similar to those in the fecal pellets, although the abundance ratios were significantly different. This study is the first to demonstrate the potential of giant clam fecal pellets as symbiont vectors to giant clam larvae. These results also demonstrate the possibility that fecal pellets are a source of zooxanthellae in coral reefs.

Inter-partner specificity limits the acquisition of thermotolerant symbionts in a model cnidarian-dinoflagellate symbiosis

The ability of corals and other cnidarians to survive climate change depends partly on the composition of their endosymbiont communities. The dinoflagellate family Symbiodiniaceae is genetically and physiologically diverse, and one proposed mechanism for cnidarians to acclimate to rising temperatures is to acquire more thermally tolerant symbionts. However, cnidarian-dinoflagellate associations vary in their degree of specificity, which may limit their capacity to alter symbiont communities. Here, we inoculated symbiont-free polyps of the sea anemone Exaiptasia pallida (commonly referred to as 'Aiptasia'), a model system for the cnidarian-dinoflagellate symbiosis, with simultaneous or sequential mixtures of thermally tolerant and thermally sensitive species of Symbiodiniaceae. We then monitored symbiont success (relative proportional abundance) at normal and elevated temperatures across two to four weeks. All anemones showed signs of bleaching at high temperature. During simultaneous inoculations, the native, thermally sensitive Breviolum minutum colonized polyps most successfully regardless of temperature when paired against the non-native but more thermally tolerant Symbiodinium microadriaticum or Durusdinium trenchii. Furthermore, anemones initially colonized with B. minutum and subsequently exposed to S. microadriaticum failed to acquire the new symbiont. These results highlight how partner specificity may place strong limitations on the ability of certain cnidarians to acquire more thermally tolerant symbionts, and hence their adaptive potential under climate change.

Thermal Tolerance in Green Hydra: Identifying the Roles of Algal Endosymbionts and Hosts in a Freshwater Holobiont Under Stress

It has been proposed that holobionts (host-symbiont units) could swap endosymbionts, rapidly alter the hologenome (host plus symbiont genome), and increase their stress tolerance. However, experimental tests of individual and combined contributions of hosts and endosymbionts to holobiont stress tolerance are needed to test this hypothesis. Here, we used six green hydra (Hydra viridissima) strains to tease apart host (hydra) and symbiont (algae) contributions to thermal tolerance. Heat shock experiments with (1) hydra with their original symbionts, (2) aposymbiotic hydra (algae removed), (3) novel associations (a single hydra strain hosting different algae individually), and (4) control hydra (aposymbiotic hydra re-associated with their original algae) showed high variation in thermal tolerance in each group. Relative tolerances of strains were the same within original, aposymbiotic, and control treatments, but reversed in the novel associations group. Aposymbiotic hydra had similar or higher thermal tolerance than hydra with algal symbionts. Selection on the holobiont appears to be stronger than on either partner alone, suggesting endosymbiosis could become an evolutionary trap under climate change. Our results suggest that green hydra thermal tolerance is strongly determined by the host, with a smaller, non-positive role for the algal symbiont. Once temperatures exceed host tolerance limits, swapping symbionts is unlikely to allow these holobionts to persist. Rather, increases in host tolerance through in situ adaptation or migration of pre-adapted host strains appear more likely to increase local thermal tolerance. Overall, our results indicate green hydra is a valuable system for studying aquatic endosymbiosis under changing environmental conditions, and demonstrate how the host and the endosymbiont contribute to holobiont stress tolerance.

Green fluorescence from cnidarian hosts attracts symbiotic algae

Reef-building corals cannot survive without symbiotic algae, Symbiodinium, on which they depend for most of their energy. Most coral species gain symbionts from the environment early in life, and possibly after bleaching (i.e., the loss of symbionts in response to stress). However, Symbiodinium density on coral reefs is very low. Although it has long been hypothesized that corals must be able to attract free-living Symbiodinium, such a mechanism has yet to be identified. Here, we use a series of experiments to demonstrate that corals attract Symbiodinium using their endogenous GFP-related green fluorescence, revealing a biological signaling mechanism that underlies the success of this symbioses that is the building block of coral reef ecosystems.

Reef-building corals thrive in nutrient-poor marine environments because of an obligate symbiosis with photosynthetic dinoflagellates of the genus Symbiodinium. Symbiosis is established in most corals through the uptake of Symbiodinium from the environment. Corals are sessile for most of their life history, whereas free-living Symbiodinium are motile; hence, a mechanism to attract Symbiodinium would greatly increase the probability of encounter between host and symbiont. Here, we examined whether corals can attract free-living motile Symbiodinium by their green fluorescence, emitted by the excitation of endogenous GFP by purple-blue light. We found that Symbiodinium have positive and negative phototaxis toward weak green and strong purple-blue light, respectively. Under light conditions that cause corals to emit green fluorescence, (e.g., strong blue light), Symbiodinium were attracted toward live coral fragments. Symbiodinium were also attracted toward an artificial green fluorescence dye with similar excitation and emission spectra to coral-GFP. In the field, more Symbiodinium were found in traps painted with a green fluorescence dye than in controls. Our results revealed a biological signaling mechanism between the coral host and its potential symbionts.

Shift of symbiont communities in Acropora tenuis juveniles under heat stress

Ocean warming is a major threat to coral reefs, leading to an increasing frequency and amplitude of coral bleaching events, where the coral and its algal symbiont associations breakdown. Long-term change and resilience of a symbiont community in coral juveniles is thought to be one of the most important aspects for determining thermal tolerance of the coral holobionts; however, despite its importance, they are not well documented in both under elevated temperature and even under natural condition. Here we investigated changes in symbiont communities in juveniles of the coral Acropora tenuis under controlled heat stress conditions (30 °C, 31/32 °C) and natural variations in seawater temperatures (19-30 °C) for up to four months. Compared with the ambient temperature conditions, coral survival rates were higher when exposed to 30 °C, but survival rates decreased when exposed to 31/32 °C. Symbiodinium types A3, C1, and D1-4 were detected in the juveniles under all thermal conditions; however, in higher water temperatures (31/32 °C), both the prevalence of D1-4 Symbiodinium and the number of juveniles harboring only this type of symbiont increased after two to four months later. In contrast, colonies at lower temperatures (30 °C and ambient) harbored multiple clades of symbionts over the same experimental period. These results highlight the flexibility of the coral-Symbiodinium symbiosis for juvenile A. tenuis under variable thermal conditions. In particular, the benefit of the preferential association with type D1-4 can be considered as a response when under heat-stress conditions, and that could help corals to cope with ocean warming.

An updated assessment of Symbiodinium spp. that associate with common scleractinian corals from Moorea (French Polynesia) reveals high diversity among background symbionts and a novel finding of clade B

The adaptative bleaching hypothesis (ABH) states that, depending on the symbiotic flexibility of coral hosts (i.e., the ability of corals to “switch” or “shuffle” their algal symbionts), coral bleaching can lead to a change in the composition of their associated Symbiodinium community and, thus, contribute to the coral’s overall survival. In order to determine the flexibility of corals, molecular tools are required to provide accurate species delineations and to detect low levels of coral-associated Symbiodinium. Here, we used highly sensitive quantitative (real-time) PCR (qPCR) technology to analyse five common coral species from Moorea (French Polynesia), previously screened using only traditional molecular methods, to assess the presence of low-abundance (background) Symbiodinium spp. Similar to other studies, each coral species exhibited a strong specificity to a particular clade, irrespective of the environment. In addition, however, each of the five species harboured at least one additional Symbiodinium clade, among clades A–D, at background levels. Unexpectedly, and for the first time in French Polynesia, clade B was detected as a coral symbiont. These results increase the number of known coral-Symbiodinium associations from corals found in French Polynesia, and likely indicate an underestimation of the ability of the corals in this region to associate with and/or “shuffle” different Symbiodinium clades. Altogether our data suggest that corals from French Polynesia may favor a trade-off between optimizing symbioses with a specific Symbiodinium clade(s), maintaining associations with particular background clades that may play a role in the ability of corals to respond to environmental change.

Transmission of a heterologous clade C Symbiodinium in a model anemone infection system via asexual reproduction

Anemones of genus Exaiptasia are used as model organisms for the study of cnidarian-dinoflagellate (genus Symbiodinium) endosymbiosis. However, while most reef-building corals harbor Symbiodinium of clade C, Exaiptasia spp. anemones mainly harbor clade B Symbiodinium (ITS2 type B1) populations. In this study, we reveal for the first time that bleached Exaiptasia pallida anemones can establish a symbiotic relationship with a clade C Symbiodinium (ITS2 type C1). We further found that anemones can transmit the exogenously supplied clade C Symbiodinium cells to their offspring by asexual reproduction (pedal laceration). In order to corroborate the establishment of stable symbiosis, we used microscopic techniques and genetic analyses to examine several generations of anemones, and the results of these endeavors confirmed the sustainability of the system. These findings provide a framework for understanding the differences in infection dynamics between homologous and heterologous dinoflagellate types using a model anemone infection system.

Phylogeny of Symbiodinium populations in zoantharians of the northern Persian Gulf

Zoantharians of the Persian Gulf (PG) experience periods of anomalous high temperature, irradiance and desiccation. Their survival largely relies on the symbiotic relationship with single celled dinoflagellates of the genus Symbiodinium. However, the phylogeny of symbionts of zoantharians has not been investigated in the region. In this study, the second internal transcribed spacer region of ribosomal DNA (ITS2) was used to recognize in hospite populations of Symbiodinium in Palythoa aff. mutuki, Palythoa tuberculosa and Zoanthus sansibaricus colonies from Hengam, Kish, Larak, and Qeshm Islands, in the PG. The results showed subclade D1-4 and a variant of A1, were the most prevalent subclades of Symbiodinium. Predominance of stress tolerant subclade D1-4 and putatively radiation tolerant variant of A1 of Symbiodinium in zoantharian species might suggest an adaptation strategy to the extreme physical environment of the PG.

Symbiodinium clades A and D differentially predispose Acropora cytherea to disease and Vibrio spp. colonization

Coral disease outbreaks have increased over the last three decades, but their causal agents remain mostly unclear (e.g., bacteria, viruses, fungi, protists). This study details a 14-month-long survey of coral colonies in which observations of the development of disease was observed in nearly half of the sampled colonies. A bimonthly qPCR method was used to quantitatively and qualitatively evaluate Symbiodinium assemblages of tagged colonies, and to detect the presence of Vibrio spp. Firstly, our data showed that predisposition to disease development in general, and, more specifically, infection by Vibrio spp. in Acropora cytherea depended on which clades of Symbiodinium were harbored. In both cases, harboring clade D rather than A was beneficial to the coral host. Secondly, the detection of Vibrio spp. in only colonies that developed disease strongly suggests opportunistic traits of the bacteria. Finally, even if sporadic cases of switching and probably shuffling were observed, this long-term survey does not suggest specific-clade recruitment in response to stressors. Altogether, our results demonstrate that the fitness of the coral holobiont depends on its initial consortium of Symbiodinium, which is distinct among colonies, rather than a temporary adaptation achieved through acquiring different Symbiodinium clades.

Combined thermal and herbicide stress in functionally diverse coral symbionts

Most reef building corals rely on symbiotic microalgae (genus Symbiodinium) to supply a substantial proportion of their energy requirements. Functional diversity of different Symbiodinium genotypes, endorsing the host with physiological advantages, has been widely reported. Yet, the influence of genotypic specificity on the symbiont's susceptibility to contaminants or cumulative stressors is unknown. Cultured Symbiodinium of presumed thermal-tolerant clade D tested especially vulnerable to the widespread herbicide diuron, suggesting important free-living populations may be at risk in areas subjected to terrestrial runoff. Co-exposure experiments where cultured Symbiodinium were exposed to diuron over a thermal stress gradient demonstrated how fast-growing clade C1 better maintained photosynthetic capability than clade D. The mixture toxicity model of Independent Action, considering combined thermal stress and herbicide contamination, revealed response additivity for inhibition of photosynthetic yield in both tested cultures, emphasizing the need to account for cumulative stressor impacts in ecological risk assessment and resource management.

The 2014 coral bleaching and freshwater flood events in Kāne'ohe Bay, Hawai'i

Until recently, subtropical Hawaiʻi escaped the major bleaching events that have devastated many tropical regions, but the continued increases in global long-term mean temperatures and the apparent ending of the Pacific Decadal Oscillation (PDO) cool phase have increased the risk of bleaching events. Climate models and observations predict that bleaching in Hawaiʻi will occur with increasing frequency and increasing severity over future decades. A freshwater “kill” event occurred during July 2014 in the northern part of Kāneʻohe Bay that reduced coral cover by 22.5% in the area directly impacted by flooding. A subsequent major bleaching event during September 2014 caused extensive coral bleaching and mortality throughout the bay and further reduced coral cover in the freshwater kill area by 60.0%. The high temperature bleaching event only caused a 1.0% reduction in live coral throughout the portion of the bay not directly impacted by the freshwater event. Thus, the combined impact of the low salinity event and the thermal bleaching event appears to be more than simply additive. The temperature regime during the September 2014 bleaching event was analogous in duration and intensity to that of the large bleaching event that occurred previously during August 1996, but resulted in a much larger area of bleaching and coral mortality. Apparently seasonal timing as well as duration and magnitude of heating is important. Coral spawning in the dominant coral species occurs early in the summer, so reservoirs of stored lipid in the corals had been depleted by spawning prior to the September 2014 event. Warm months above 27 °C result in lower coral growth and presumably could further decrease lipid reserves, leading to a bleaching event that was more severe than would have happened if the high temperatures occurred earlier in the summer. Hawaiian reef corals decrease skeletal growth at temperatures above 27 °C, so perhaps the “stress period” actually started long before the bleaching threshold of 29 °C was reached. Hawaiʻi is directly influenced by the PDO which may become a factor influencing bleaching events in subtropical Hawaiʻi in much the same manner as variations in the El Niño Southern Oscillation (ENSO) influences bleaching events at low latitudes in the tropical Pacific. Records show that offshore temperatures measured by satellite will not always predict inshore bleaching because other factors (high cloud cover, high wind and wave action, tidal exchange rate) can limit inshore heating and prevent temperatures in the bay from reaching the bleaching threshold. Low light levels due to cloud cover or high turbidity can also serve to prevent bleaching.

Taxonomic and environmental variation of metabolite profiles in marine dinoflagellates of the genus symbiodinium

Microorganisms in terrestrial and marine ecosystems are essential to environmental sustainability. In the marine environment, invertebrates often depend on metabolic cooperation with their endosymbionts. Coral reefs, one of the most important marine ecosystems, are based on the symbiosis between a broad diversity of dinoflagellates of the genus Symbiodinium and a wide phyletic diversity of hosts (i.e., cnidarian, molluscan, poriferan). This diversity is reflected in the ecology and physiology of the symbionts, yet the underlying biochemical mechanisms are still poorly understood. We examined metabolite profiles of four cultured species of Symbiodinium known to form viable symbioses with reef-building corals, S. microadriaticum (cp-type A194), S. minutum (cp-type B184), S. psygmophilum (cp-type B224) and S. trenchii (cp-type D206). Metabolite profiles were shown to differ among Symbiodinium species and were found to be affected by their physiological response to growth in different temperatures and light regimes. A combined Random Forests and Bayesian analysis revealed that the four Symbiodinium species examined primarily differed in their production of sterols and sugars, including a C₂₉ stanol and the two sterols C₂₈Δ⁵ and C₂₈Δ^5,22, as well as differences in metabolite abundances of a hexose and inositol. Inositol levels were also strongly affected by changes in temperature across all Symbiodinium species. Our results offer a detailed view of the metabolite profile characteristic of marine symbiotic dinoflagellates of the genus Symbiodinium, and identify patterns of metabolites related to several growth conditions.

Community structure and coral status across reef fishing intensity gradients in Palk Bay reef, southeast coast of India

Coral reef fishes are exploited without the knowledge of their sustainability and their possible effect in altering the community structure of a coral reef ecosystem. Alteration of the community structure could cause a decline in the health of coral reefs and its services. We documented the coral community structure, status of live corals and reef fish assemblages in Palk Bay at the reef fishing hotspots and its nearby reef area with minimum fishing pressure and compared it with a control reef area where reef fishing was banned for more than two decades. The comparison was based on the percent cover of different forms of live corals, their diversity and the density and diversity of reef fishes. The reef fish stock in the reef fishing hotspots and its neighbouring reef was lower by 61 and 38%, respectively compared to the control reef. The herbivore fish Scarus ghobban and Siganus javus were exploited at a rate of 250 and 105 kg month(-1) fishermen(-1), respectively, relatively high comparing the small reef area. Live and dead corals colonized by turf algae were predominant in both the reef fishing hotspots and its nearby coral ecosystems. The percent cover of healthy live corals and live corals colonized by turf algae was <10 and >80%, respectively, in the intensively fished coral ecosystems. The corals were less diverse and the massive Porites and Favia colonies were abundant in the intensive reef fishing sites. Results of this study suggest that the impact of reef fish exploitation was not solely restricted to the intensively fished reefs, but also to the nearby reefs which play a critical role in the resilience of degraded reef ecosystems.

Emerging importance of holobionts in evolution and in probiotics

The existence of microbe free animals or plants in nature is virtually impossible as they and plants have a certain degree of symbiotic association with microbes. This symbiotic association leads to the formation of holobiont (host and its symbionts). This mutual coexistence is not merely at the physical or chemical level but also at the genetic level leading to the emergence of the concept of hologenome (gene pool of host and its associated symbionts). The abundance of symbionts with the associated gene diversity contributes to the fitness of the holobiont under varying environmental conditions. The hologenome theory of evolution considers the dynamic holobiont as a single unit for natural selection and provides a more accommodating view of evolution blending Darwinism and Lamarkism. Additionally, holobionts are providing scientific basis to our understanding of the growing importance of probiotics in human health and in disease management.

Coral Reef Resilience through Biodiversity

Irrefutable evidence of coral reef degradation worldwide and increasing pressure from rising seawater temperatures and ocean acidification associated with climate change have led to a focus on reef resilience and a call to “manage” coral reefs for resilience. Ideally, global action to reduce emission of carbon dioxide and other greenhouse gases will be accompanied by local action. Effective management requires reduction of local stressors, identification of the characteristics of resilient reefs, and design of marine protected area networks that include potentially resilient reefs. Future research is needed on how stressors interact, on how climate change will affect corals, fish, and other reef organisms as well as overall biodiversity, and on basic ecological processes such as connectivity. Not all reef species and reefs will respond similarly to local and global stressors. Because reef-building corals and other organisms have some potential to adapt to environmental changes, coral reefs will likely persist in spite of the unprecedented combination of stressors currently affecting them. The biodiversity of coral reefs is the basis for their remarkable beauty and for the benefits they provide to society. The extraordinary complexity of these ecosystems makes it both more difficult to predict their future and more likely they will have a future.

Ancient DNA from coral-hosted Symbiodinium reveal a static mutualism over the last 172 years

Ancient DNA (aDNA) provides powerful evidence for detecting the genetic basis for adaptation to environmental change in many taxa. Among the greatest of changes in our biosphere within the last century is rapid anthropogenic ocean warming. This phenomenon threatens corals with extinction, evidenced by the increasing observation of widespread mortality following mass bleaching events. There is some evidence and conjecture that coral-dinoflagellate symbioses change partnerships in response to changing external conditions over ecological and evolutionary timescales. Until now, we have been unable to ascertain the genetic identity of Symbiodinium hosted by corals prior to the rapid global change of the last century. Here, we show that Symbiodinium cells recovered from dry, century old specimens of 6 host species of octocorals contain sufficient DNA for amplification of the ITS2 subregion of the nuclear ribosomal DNA, commonly used for genotyping within this genus. Through comparisons with modern specimens sampled from similar locales we show that symbiotic associations among several species have been static over the last century, thereby suggesting that adaptive shifts to novel symbiont types is not common among these gorgonians, and perhaps, symbiotic corals in general.

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.

Impact of light and temperature on the uptake of algal symbionts by coral juveniles

The effects of temperature and light on the breakdown of the coral-Symbiodinium symbiosis are well documented but current understanding of their roles during initial uptake and establishment of symbiosis is limited. In this study, we investigate how temperature and light affect the uptake of the algal symbionts, ITS1 types C1 and D, by juveniles of the broadcast-spawning corals Acropora tenuis and A. millepora. Elevated temperatures had a strong negative effect on Symbiodinium uptake in both coral species, with corals at 31°C showing as little as 8% uptake compared to 87% at 28°C. Juveniles in high light treatments (390 µmol photons m⁻² s⁻¹) had lower cell counts across all temperatures, emphasizing the importance of the light environment during the initial uptake phase. The proportions of the two Symbiodinium types taken up, as quantified by a real time PCR assay using clade C- and D-specific primers, were also influenced by temperature, although variation in uptake dynamics between the two coral species indicates a host effect. At 28°C, A. tenuis juveniles were dominated by C1 Symbiodinium, and while the number of D Symbiodinium cells increased at 31°C, they never exceeded the number of C1 cells. In contrast, juveniles of A. millepora had approximately equal numbers of C1 and D cells at 28°C, but were dominated by D at 30°C and 31°C. This study highlights the significant role that environmental factors play in the establishment of coral-Symbiodinium symbiosis and provides insights into how potentially competing Symbiodinium types take up residence in coral juveniles.

Variations in reactive oxygen release and antioxidant activity in multiple Symbiodinium types in response to elevated temperature

As ocean temperatures rise, investigations into what the physiological effects will be on the symbiotic microalga Symbiodinium, and how these may play into the cnidarian bleaching response, have highlighted the contribution of reactive oxygen species (ROS). Previous studies have laid this groundwork using a limited number of Symbiodinium phylotypes, and so this study aims to expand this understanding by exploring the effects of sub-lethal elevated temperatures on the physiological response of seven genetically distinct types of Symbiodinium, including A1, B1, B2, C1, D, E1, and F2. The production of ROS (at 26 °C, 29 °C, 30 °C, and 31 °C) and activity of the antioxidants catalase (CAT) and superoxide dismutase (SOD) (at 26 °C and 31 °C) were measured as indicators of sensitivity or tolerance to heat stress. Symbiodinium types B1 and C1 were the most thermally sensitive, with C1 producing the highest amount of ROS at elevated temperatures. Types A1 and F2 were tolerant, having no increase in ROS production, and were the only types to increase both CAT and SOD activity with temperature stress. Type B2 had decreased ROS production and elevation of CAT activity, while type E1 had decreased levels of ROS production at elevated temperatures. Type D was the only Symbiodinium type to remain unaffected by elevated temperatures. These results are consistent with previous findings of relative sensitivity or tolerance to elevated temperatures, specifically with regards to types A1, B1, and F2. The inclusion of types B2, C1, D, and E1 provides further new evidence of how types differ in their thermal responses, suggesting differing mechanisms exist in the Symbiodnium response to higher temperature and highlighting the importance of establishing symbiont identity when exploring the response of intact associations to this type of stress.

Abrolhos bank reef health evaluated by means of water quality, microbial diversity, benthic cover, and fish biomass data

The health of the coral reefs of the Abrolhos Bank (southwestern Atlantic) was characterized with a holistic approach using measurements of four ecosystem components: (i) inorganic and organic nutrient concentrations, [1] fish biomass, [1] macroalgal and coral cover and (iv) microbial community composition and abundance. The possible benefits of protection from fishing were particularly evaluated by comparing sites with varying levels of protection. Two reefs within the well-enforced no-take area of the National Marine Park of Abrolhos (Parcel dos Abrolhos and California) were compared with two unprotected coastal reefs (Sebastião Gomes and Pedra de Leste) and one legally protected but poorly enforced coastal reef (the “paper park” of Timbebas Reef). The fish biomass was lower and the fleshy macroalgal cover was higher in the unprotected reefs compared with the protected areas. The unprotected and protected reefs had similar seawater chemistry. Lower vibrio CFU counts were observed in the fully protected area of California Reef. Metagenome analysis showed that the unprotected reefs had a higher abundance of archaeal and viral sequences and more bacterial pathogens, while the protected reefs had a higher abundance of genes related to photosynthesis. Similar to other reef systems in the world, there was evidence that reductions in the biomass of herbivorous fishes and the consequent increase in macroalgal cover in the Abrolhos Bank may be affecting microbial diversity and abundance. Through the integration of different types of ecological data, the present study showed that protection from fishing may lead to greater reef health. The data presented herein suggest that protected coral reefs have higher microbial diversity, with the most degraded reef (Sebastião Gomes) showing a marked reduction in microbial species richness. It is concluded that ecological conditions in unprotected reefs may promote the growth and rapid evolution of opportunistic microbial pathogens.

The biology and economics of coral growth

To protect natural coral reefs, it is of utmost importance to understand how the growth of the main reef-building organisms-the zooxanthellate scleractinian corals-is controlled. Understanding coral growth is also relevant for coral aquaculture, which is a rapidly developing business. This review paper provides a comprehensive overview of factors that can influence the growth of zooxanthellate scleractinian corals, with particular emphasis on interactions between these factors. Furthermore, the kinetic principles underlying coral growth are discussed. The reviewed information is put into an economic perspective by making an estimation of the costs of coral aquaculture.

Environmental symbiont acquisition may not be the solution to warming seas for reef-building corals

BACKGROUND

Coral reefs worldwide are in decline. Much of the mortality can be attributed to coral bleaching (loss of the coral's intracellular photosynthetic algal symbiont) associated with global warming. How corals will respond to increasing oceanic temperatures has been an area of extensive study and debate. Recovery after a bleaching event is dependent on regaining symbionts, but the source of repopulating symbionts is poorly understood. Possibilities include recovery from the proliferation of endogenous symbionts or recovery by uptake of exogenous stress-tolerant symbionts.

METHODOLOGY/PRINCIPAL FINDINGS

To test one of these possibilities, the ability of corals to acquire exogenous symbionts, bleached colonies of Porites divaricata were exposed to symbiont types not normally found within this coral and symbiont acquisition was monitored. After three weeks exposure to exogenous symbionts, these novel symbionts were detected in some of the recovering corals, providing the first experimental evidence that scleractinian corals are capable of temporarily acquiring symbionts from the water column after bleaching. However, the acquisition was transient, indicating that the new symbioses were unstable. Only those symbiont types present before bleaching were stable upon recovery, demonstrating that recovery was from the resident in situ symbiont populations.

CONCLUSIONS/SIGNIFICANCE

These findings suggest that some corals do not have the ability to adjust to climate warming by acquiring and maintaining exogenous, more stress-tolerant symbionts. This has serious ramifications for the success of coral reefs and surrounding ecosystems and suggests that unless actions are taken to reverse it, climate change will lead to decreases in biodiversity and a loss of coral reefs.

Improved water quality can ameliorate effects of climate change on corals

The threats of wide-scale coral bleaching and reef demise associated with anthropogenic climate change are widely known. Moreover, rates of genetic adaptation and/or changes in the coral-zooxanthella partnerships are considered unlikely to be sufficiently fast for corals to acquire increased physiological resistance to increasing sea temperatures and declining pH. However, it has been suggested that coral reef resilience to climate change may be improved by good local management of coral reefs, including management of water quality. Here, using major data sets from the Great Barrier Reef (GBR), Australia, we investigate geographic patterns of coral bleaching in 1998 and 2002 and outline a synergism between heat stress and nutrient flux as a major causative mechanism for those patterns. The study provides the first concrete evidence for the oft-expressed belief that improved coral reef management will increase the regional-scale survival prospects of coral reefs to global climate change.

The roles and interactions of symbiont, host and environment in defining coral fitness

BACKGROUND

Reef-building corals live in symbiosis with a diverse range of dinoflagellate algae (genus Symbiodinium) that differentially influence the fitness of the coral holobiont. The comparative role of symbiont type in holobiont fitness in relation to host genotype or the environment, however, is largely unknown. We addressed this knowledge gap by manipulating host-symbiont combinations and comparing growth, survival and thermal tolerance among the resultant holobionts in different environments.

METHODOLOGY/PRINCIPAL FINDINGS

Offspring of the coral, Acropora millepora, from two thermally contrasting locations, were experimentally infected with one of six Symbiodinium types, which spanned three phylogenetic clades (A, C and D), and then outplanted to the two parental field locations (central and southern inshore Great Barrier Reef, Australia). Growth and survival of juvenile corals were monitored for 31-35 weeks, after which their thermo-tolerance was experimentally assessed. Our results showed that: (1) Symbiodinium type was the most important predictor of holobiont fitness, as measured by growth, survival, and thermo-tolerance; (2) growth and survival, but not heat-tolerance, were also affected by local environmental conditions; and (3) host population had little to no effect on holobiont fitness. Furthermore, coral-algal associations were established with symbiont types belonging to clades A, C and D, but three out of four symbiont types belonging to clade C failed to establish a symbiosis. Associations with clade A had the lowest fitness and were unstable in the field. Lastly, Symbiodinium types C1 and D were found to be relatively thermo-tolerant, with type D conferring the highest tolerance in A. millepora.

CONCLUSIONS/SIGNIFICANCE

These results highlight the complex interactions that occur between the coral host, the algal symbiont, and the environment to shape the fitness of the coral holobiont. An improved understanding of the factors affecting coral holobiont fitness will assist in predicting the responses of corals to global climate change.

Reef endemism, host specificity and temporal stability in populations of symbiotic dinoflagellates from two ecologically dominant Caribbean corals

Background

The dinoflagellate genus Symbiodinium forms symbioses with numerous protistan and invertebrate metazoan hosts. However, few data on symbiont genetic structure are available, hindering predictions of how these populations and their host associations will fair in the face of global climate change.

Methodology/Principal Findings

Here, Symbiodinium population structure from two of the Caribbean's ecologically dominant scleractinian corals, Montastraea faveolata and M. annularis, was examined. Tagged colonies on Florida Keys and Bahamian (i.e., Exuma Cays) reefs were sampled from 2003–2005 and their Symbiodinium diversity assessed via internal transcribed spacer 2 (ITS2) rDNA and three Symbiodinium Clade B-specific microsatellite loci. Generally, the majority of host individuals at a site harbored an identical Symbiodinium ITS2 “type” B1 microsatellite genotype. Notably, symbiont genotypes were largely reef endemic, suggesting a near absence of dispersal between populations. Relative to the Bahamas, sympatric M. faveolata and M. annularis in the Florida Keys harbored unique Symbiodinium populations, implying regional host specificity in these relationships. Furthermore, within-colony Symbiodinium population structure remained stable through time and environmental perturbation, including a prolonged bleaching event in 2005.

Conclusions/Significance

Taken together, the population-level endemism, specificity and stability exhibited by Symbiodinium raises concerns about the long-term adaptive capacity and persistence of these symbioses in an uncertain future of climate change.

The host transcriptome remains unaltered during the establishment of coral-algal symbioses

Coral reefs are based on the symbiotic relationship between corals and photosynthetic dinoflagellates of the genus Symbiodinium. We followed gene expression of coral larvae of Acropora palmata and Montastraea faveolata after exposure to Symbiodinium strains that differed in their ability to establish symbioses. We show that the coral host transcriptome remains almost unchanged during infection by competent symbionts, but is massively altered by symbionts that fail to establish symbioses. Our data suggest that successful coral-algal symbioses depend mainly on the symbionts' ability to enter the host in a stealth manner rather than a more active response from the coral host.