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.

Unfamiliar partnerships limit cnidarian holobiont acclimation to warming.

Enhancing the resilience of corals to rising temperatures is now a matter of urgency, leading to growing efforts to explore the use of heat tolerant symbiont species to improve their thermal resilience. The notion that adaptive traits can be retained by transferring the symbionts alone, however, challenges the holobiont concept, a fundamental paradigm in coral research. Holobiont traits are products of a specific community (holobiont) and all its co-evolutionary and local adaptations, which might limit the retention or transference of holobiont traits by exchanging only one partner. Here we evaluate how interchanging partners affect the short- and long-term performance of holobionts under heat stress using clonal lineages of the cnidarian model system Aiptasia (host and Symbiodiniaceae strains) originating from distinct thermal environments. Our results show that holobionts from more thermally variable environments have higher plasticity to heat stress, but this resilience could not be transferred to other host genotypes through the exchange of symbionts. Importantly, our findings highlight the role of the host in determining holobiont productivity in response to thermal stress and indicate that local adaptations of holobionts will likely limit the efficacy of interchanging unfamiliar compartments to enhance thermal tolerance.

N-Linked Surface Glycan Biosynthesis, Composition, Inhibition, and Function in Cnidarian-Dinoflagellate Symbiosis.

The success of symbioses between cnidarian hosts (e.g., corals and sea anemones) and micro-algal symbionts hinges on the molecular interactions that govern the establishment and maintenance of intracellular mutualisms. As a fundamental component of innate immunity, glycan-lectin interactions impact the onset of marine endosymbioses, but our understanding of the effects of cell surface glycome composition on symbiosis establishment remains limited. In this study, we examined the canonical N-glycan biosynthesis pathway in the genome of the dinoflagellate symbiont Breviolum minutum (family Symbiodiniaceae) and found it to be conserved with the exception of the transferase GlcNAc-TII (MGAT2). Using coupled liquid chromatography-mass spectrometry (LC-MS/MS), we characterized the cell surface N-glycan content of B. minutum, providing the first insight into the molecular composition of surface glycans in dinoflagellates. We then used the biosynthesis inhibitors kifunensine and swainsonine to alter the glycan composition of B. minutum. Successful high-mannose enrichment via kifunensine treatment resulted in a significant decrease in colonization of the model sea anemone Aiptasia (Exaiptasia pallida) by B. minutum. Hybrid glycan enrichment via swainsonine treatment, however, could not be confirmed and did not impact colonization. We conclude that functional Golgi processing of N-glycans is critical for maintaining appropriate cell surface glycan composition and for ensuring colonization success by B. minutum.

Warm preconditioning protects against acute heat-induced respiratory dysfunction and delays bleaching in a symbiotic sea anemone.

Preconditioning to non-stressful warming can protect some symbiotic cnidarians against the high temperature-induced collapse of their mutualistic endosymbiosis with photosynthetic dinoflagellates (Symbiodinium spp.), a process known as bleaching. Here, we sought to determine whether such preconditioning is underpinned by differential regulation of aerobic respiration. We quantified in vivo metabolism and mitochondrial respiratory enzyme activity in the naturally symbiotic sea anemone Exaiptasia pallida preconditioned to 30°C for >7 weeks as well as anemones kept at 26°C. Preconditioning resulted in increased Symbiodinium photosynthetic activity and holobiont (host+symbiont) respiration rates. Biomass-normalised activities of host respiratory enzymes [citrate synthase and the mitochondrial electron transport chain (mETC) complexes I and IV] were higher in preconditioned animals, suggesting that increased holobiont respiration may have been due to host mitochondrial biogenesis and/or enlargement. Subsequent acute heating of preconditioned and 'thermally naive' animals to 33°C induced dramatic increases in host mETC complex I and Symbiodinium mETC complex II activities only in thermally naive E. pallida These changes were not reflected in the activities of other respiratory enzymes. Furthermore, bleaching in preconditioned E. pallida (defined as the significant loss of symbionts) was delayed by several days relative to the thermally naive group. These findings suggest that changes to mitochondrial biogenesis and/or function in symbiotic cnidarians during warm preconditioning might play a protective role during periods of exposure to stressful heating.

Elucidating the evolutionary relationships of the Aiptasiidae, a widespread cnidarian-dinoflagellate model system (Cnidaria: Anthozoa: Actiniaria: Metridioidea).

Sea anemones of the family Aiptasiidae sensu Grajales and Rodríguez (2014) are conspicuous members of shallow-water environments, including several species widely used as model systems for the study of cnidarian-dinoflagellate symbiosis and coral bleaching. Although previously published phylogenetic studies of sea anemones recovered Aiptasiidae as polyphyletic, they only included a sparse sample in terms of its taxonomic diversity and membership of the family had not been yet revised. This study explores the phylogenetic relationships of this family using five molecular markers and including newly collected material from the geographical distribution of most of the currently described genera and species. We find a monophyletic family Aiptasiidae. All the currently proposed genera were recovered as monophyletic units, a finding also supported by diagnostic morphological characters. Our results confirm Bellactis and Laviactis as members of Aiptasiidae, also in agreement with previous morphological studies. The monophyly of the group is congruent with the morphological homogeneity of the members of this family. The obtained results also allow discussing the evolution of morphological characters within the family. Furthermore, we find evidence for and describe a new cryptic species, Exaiptasia brasiliensis sp. nov., based on molecular data, geographical distribution, and the identity of its endosymbiotic dinoflagellate.

Large-scale deorphanization of Nematostella vectensis neuropeptide G protein-coupled receptors supports the independent expansion of bilaterian and cnidarian peptidergic systems.

Neuropeptides are ancient signaling molecules in animals but only few peptide receptors are known outside bilaterians. Cnidarians possess a large number of G protein-coupled receptors (GPCRs) - the most common receptors of bilaterian neuropeptides - but most of these remain orphan with no known ligands. We searched for neuropeptides in the sea anemone Nematostella vectensis and created a library of 64 peptides derived from 33 precursors. In a large-scale pharmacological screen with these peptides and 161 N. vectensis GPCRs, we identified 31 receptors specifically activated by 1 to 3 of 14 peptides. Mapping GPCR and neuropeptide expression to single-cell sequencing data revealed how cnidarian tissues are extensively connected by multilayer peptidergic networks. Phylogenetic analysis identified no direct orthology to bilaterian peptidergic systems and supports the independent expansion of neuropeptide signaling in cnidarians from a few ancestral peptide-receptor pairs.

Abundance of Oligoflexales bacteria is associated with algal symbiont density, independent of thermal stress in Aiptasia anemones.

Many multicellular organisms, such as humans, plants, and invertebrates, depend on symbioses with microbes for metabolic cooperation and exchange. Reef-building corals, an ecologically important order of invertebrates, are particularly vulnerable to environmental stress in part because of their nutritive symbiosis with dinoflagellate algae, and yet also benefit from these and other microbial associations. While coral microbiomes remain difficult to study because of their complexity, the anemone Aiptasia is emerging as a simplified model. Research has demonstrated co-occurrences between microbiome composition and the abundance and type of algal symbionts in cnidarians. However, whether these patterns are the result of general stress-induced shifts or depletions of algal-associated bacteria remains unclear. Our study aimed to distinguish the effect of changes in symbiont density and thermal stress on the microbiome of symbiotic Aiptasia strain CC7 by comparing them with aposymbiotic anemones, depleted of their native symbiont, Symbiodinium linucheae. Our analysis indicated that overall thermal stress had the greatest impact on disrupting the microbiome. We found that three bacterial classes made up most of the relative abundance (60%-85%) in all samples, but the rare microbiome fluctuated between symbiotic states and following thermal stress. We also observed that S. linucheae density correlated with abundance of Oligoflexales, suggesting these bacteria may be primary symbionts of the dinoflagellate algae. The findings of this study help expand knowledge on prospective multipartite symbioses in the cnidarian holobiont and how they respond to environmental disturbance.

Impact of rising seawater temperature on a phagocytic cell population during V. parahaemolyticus infection in the sea anemone E. pallida.

Climate change is increasing ocean temperatures and consequently impacts marine life (e.g., bacterial communities). In this context, studying host-pathogen interactions in marine organisms is becoming increasingly important, not only for ecological conservation, but also to reduce economic loss due to mass mortalities in cultured species. In this study, we used Exaiptasia pallida (E. pallida), an anemone, as an emerging marine model to better understand the effect of rising temperatures on the infection induced by the pathogenic marine bacterium Vibrio parahaemolyticus. The effect of temperature on E. pallida was examined at 6, 24, or 30 h after bath inoculation with 108 CFU of V. parahaemolyticus expressing GFP (Vp-GFP) at 27°C (husbandry temperature) or 31°C (heat stress). Morphological observations of E. pallida and their Hsps expression demonstrated heat stress induced increasing damage to anemones. The kinetics of the infections revealed that Vp-GFP were localized on the surface of the ectoderm and in the mucus during the first hours of infection and in the mesenterial filaments thereafter. To better identify the E. pallida cells targeted by Vp-GFP infection, we used spectral flow cytometry. E. pallida cell types were identified based on their autofluorescent properties. corresponding to different cell types (algae and cnidocytes). We identified an AF10 population whose autofluorescent spectrum was identical to that of human monocytes/macrophage, suggesting that this spectral print could be the hallmark of phagocytic cells called "amebocytes''. AF10 autofluorescent cells had a high capacity to phagocytize Vp-GFP, suggesting their possible role in fighting infection. This was confirmed by microscopy using sorted AF10 and GFP-positive cells (AF10+/GFP+). The number of AF10+/GFP+ cells were reduced at 31°C, demonstrating that increased temperature not only damages tissue but also affects the immune response of E. pallida. In conclusion, our study provides a springboard for more comprehensive studies of immune defense in marine organisms and paves the way for future studies of the dynamics, activation patterns, and functional responses of immune cells when encountering pathogens.

Cnidarian-Symbiodiniaceae symbiosis establishment is independent of photosynthesis.

Photosynthesis shapes the symbiotic relationships between cnidarians and Symbiodiniaceae algae-with many cnidarian hosts requiring symbiont photosynthate for survival-but little is known about how photosynthesis impacts symbiosis establishment. Here, we show that during symbiosis establishment, infection, proliferation, and maintenance can proceed without photosynthesis, but the ability to do so is dependent on specific cnidarian-Symbiodiniaceae relationships. The evaluation of 31 pairs of symbiotic relationships (five species of Symbiodiniaceae in sea anemone, coral, and jellyfish hosts) revealed that infection can occur without photosynthesis. A UV mutagenesis method for Symbiodiniaceae was established and used to generate six photosynthetic mutants that can infect these hosts. Without photosynthesis, Symbiodiniaceae cannot proliferate in the sea anemone Aiptasia or jellyfish Cassiopea but can proliferate in the juvenile polyps of the coral Acropora. After 6 months of darkness, Breviolum minutum is maintained within Aiptasia, indicating that Symbiodiniaceae maintenance can be independent of photosynthesis. Manipulating photosynthesis provides insights into cnidarian-Symbiodiniaceae symbiosis.

A shift away from mutualism under food-deprived conditions in an anemone-dinoflagellate association.

The mutualistic symbiosis between anthozoans and intra-gastrodermal dinoflagellates of the family Symbiodiniaceae is the functional basis of all coral reef ecosystems, with the latter providing up to 95% of their fixed photosynthate to their hosts in exchange for nutrients. However, recent studies of sponges, jellyfish, and anemones have revealed the potential for this mutualistic relationship to shift to parasitism under stressful conditions. Over a period of eight weeks, we compared the physiological conditions of both inoculated and aposymbiotic anemones (Exaiptasia pallida) that were either fed or starved. By the sixth week, both fed groups of anemones were significantly larger than their starved counterparts. Moreover, inoculated and starved anemones tended to disintegrate into "tissue balls" within eight weeks, and 25% of the samples died; in contrast, starved aposymbiotic anemones required six months to form tissue balls, and no anemones from this group died. Our results show that the dinoflagellates within inoculated anemones may have posed a fatal metabolic burden on their hosts during starvation; this may be because of the need to prioritize their own metabolism and nourishment at the expense of their hosts. Collectively, our study reveals the potential of this dynamic symbiotic association to shift away from mutualism during food-deprived conditions.

Host and Symbiont Cell Cycle Coordination Is Mediated by Symbiotic State, Nutrition, and Partner Identity in a Model Cnidarian-Dinoflagellate Symbiosis.

The cell cycle is a critical component of cellular proliferation, differentiation, and response to stress, yet its role in the regulation of intracellular symbioses is not well understood. To explore host-symbiont cell cycle coordination in a marine symbiosis, we employed a model for coral-dinoflagellate associations: the tropical sea anemone Aiptasia (Exaiptasia pallida) and its native microalgal photosymbionts (Breviolum minutum and Breviolum psygmophilum). Using fluorescent labeling and spatial point-pattern image analyses to characterize cell population distributions in both partners, we developed protocols that are tailored to the three-dimensional cellular landscape of a symbiotic sea anemone tentacle. Introducing cultured symbiont cells to symbiont-free adult hosts increased overall host cell proliferation rates. The acceleration occurred predominantly in the symbiont-containing gastrodermis near clusters of symbionts but was also observed in symbiont-free epidermal tissue layers, indicating that the presence of symbionts contributes to elevated proliferation rates in the entire host during colonization. Symbiont cell cycle progression differed between cultured algae and those residing within hosts; the endosymbiotic state resulted in increased S-phase but decreased G2/M-phase symbiont populations. These phenotypes and the deceleration of cell cycle progression varied with symbiont identity and host nutritional status. These results demonstrate that host and symbiont cells have substantial and species-specific effects on the proliferation rates of their mutualistic partners. This is the first empirical evidence to support species-specific regulation of the symbiont cell cycle within a single cnidarian-dinoflagellate association; similar regulatory mechanisms likely govern interpartner coordination in other coral-algal symbioses and shape their ecophysiological responses to a changing climate.IMPORTANCE Biomass regulation is critical to the overall health of cnidarian-dinoflagellate symbioses. Despite the central role of the cell cycle in the growth and proliferation of cnidarian host cells and dinoflagellate symbionts, there are few studies that have examined the potential for host-symbiont coregulation. This study provides evidence for the acceleration of host cell proliferation when in local proximity to clusters of symbionts within cnidarian tentacles. The findings suggest that symbionts augment the cell cycle of not only their enveloping host cells but also neighboring cells in the epidermis and gastrodermis. This provides a possible mechanism for rapid colonization of cnidarian tissues. In addition, the cell cycles of symbionts differed depending on nutritional regime, symbiotic state, and species identity. The responses of cell cycle profiles to these different factors implicate a role for species-specific regulation of symbiont cell cycles within host cnidarian tissues.

Microbiota characterization of Exaiptasia diaphana from the Great Barrier Reef.

BACKGROUND: Coral reefs have sustained damage of increasing scale and frequency due to climate change, thereby intensifying the need to elucidate corals' biological characteristics, including their thermal tolerance and microbial symbioses. The sea anemone, Exaiptasia diaphana, has proven an ideal coral model for many studies due to its close phylogenetic relationship and shared traits, such as symbiosis with algae of the family Symbiodiniaceae. However, established E. diaphana clonal lines are not available in Australia thus limiting the ability of Australian scientists to conduct research with this model. To help address this, the bacterial and Symbiodiniaceae associates of four Great Barrier Reef (GBR)-sourced E. diaphana genotypes established in laboratory aquaria and designated AIMS1-4, and from proxies of wild GBR E. diaphana were identified by metabarcoding of the bacterial 16S rRNA gene and eukaryotic rRNA gene ITS2 region. The relationship between AIMS1-4 and their bacterial associates was investigated, as was bacterial community phenotypic potential. Existing data from two existing anemone clonal lines, CC7 and H2, were included for comparison. RESULTS: Overall, 2238 bacterial amplicon sequence variants (ASVs) were observed in the AIMS1-4 bacterial communities, which were dominated by Proteobacteria and Bacteroidetes, together comprising > 90% relative abundance. Although many low abundance bacterial taxa varied between the anemone genotypes, the AIMS1-4 communities did not differ significantly. A significant tank effect was identified, indicating an environmental effect on the microbial communities. Bacterial community richness was lower in all lab-maintained E. diaphana compared to the wild proxies, suggesting a reduction in bacterial diversity and community phenotypic potential due to culturing. Seventeen ASVs were common to every GBR lab-cultured anemone, however five were associated with the Artemia feedstock, making their specific association to E. diaphana uncertain. The dominant Symbiodiniaceae symbiont in all GBR anemones was Breviolum minutum. CONCLUSION: Despite differences in the presence and abundance of low abundance taxa, the bacterial communities of GBR-sourced lab-cultured E. diaphana are generally uniform and comparable to communities reported for other lab-cultured E. diaphana. The data presented here add to the global E. diaphana knowledge base and make an important contribution to the establishment of a GBR-sourced coral model organism.

The Effect of Thermal Stress on the Bacterial Microbiome of Exaiptasia diaphana.

Coral bleaching linked to climate change has generated interest in the response of coral's bacterial microbiome to thermal stress. The sea anemone, Exaiptasia diaphana, is a popular coral model, but the response of its bacteria to thermal stress has been barely explored. To address this, we compared the bacterial communities of Great Barrier Reef (GBR) E. diaphana maintained at 26 °C or exposed to increasing temperature (26-33 °C) over two weeks. Communities were analyzed by metabarcoding of the bacterial 16S rRNA gene. Bleaching and Symbiodiniaceae health were assessed by Symbiodiniaceae cell density and dark-adapted quantum yield (Fv/Fm), respectively. Significant bleaching and reductions in Fv/Fm occurred in the heat-treated anemones above 29 °C. Overall declines in bacterial alpha diversity in all anemones were also observed. Signs of bacterial change emerged above 31 °C. Some initial outcomes may have been influenced by relocation or starvation, but collectively, the bacterial community and taxa-level data suggested that heat was the primary driver of change above 32 °C. Six bacterial indicator species were identified as potential biomarkers for thermal stress. We conclude that the bacterial microbiome of GBR E. diaphana is generally stable until a thermal threshold is surpassed, after which significant changes occur.

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.

Subtle Differences in Symbiont Cell Surface Glycan Profiles Do Not Explain Species-Specific Colonization Rates in a Model Cnidarian-Algal Symbiosis.

Mutualisms between cnidarian hosts and dinoflagellate endosymbionts are foundational to coral reef ecosystems. These symbioses are often re-established every generation with high specificity, but gaps remain in our understanding of the cellular mechanisms that control symbiont recognition and uptake dynamics. Here, we tested whether differences in glycan profiles among different symbiont species account for the different rates at which they initially colonize aposymbiotic polyps of the model sea anemone Aiptasia (Exaiptasia pallida). First, we used a lectin array to characterize the glycan profiles of colonizing Symbiodinium minutum (ITS2 type B1) and noncolonizing Symbiodinium pilosum (ITS2 type A2), finding subtle differences in the binding of lectins Euonymus europaeus lectin (EEL) and Urtica dioica agglutinin lectin (UDA) that distinguish between high-mannoside and hybrid-type protein linked glycans. Next, we enzymatically cleaved glycans from the surfaces of S. minutum cultures and followed their recovery using flow cytometry, establishing a 48-72 h glycan turnover rate for this species. Finally, we exposed aposymbiotic host polyps to cultured S. minutum cells masked by EEL or UDA lectins for 48 h, then measured cell densities the following day. We found no effect of glycan masking on symbiont density, providing further support to the hypothesis that glycan-lectin interactions are more important for post-phagocytic persistence of specific symbionts than they are for initial uptake. We also identified several methodological and biological factors that may limit the utility of studying glycan masking in the Aiptasia system.

A detailed observation of the ejection and retraction of defense tissue acontia in sea anemone (Exaiptasia pallida).

Acontia, located in the gastrovascular cavity of anemone, are thread-like tissue containing numerous stinging cells which serve as a unique defense tissue against predators of the immobile acontiarian sea anemone. Although its morphology and biological functions, such as defense and digestion, have been studied, the defense behavior and the specific events of acontia ejection and retraction are unclear. The aim of this study is to observe and record the detailed process of acontia control in anemones. Observations reveal that the anemone, Exaiptasia pallida, possibly controls a network of body muscles and manipulates water pressure in the gastrovascular cavity to eject and retract acontia. Instead of resynthesizing acontia after each ejection, the retraction and reuse of acontia enables the anemone to respond quickly at any given time, thus increasing its overall survivability. Since the Exaiptasia anemone is an emerging model for coral biology, this study provides a foundation to further investigate the biophysics, neuroscience, and defense biology of this marine model organism.

Partitioning of Respiration in an Animal-Algal Symbiosis: Implications for Different Aerobic Capacity between Symbiodinium spp.

Cnidarian-dinoflagellate symbioses are ecologically important and the subject of much investigation. However, our understanding of critical aspects of symbiosis physiology, such as the partitioning of total respiration between the host and symbiont, remains incomplete. Specifically, we know little about how the relationship between host and symbiont respiration varies between different holobionts (host-symbiont combinations). We applied molecular and biochemical techniques to investigate aerobic respiratory capacity in naturally symbiotic Exaiptasia pallida sea anemones, alongside animals infected with either homologous ITS2-type A4 Symbiodinium or a heterologous isolate of Symbiodinium minutum (ITS2-type B1). In naturally symbiotic anemones, host, symbiont, and total holobiont mitochondrial citrate synthase (CS) enzyme activity, but not host mitochondrial copy number, were reliable predictors of holobiont respiration. There was a positive association between symbiont density and host CS specific activity (mg protein(-1)), and a negative correlation between host- and symbiont CS specific activities. Notably, partitioning of total CS activity between host and symbiont in this natural E. pallida population was significantly different to the host/symbiont biomass ratio. In re-infected anemones, we found significant between-holobiont differences in the CS specific activity of the algal symbionts. Furthermore, the relationship between the partitioning of total CS activity and the host/symbiont biomass ratio differed between holobionts. These data have broad implications for our understanding of cnidarian-algal symbiosis. Specifically, the long-held assumption of equivalency between symbiont/host biomass and respiration ratios can result in significant overestimation of symbiont respiration and potentially erroneous conclusions regarding the percentage of carbon translocated to the host. The interspecific variability in symbiont aerobic capacity provides further evidence for distinct physiological differences that should be accounted for when studying diverse host-symbiont combinations.

Unravelling the role of zooxanthellae in the uptake and depuration of an essential metal in Exaiptasia pallida; an experiment using a model cnidarian.

Coral skeletons record historical trace metal levels in the environment, however, the use of coral skeletal records for biomonitoring studies mostly fail to consider the influence of metal regulation by the living components of coral and subsequent incorporation into the skeleton. This study presents Exaiptasia pallida as a representative of the living components of coral and shows metal partitioning between the tissue and zooxanthellae after chronic exposure to Zn. A strong tendency for preferential accumulation in the zooxanthellae occurred after 32 days exposure and Zn concentrations in tissue and zooxanthellae were 123.3±0.7 mg kg(-1) and 294.9±8.5 respectively. This study shows zooxanthellae density plays an important role in controlling Zn loading in whole anemones and must be considered when investigating metal uptake and loading in zooxanthellate organisms. Further studies that investigate links between aragonite deposition rates and zooxanthellae density and incorporation pathways of metals into skeleton are warranted.