Cnidarian Hosts Research Articles

Unique photosynthetic strategies employed by closely related Breviolum minutum strains under rapid short-term cumulative heat stress.

The thermal tolerance of symbiodiniacean photo-endosymbionts largely underpins the thermal bleaching resilience of their cnidarian hosts such as corals and the coral model Exaiptasia diaphana. While variation in thermal tolerance between species is well documented, variation between conspecific strains is understudied. We compared the thermal tolerance of three closely related strains of Breviolum minutum represented by two internal transcribed spacer region 2 profiles (one strain B1-B1o-B1g-B1p and the other two strains B1-B1a-B1b-B1g) and differences in photochemical and non-photochemical quenching, de-epoxidation state of photopigments, and accumulation of reactive oxygen species under rapid short-term cumulative temperature stress (26-40 °C). We found that B. minutum strains employ distinct photoprotective strategies, resulting in different upper thermal tolerances. We provide evidence for previously unknown interdependencies between thermal tolerance traits and photoprotective mechanisms that include a delicate balancing of excitation energy and its dissipation through fast relaxing and state transition components of non-photochemical quenching. The more thermally tolerant B. minutum strain (B1-B1o-B1g-B1p) exhibited an enhanced de-epoxidation that is strongly linked to the thylakoid membrane melting point and possibly membrane rigidification minimizing oxidative damage. This study provides an in-depth understanding of photoprotective mechanisms underpinning thermal tolerance in closely related strains of B. minutum.

Comparative physiology reveals heat stress disrupts acid-base homeostasis independent of symbiotic state in the model cnidarian Exaiptasia diaphana.

Climate change threatens the survival of symbiotic cnidarians by causing photosymbiosis breakdown in a process known as bleaching. Direct effects of temperature on cnidarian host physiology remain difficult to describe because heatwaves depress symbiont performance, leading to host stress and starvation. The symbiotic sea anemone Exaiptasia diaphana provides an opportune system to disentangle direct versus indirect heat effects on the host, as it can survive indefinitely without symbionts. We tested the hypothesis that heat directly impairs cnidarian physiology by comparing symbiotic and aposymbiotic individuals of two laboratory subpopulations of a commonly used clonal strain of E. diaphana, CC7. We exposed anemones to a range of temperatures (ambient, +2°C, +4°C and +6°C) for 15-18 days, then measured their symbiont population densities, autotrophic carbon assimilation and translocation, photosynthesis, respiration and host intracellular pH (pHi). Symbiotic anemones from the two subpopulations differed in size and symbiont density and exhibited distinct heat stress responses, highlighting the importance of acclimation to different laboratory conditions. Specifically, the cohort with higher initial symbiont densities experienced dose-dependent symbiont loss with increasing temperature and a corresponding decline in host photosynthate accumulation. In contrast, the cohort with lower initial symbiont densities did not lose symbionts or assimilate less photosynthate when heated, similar to the response of aposymbiotic anemones. However, anemone pHi decreased at higher temperatures regardless of cohort, symbiont presence or photosynthate translocation, indicating that heat consistently disrupts cnidarian acid-base homeostasis independent of symbiotic status or mutualism breakdown. Thus, pH regulation may be a critical vulnerability for cnidarians in a changing climate.

Spicy food for the egg-cowries: the evolution of corallivory in the Ovulidae (Gastropoda: Cypraeoidea)

IntroductionHost-parasite associations provide very useful models to study adaptive processes. We investigated the interaction between carnivorous marine gastropods, the Ovulidae or egg-cowries, and their cnidarian food targets. Ovulidae (Fleming, 1828), is a family of specialized carnivorous caenogastropods that feed by browsing on octocorals (Anthozoa: Octocorallia: Malacalcyonacea and Scleralcyonacea) or, to a much lesser degree, on antipatharians (Anthozoa: Hexacorallia: Antipatharia) and Stylasteridae (Hydrozoa: Hydroidolina: Anthoathecata). Very scanty information is available on the phylogenetic relationships and the degree of specificity of the relationship with the cnidarians of this corallivorous lineage, especially for deep-water taxa.MethodsTo assess taxonomic identifications and investigate cnidarian/ovulid relationships in the context of their evolution, we generated an extensive molecular dataset comprising two mitochondrial (cox1 and 16S rDNA) and one nuclear gene (28S rDNA) from 524 specimens collected worldwide. The coral hosts of the ovulid species have been identified by integrating literature data with new records, employing morphological and/or molecular (the mitochondrial 16S rDNA and mtMSH, and the nuclear ITS2) markers.ResultsWe obtained a molecular phylogenetic framework for the Ovulidae, time-calibrated with nine reliable fossil records. An ancestral state reconstruction allowed to identify Hexacorallia or Hydroidolina as the most likely ancestral cnidarian host for the Ovulidae.DiscussionOur phylogenetic hypothesis revealed the existence of groups that do not completely correspond to the currently employed subfamilial arrangement. Concerning trophic ecology, while only pediculariines (Pedicularia and allied) are associated with hydrozoans (Stylasteridae), our results suggest that some ovulid lineages shifted independently between octocorals and hexacorals.

Chlamydiae in corals: shared functional potential despite broad taxonomic diversity.

Cnidarians, such as corals and sea anemones, associate with a wide range of bacteria that have essential functions, including nutrient cycling and the production of antimicrobial compounds. Within cnidarians, bacteria can colonize all microhabitats including the tissues. Among them are obligate intracellular bacteria of the phylum Chlamydiota (chlamydiae) whose impact on cnidarian hosts and holobionts, especially corals, remain unknown. Here, we conducted a meta-analysis of previously published 16S rRNA gene metabarcoding data from cnidarians (e.g. coral, jellyfish, and anemones), eight metagenome-assembled genomes (MAGs) of coral-associated chlamydiae, and one MAG of jellyfish-associated chlamydiae to decipher their diversity and functional potential. While the metabarcoding dataset showed an enormous diversity of cnidarian-associated chlamydiae, six out of nine MAGs were affiliated with the Simkaniaceae family. The other three MAGs were assigned to the Parasimkaniaceae, Rhabdochlamydiaceae, and Anoxychlamydiaceae, respectively. All MAGs lacked the genes necessary for an independent existence, lacking any nucleotide or vitamin and most amino acid biosynthesis pathways. Hallmark chlamydial genes, such as a type III secretion system, nucleotide transporters, and genes for host interaction, were encoded in all MAGs. Together these observations suggest an obligate intracellular lifestyle of coral-associated chlamydiae. No unique genes were found in coral-associated chlamydiae, suggesting a lack of host specificity. Additional studies are needed to understand how chlamydiae interact with their coral host, and other microbes in coral holobionts. This first study of the diversity and functional potential of coral-associated chlamydiae improves our understanding of both the coral microbiome and the chlamydial lifestyle and host range.

A carbon-nitrogen negative feedback loop underlies the repeated evolution of cnidarian–Symbiodiniaceae symbioses

Symbiotic associations with Symbiodiniaceae have evolved independently across a diverse range of cnidarian taxa including reef-building corals, sea anemones, and jellyfish, yet the molecular mechanisms underlying their regulation and repeated evolution are still elusive. Here, we show that despite their independent evolution, cnidarian hosts use the same carbon-nitrogen negative feedback loop to control symbiont proliferation. Symbiont-derived photosynthates are used to assimilate nitrogenous waste via glutamine synthetase–glutamate synthase-mediated amino acid biosynthesis in a carbon-dependent manner, which regulates the availability of nitrogen to the symbionts. Using nutrient supplementation experiments, we show that the provision of additional carbohydrates significantly reduces symbiont density while ammonium promotes symbiont proliferation. High-resolution metabolic analysis confirmed that all hosts co-incorporated glucose-derived 13C and ammonium-derived 15N via glutamine synthetase–glutamate synthase-mediated amino acid biosynthesis. Our results reveal a general carbon-nitrogen negative feedback loop underlying these symbioses and provide a parsimonious explanation for their repeated evolution.

Coupled carbon and nitrogen cycling regulates the cnidarian–algal symbiosis

Efficient nutrient recycling underpins the ecological success of cnidarian-algal symbioses in oligotrophic waters. In these symbioses, nitrogen limitation restricts the growth of algal endosymbionts in hospite and stimulates their release of photosynthates to the cnidarian host. However, the mechanisms controlling nitrogen availability and their role in symbiosis regulation remain poorly understood. Here, we studied the metabolic regulation of symbiotic nitrogen cycling in the sea anemone Aiptasia by experimentally altering labile carbon availability in a series of experiments. Combining 13C and 15N stable isotope labeling experiments with physiological analyses and NanoSIMS imaging, we show that the competition for environmental ammonium between the host and its algal symbionts is regulated by labile carbon availability. Light regimes optimal for algal photosynthesis increase carbon availability in the holobiont and stimulate nitrogen assimilation in the host metabolism. Consequently, algal symbiont densities are lowest under optimal environmental conditions and increase toward the lower and upper light tolerance limits of the symbiosis. This metabolic regulation promotes efficient carbon recycling in a stable symbiosis across a wide range of environmental conditions. Yet, the dependence on resource competition may favor parasitic interactions, explaining the instability of the cnidarian-algal symbiosis as environmental conditions in the Anthropocene shift towards its tolerance limits.

Gas Chromatography-Mass Spectrometry-Based Targeted Metabolomics of Hard Coral Samples.

Gas chromatography-mass spectrometry (GC-MS)-based approaches have proven to be powerful for elucidating the metabolic basis of the cnidarian-dinoflagellate symbiosis and how coral responds to stress (i.e., during temperature-induced bleaching). Steady-state metabolite profiling of the coral holobiont, which comprises the cnidarian host and its associated microbes (Symbiodiniaceae and other protists, bacteria, archaea, fungi, and viruses), has been successfully applied under ambient and stress conditions to characterize the holistic metabolic status of the coral. However, to answer questions surrounding the symbiotic interactions, it is necessary to analyze the metabolite profiles of the coral host and its algal symbionts independently, which can only be achieved by physical separation and isolation of the tissues, followed by independent extraction and analysis. While the application of metabolomics is relatively new to the coral field, the sustained efforts of research groups have resulted in the development of robust methods for analyzing metabolites in corals, including the separation of the coral host tissue and algal symbionts. This paper presents a step-by-step guide for holobiont separation and the extraction of metabolites for GC-MS analysis, including key optimization steps for consideration. We demonstrate how, once analyzed independently, the combined metabolite profile of the two fractions (coral and Symbiodiniaceae) is similar to the profile of the whole (holobiont), but by separating the tissues, we can also obtain key information about the metabolism of and interactions between the two partners that cannot be obtained from the whole alone.

Single-polyp metabolomics reveals biochemical structuring of the coral holobiont at multiple scales

All biology happens in space, and spatial structuring plays an important role in mediating biological processes at all scales from cells to ecosystems. However, the metabolomic structuring of the coral holobiont has yet to be fully explored. Here, we present a method to detect high-quality metabolomic data from individual coral polyps and apply this method to study the patterning of biochemicals across multiple spatial (~1 mm - ~100 m) and organizational scales (polyp to population). The data show a strong signature for individual coral colonies, a weaker signature of branches within colonies, and variation at the polyp level related to the polyps’ location along a branch. Mapping metabolites to either the coral or algal components of the holobiont reveals that polyp-level variation along the length of a branch was largely driven by molecules associated with the cnidarian host as opposed to the algal symbiont, predominantly putative sulfur-containing metabolites. This work yields insights on the spatial structuring of biochemicals in the coral holobiont, which is critical for design, analysis, and interpretation of studies on coral reef biochemistry.

Spatial metabolomics for symbiotic marine invertebrates.

Microbial symbionts frequently localize within specific body structures or cell types of their multicellular hosts. This spatiotemporal niche is critical to host health, nutrient exchange, and fitness. Measuring host-microbe metabolite exchange has conventionally relied on tissue homogenates, eliminating dimensionality and dampening analytical sensitivity. We have developed a mass spectrometry imaging workflow for a soft- and hard-bodied cnidarian animal capable of revealing the host and symbiont metabolome in situ, without the need for a priori isotopic labelling or skeleton decalcification. The mass spectrometry imaging method provides critical functional insights that cannot be gleaned from bulk tissue analyses or other presently available spatial methods. We show that cnidarian hosts may regulate microalgal symbiont acquisition and rejection through specific ceramides distributed throughout the tissue lining the gastrovascular cavity. The distribution pattern of betaine lipids showed that once resident, symbionts primarily reside in light-exposed tentacles to generate photosynthate. Spatial patterns of these metabolites also revealed that symbiont identity can drive host metabolism.

High-frequency imagery to capture coral tissue (Montipora capricornis) response to environmental stress, a pilot study.

Environment stress is a major threat to the existence of coral reefs and has generated a lot of interest in the coral research community. Under the environmental stress, corals can experience tissue loss and/or the breakdown of symbiosis between the cnidarian host and its symbiotic algae causing the coral tissue to appear white as the skeleton can be seen by transparency. Image analysis is a common method used to assess tissue response under the environmental stress. However, the traditional approach is limited by the dynamic nature of the coral-algae symbiosis. Here, we observed coral tissue response in the scleractinian coral, Montipora capricornis, using high frequency image analysis throughout the experiment, as opposed to the typical start/end point assessment method. Color analysis reveals that the process can be divided into five stages with two critical stages according to coral tissue morphology and color ratio. We further explore changes to the morphology of individual polyps by means of the Pearson correlation coefficient and recurrence plots, where the quasi-periodic and nonstationary dynamics can be identified. The recurrence quantification analysis also allows the comparison between the different polyps. Our research provides a detailed visual and mathematical analysis of coral tissue response to environmental stress, which potentially shows universal applicability. Moreover, our approach provides a robust quantitative advancement for improving our insight into a suite of biotic responses in the perspective of coral health evaluation and fate prediction.

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

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

A candidate transporter allowing symbiotic dinoflagellates to feed their coral hosts

The symbiotic partnership between corals and dinoflagellate algae is crucial to coral reefs. Corals provide their algal symbionts with shelter, carbon dioxide and nitrogen. In exchange, the symbiotic algae supply their animal hosts with fixed carbon in the form of glucose. But how glucose is transferred from the algal symbiont to the animal host is unknown. We reasoned that a transporter resident in the dinoflagellate cell membrane would facilitate outward transfer of glucose to the surrounding host animal tissue. We identified a candidate transporter in the cnidarian symbiont dinoflagellate Breviolum minutum that belongs to the ubiquitous family of facilitative sugar uniporters known as SWEETs (sugars will eventually be exported transporters). Previous gene expression analyses had shown that BmSWEET1 is upregulated when the algae are living symbiotically in a cnidarian host by comparison to the free-living state [1, 2]. We used immunofluorescence microscopy to localise BmSWEET1 in the dinoflagellate cell membrane. Substrate preference assays in a yeast surrogate transport system showed that BmSWEET1 transports glucose. Quantitative microscopy showed that symbiotic B. minutum cells have significantly more BmSWEET1 protein than free-living cells of the same strain, consistent with export during symbiosis but not during the free-living, planktonic phase. Thus, BmSWEET1 is in the right place, at the right time, and has the right substrate to be the transporter with which symbiotic dinoflagellate algae feed their animal hosts to power coral reefs.

Multi-macromolecular Extraction from Endosymbiotic Anthozoans.

Obligately symbiotic associations between reef-building corals (anthozoan cnidarians) and photosynthetically active dinoflagellates of the family Symbiodiniaceae comprise the functional basis of all coral reef ecosystems. Given the existential threats of global climate change toward these thermo-sensitive entities, there is an urgent need to better understand the physiological implications of changes in the abiotic milieu of scleractinian corals and their mutualistic algal endosymbionts. Although initially slow to leverage the immense breakthroughs in molecular biotechnology that have benefited humankind, coral biologists are making up for lost time in exploiting an array of ever-advancing molecular tools for answering key questions pertaining to the survival of corals in an ever-changing world. In order to comprehensively characterize the multi-omic landscape of the coral holobiont-the cnidarian host, its intracellular dinoflagellates, and a plethora of other microbial constituents-I introduce a series of protocols herein that yield large quantities of high-quality RNA, DNA, protein, lipids, and polar metabolites from a diverse array of reef corals and endosymbiotic sea anemones. Although numerous published articles in the invertebrate zoology field feature protocols that lead to sufficiently high yield of intact host coral macromolecules, through using the approach outlined herein one may simultaneously acquire a rich, multi-compartmental biochemical pool that truly reflects the complex and dynamic nature of these animal-plant chimeras.

Octocorals in the Gulf of Aqaba exhibit high photosymbiont fidelity.

Symbiotic associations, widespread in terrestrial and marine ecosystems, are of considerable ecological importance. Many tropical coral species are holobionts, formed by the obligate association between a cnidarian host and endosymbiotic dinoflagellates of the family Symbiodiniaceae. The latter are abundant on coral reefs from very shallow water down to the upper mesophotic zone (30-70 m). The research on scleractinians has revealed that the photosymbiont lineages present in the cnidarian host play an important role in the coral's ability to thrive under different environmental conditions, such as light regime and temperature. However, little is known regarding octocoral photosymbionts, and in particular regarding those found deeper than 30 m. Here, we used ribosomal (ITS2) and chloroplast (23S) markers to uncover, for the first time, the dominant Symbiodiniaceae taxa present in 19 mesophotic octocoral species (30-70 m depth) from the Gulf of Aqaba/Eilat (northern Red Sea). In addition, using high-throughput sequencing of the ITS2 region we characterized both the dominant and the rare Symbiodiniaceae lineages found in several species across depth. The phylogenetic analyses of both markers were in agreement and revealed that most of the studied mesophotic octocorals host the genus Cladocopium. Litophyton spp. and Klyxum utinomii were exceptions, as they harbored Symbiodinium and Durusdinium photosymbionts, respectively. While the dominant algal lineage of each coral species did not vary across depth, the endosymbiont community structure significantly differed between host species, as well as between different depths for some host species. The findings from this study contribute to the growing global-catalogue of Cnidaria-Symbiodiniaceae associations. Unravelling the Symbiodiniaceae composition in octocoral holobionts across environmental gradients, depth in particular, may enable a better understanding of how specialized those associations are, and to what extent coral holobionts are able to modify their photosymbionts.

Symbiosis induces unique volatile profiles in the model cnidarian Aiptasia.

The establishment and maintenance of the symbiosis between a cnidarian host and its dinoflagellate symbionts is central to the success of coral reefs. To explore the metabolite production underlying this symbiosis, we focused on a group of low molecular weight secondary metabolites, biogenic volatile organic compounds (BVOCs). BVOCs are released from an organism or environment, and can be collected in the gas phase, allowing non-invasive analysis of an organism's metabolism (i.e. 'volatilomics'). We characterised volatile profiles of the sea anemone Aiptasia (Exaiptasia diaphana), a model system for cnidarian-dinoflagellate symbiosis, using comprehensive two-dimensional gas chromatography coupled with time-of-flight mass spectrometry. We compared volatile profiles between: (1) symbiotic anemones containing their native symbiont, Breviolum minutum; (2) aposymbiotic anemones; and (3) cultured isolates of B. minutum. Overall, 152 BVOCs were detected, and classified into 14 groups based on their chemical structure, the most numerous groups being alkanes and aromatic compounds. A total of 53 BVOCs were differentially abundant between aposymbiotic anemones and B. minutum cultures; 13 between aposymbiotic and symbiotic anemones; and 60 between symbiotic anemones and cultures of B. minutum. More BVOCs were differentially abundant between cultured and symbiotic dinoflagellates than between aposymbiotic and symbiotic anemones, suggesting that symbiosis may modify symbiont physiology more than host physiology. This is the first volatilome analysis of the Aiptasia model system and provides a foundation from which to explore how BVOC production is perturbed under environmental stress, and ultimately the role they play in this important symbiosis.

Engineered nanoceria alleviates thermally induced oxidative stress in free-living Breviolum minutum (Symbiodiniaceae, formerly Clade B)

The breakdown of symbiotic mutualism between cnidarian hosts and dinoflagellate algae partners (i.e., bleaching) has been linked to an immune-like response pathway brought on by a nitro-oxidative burst, a symptom of thermal stress. Stress induced by reactive oxygen species (ROS)/reactive nitrogen species is a problem common to aerobic systems. In this study, we tested the antioxidant effects of engineered poly(acrylic acid)-coated cerium dioxide nanoparticles (CeO2, nanoceria) on free-living Symbiodiniaceae (Breviolum minutum), a dinoflagellate alga that forms symbiotic relationships with reef-building corals and anemones. Results show that poly(acrylic acid)-coated CeO2 with hydrodynamic diameters of ~4 nm are internalized by B. minutum in under 30 min and subsequently localized in the cytosol. Nanoceria exposure does not inhibit cell growth over time, with the treated cultures showing a similar growth trend over the 25-day exposure. Aerobic activity and thermal stress when held at 34°C for 1 h (+6°C above control) led to increased intracellular ROS concentration with time. A clear ROS scavenging effect of the nanoceria was observed, with a 5-fold decrease in intracellular ROS levels during thermal stress. The nitric oxide (NO) concentration decreased by ~17% with thermal stress, suggesting the rapid involvement of NO scavenging enzymes or proteins within 1 h of stress onset. The presence of nanoceria did not appear to influence NO concentration. Furthermore, aposymbiotic anemones (Exaiptasia diaphana, ex Aiptasia pallida) were successfully infected with nanoceria-loaded B. minutum, demonstrating that inoculation could serve as a delivery method. The ability of nanoceria to be taken up by Symbiodiniaceae and reduce ROS production could be leveraged as a potential mitigation strategy to reduce coral bleaching.

Cryptic diversity and spatial genetic variation in the coral Acropora tenuis and its endosymbionts across the Great Barrier Reef.

Genomic studies are uncovering extensive cryptic diversity within reef-building corals, suggesting that evolutionarily and ecologically relevant diversity is highly underestimated in the very organisms that structure coral reefs. Furthermore, endosymbiotic algae within coral host species can confer adaptive responses to environmental stress and may represent additional axes of coral genetic variation that are not constrained by taxonomic divergence of the cnidarian host. Here, we examine genetic variation in a common and widespread, reef-building coral, Acropora tenuis, and its associated endosymbiotic algae along the entire expanse of the Great Barrier Reef (GBR). We use SNPs derived from genome-wide sequencing to characterize the cnidarian coral host and organelles from zooxanthellate endosymbionts (genus Cladocopium). We discover three distinct and sympatric genetic clusters of coral hosts, whose distributions appear associated with latitude and inshore-offshore reef position. Demographic modelling suggests that the divergence history of the three distinct host taxa ranges from 0.5 to 1.5million years ago, preceding the GBR's formation, and has been characterized by low-to-moderate ongoing inter-taxon gene flow, consistent with occasional hybridization and introgression typifying coral evolution. Despite this differentiation in the cnidarian host, A.tenuis taxa share a common symbiont pool, dominated by the genus Cladocopium (Clade C). Cladocopium plastid diversity is not strongly associated with host identity but varies with reef location relative to shore: inshore colonies contain lower symbiont diversity on average but have greater differences between colonies as compared with symbiont communities from offshore colonies. Spatial genetic patterns of symbiont communities could reflect local selective pressures maintaining coral holobiont differentiation across an inshore-offshore environmental gradient. The strong influence of environment (but not host identity) on symbiont community composition supports the notion that symbiont community composition responds to habitat and may assist in the adaptation of corals to future environmental change.

Fitness of Cassiopea polyps Inoculated with Different Types of Symbionts

The specificity of the relationship between cnidarian hosts and symbiotic dinoflagellates (zooxanthellae) differs among host species. Some cnidarian hosts can establish symbiotic relationship with various types of zooxanthellae, while others exhibit high fidelity to specific symbiont type. It is not known how compatibility or specificity of the relationship is determined. We hypothesized that some cnidarian hosts select symbiont type that leads to highest fitness when the host is flexible with symbiont type and more than one types of symbionts are available. As a first step to study this possibility, compatibility of clonal polyps of Cassiopea sp. with six strains of cultured zooxanthellae and the fitness of the host associated with different types of symbionts were studied. Polyp diameter was measured and the number of asexual buds were calculated as a measure of host fitness. The number of zooxanthellae in host and in asexual buds was also measured as a measure of symbiont fitness. Three strains KB8 (clade A), Y106 (clade A), and K100 (clade B) were compatible with the Cassiopea polyps, while other three strains, Y103 (clade C), K111 (clade D), and K102 (clade F) were incompatible. No clear difference in the fitness was found among the polyps inoculated with compatible and incompatible symbiont strains. In one experiment, a compatible strain Y106 seemed to decrease host fitness, but this should be checked by further studies. This study suggests that feeding regimes and long observation period might be important when fitness of hosts associated with different types of symbionts is investigated.

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 6months 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.

High light quantity suppresses locomotion in symbiotic Aiptasia

Many cnidarians engage in endosymbioses with microalgae of the family Symbiodiniaceae. In this association, the fitness of the cnidarian host is closely linked to the photosynthetic performance of its microalgal symbionts. Phototaxis may enable semi-sessile cnidarians to optimize the light regime for their microalgal symbionts. Indeed, phototaxis and phototropism have been reported in the photosymbiotic sea anemone Aiptasia. However, the influence of light quantity on the locomotive behavior of Aiptasia remains unknown. Here we show that light quantity and the presence of microalgal symbionts modulate the phototactic behavior in Aiptasia. Although photosymbiotic Aiptasia were observed to move in seemingly random directions along an experimental light gradient, their probability of locomotion depended on light quantity. As photosymbiotic animals were highly mobile in low light but almost immobile at high light quantities, photosymbiotic Aiptasia at low light quantities exhibited an effective net movement towards light levels sufficient for positive net photosynthesis. In contrast, aposymbiotic Aiptasia exhibited greater mobility than their photosymbiotic counterparts, regardless of light quantity. Our results suggest that photosynthetic activity of the microalgal symbionts suppresses locomotion in Aiptasia, likely by supporting a positive energy balance in the host. We propose that motile photosymbiotic organisms can develop phototactic behavior as a consequence of starvation linked to symbiotic nutrient cycling.