New insights into the dynamics between reef corals and their associated dinoflagellate endosymbionts from population genetic studies

The mutualistic symbioses between reef‐building corals and micro‐algae form the basis of coral reef ecosystems, yet recent environmental changes threaten their survival. Diversity in host‐symbiont pairings on the sub‐species level could be an unrecognized source of functional variation in response to stress. The Caribbean elkhorn coral, Acropora palmata, associates predominantly with one symbiont species (Symbiodinium ‘fitti’), facilitating investigations of individual‐level (genotype) interactions. Individual genotypes of both host and symbiont were resolved across the entire species’ range. Most colonies of a particular animal genotype were dominated by one symbiont genotype (or strain) that may persist in the host for decades or more. While Symbiodinium are primarily clonal, the occurrence of recombinant genotypes indicates sexual recombination is the source of this genetic variation, and some evidence suggests this happens within the host. When these data are examined at spatial scales spanning the entire distribution of A. palmata, gene flow among animal populations was an order of magnitude greater than among populations of the symbiont. This suggests that independent micro‐evolutionary processes created dissimilar population genetic structures between host and symbiont. The lower effective dispersal exhibited by the dinoflagellate raises questions regarding the extent to which populations of host and symbiont can co‐evolve during times of rapid and substantial climate change. However, these findings also support a growing body of evidence, suggesting that genotype‐by‐genotype interactions may provide significant physiological variation, influencing the adaptive potential of symbiotic reef corals to severe selection.     

Chronic parrotfish grazing impedes coral recovery after bleaching

Coral bleaching, in which corals become visibly pale and typically lose their endosymbiotic zooxanthellae (Symbiodinium spp.), increasingly threatens coral reefs worldwide. While the proximal environmental triggers of bleaching are reasonably well understood, considerably less is known concerning physiological and ecological factors that might exacerbate coral bleaching or delay recovery. We report a bleaching event in Belize during September 2004 in which Montastraea spp. corals that had been previously grazed by corallivorous parrotfishes showed a persistent reduction in symbiont density compared to intact colonies. Additionally, grazed corals exhibited greater diversity in the genetic composition of their symbiont communities, changing from uniform ITS2 type C7 Symbiodinium prior to bleaching to mixed assemblages of Symbiodinium types post-bleaching. These results suggest that chronic predation may exacerbate the influence of environmental stressors and, by altering the coral-zooxanthellae symbiosis, such abiotic-biotic interactions may contribute to spatial variation in bleaching processes.     

High genetic differentiation and cross-shelf patterns of genetic diversity among Great Barrier Reef populations of Symbiodinium

The resilience of Symbiodinium harboured by corals is dependent on the genetic diversity and extent of connectivity among reef populations. This study presents genetic analyses of Great Barrier Reef (GBR) populations of clade C Symbiodinium hosted by the alcyonacean coral, Sinularia flexibilis. Allelic variation at four newly developed microsatellite loci demonstrated that Symbiodinium populations are genetically differentiated at all spatial scales from 16 to 1,360 km (pairwise Φ_ST = 0.01–0.47, mean = 0.22); the only exception being two neighbouring populations in the Cairns region separated by 17 km. This indicates that gene flow is restricted for Symbiodinium C hosted by S. flexibilis on the GBR. Patterns of population structure reflect longshore circulation patterns and limited cross-shelf mixing, suggesting that passive transport by currents is the primary mechanism of dispersal in Symbiodinium types that are acquired horizontally. There was no correlation between the genetic structure of Symbiodinium populations and their host S. flexibilis, most likely because different factors affect the dispersal and recruitment of each partner in the symbiosis. The genetic diversity of these Symbiodinium reef populations is on average 1.5 times lower on inshore reefs than on offshore reefs. Lower inshore diversity may reflect the impact of recent bleaching events on Sinularia assemblages, which have been more widespread and severe on inshore reefs, but may also have been shaped by historical sea level fluctuations or recent migration patterns. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. Communicated by Biology Editor Dr. Ruth Gates.     

Assessing Symbiodinium diversity in scleractinian corals via next‐generation sequencing‐based genotyping of the ITS2 rDNA region

The persistence of coral reef ecosystems relies on the symbiotic relationship between scleractinian corals and intracellular, photosynthetic dinoflagellates in the genus Symbiodinium. Genetic evidence indicates that these symbionts are biologically diverse and exhibit discrete patterns of environmental and host distribution. This makes the assessment of Symbiodinium diversity critical to understanding the symbiosis ecology of corals. Here, we applied pyrosequencing to the elucidation of Symbiodinium diversity via analysis of the internal transcribed spacer 2 (ITS2) region, a multicopy genetic marker commonly used to analyse Symbiodinium diversity. Replicated data generated from isoclonal Symbiodinium cultures showed that all genomes contained numerous, yet mostly rare, ITS2 sequence variants. Pyrosequencing data were consistent with more traditional denaturing gradient gel electrophoresis (DGGE) approaches to the screening of ITS2 PCR amplifications, where the most common sequences appeared as the most intense bands. Further, we developed an operational taxonomic unit (OTU)‐based pipeline for Symbiodinium ITS2 diversity typing to provisionally resolve ecologically discrete entities from intragenomic variation. A genetic distance cut‐off of 0.03 collapsed intragenomic ITS2 variants of isoclonal cultures into single OTUs. When applied to the analysis of field‐collected coral samples, our analyses confirm that much of the commonly observed Symbiodinium ITS2 diversity can be attributed to intragenomic variation. We conclude that by analysing Symbiodinium populations in an OTU‐based framework, we can improve objectivity, comparability and simplicity when assessing ITS2 diversity in field‐based studies.     

Phylogenetic Identification of Symbiotic Dinoflagellates via Length Heteroplasmy in Domain V of Chloroplast Large Subunit (cp23S)—Ribosomal DNA Sequences

A protocol that takes advantage of length heteroplasmy in domain V of chloroplast large subunit (cp23S)–ribosomal DNA to identify members of the symbiotic dinoflagellate genus Symbiodinium is presented. This protocol is highly specific for Symbiodinium, can provide intercladal and intracladal identification of a particular Symbiodinium isolate, and can detect multiple Symbiodinium chloroplast genotypes simultaneously in the same isolate, making his technique attractive for a variety of research questions. We used this technique to characterize variation among Symbiodinium populations associated with a range of phylogenetically diverse and geographically discrete hosts. We also examined symbiont variation within a single host, the Caribbean gorgonian Pseudopterogorgia elisabethae, from 9 sites in the Bahamas, and we report a previously undocumented level of symbiont specificity for particular members of Symbiodinium clade B in this gorgonian. Current address of Scott R. Santos: Department of Biochemistry and Molecular Biophysics, University of Arizona, Tucson, AZ, 85721, U.S.A.     

Population genetic data of a model symbiotic cnidarian system reveal remarkable symbiotic specificity and vectored introductions across ocean basins

The Aiptasia–Symbiodinium symbiosis is a promising model for experimental studies of cnidarian–dinoflagellate associations, yet relatively little is known regarding the genetic diversity of either symbiotic partner. To address this, we collected Aiptasia from 16 localities throughout the world and examined the genetic diversity of both anemones and their endosymbionts. Based on newly developed SCAR markers, Aiptasia consisted of two genetically distinct populations: one Aiptasia lineage from Florida and a second network of Aiptasia genotypes found at other localities. These populations did not conform to the distributions of described Aiptasia species, suggesting that taxonomic re‐evaluation is needed in the light of molecular genetics. Associations with Symbiodinium further demonstrated the distinctions among Aiptasia populations. According to 18S RFLP, ITS2‐DGGE and microsatellite flanker region sequencing, Florida anemones engaged in diverse symbioses predominantly with members of Symbiodinium Clades A and B, but also C, whereas anemones from elsewhere harboured only S. minutum within Clade B. Symbiodinium minutum apparently does not form a stable symbiosis with other hosts, which implies a highly specific symbiosis. Fine‐scale differences among S. minutum populations were quantified using six microsatellite loci. Populations of S. minutum had low genotypic diversity and high clonality (R = 0.14). Furthermore, minimal population structure was observed among regions and ocean basins, due to allele and genotype sharing. The lack of genetic structure and low genotypic diversity suggest recent vectoring of Aiptasia and S. minutum across localities. This first ever molecular‐genetic study of a globally distributed cnidarian and its Symbiodinium assemblages reveals host–symbiont specificity and widely distributed populations in an important model system.     

<Emphasis Type="Italic">Symbiodinium</Emphasis> associations with diseased and healthy scleractinian corals

Despite recent advances in identifying the causative agents of disease in corals and understanding the impact of epizootics on reef communities, little is known regarding the interactions among diseases, corals, and their dinoflagellate endosymbionts (Symbiodinium spp.). Since the genotypes of both corals and their resident Symbiodinium contribute to colony-level phenotypes, such as thermotolerance, symbiont genotypes might also contribute to the resistance or susceptibility of coral colonies to disease. To explore this, Symbiodinium were identified using the internal transcribed spacer-2 region of ribosomal DNA from diseased and healthy tissues within individual coral colonies infected with black band disease (BB), dark spot syndrome (DSS), white plague disease (WP), or yellow blotch disease (YB) in the Florida Keys (USA) and the US Virgin Islands. Most of the diseased colonies sampled contained B1, B5a, or C1 (depending on host species), while apparently healthy colonies of the same coral species frequently hosted these types and/or additional symbiont diversity. No potentially “parasitic” Symbiodinium types, uniquely associated with diseased coral tissue, were detected. Within most individual colonies, the same dominant Symbiodinium type was detected in diseased and visually healthy tissues. These data indicate that specific Symbiodinium types are not correlated with the infected tissues of diseased colonies and that DSS and WP onset do not trigger symbiont shuffling within infected tissues. However, few diseased colonies contained clade D symbionts suggesting a negative correlation between hosting Symbiodinium clade D and disease incidence in scleractinian corals. Understanding the influence of Symbiodinium diversity on colony phenotypes may play a critical role in predicting disease resistance and susceptibility in scleractinian corals.     

Population genetics of reef coral endosymbionts (Symbiodinium,Dinophyceae)

Symbiodinium is a diverse genus of unicellular dinoflagellate symbionts associating with various marine protists and invertebrates. Although the broadscale diversity and phylogenetics of the Symbiodinium complex is well established, there have been surprisingly few data on fine‐scale population structure and biogeography of these dinoflagellates. Yet population‐level processes contribute strongly to the biology of Symbiodinium, including how anthropogenic‐driven global climate change impacts these symbionts and their host associations. Here, we present a synthesis of population‐level characteristics for Symbiodinium, with an emphasis on how phylogenetic affinities, dynamics within and among host individuals, and a propensity towards clonality shape patterns on and across reefs. Major inferences include the following: (i) Symbiodinium populations within individual hosts are comprised mainly of cells belonging to a single or few genetic clones. (ii) Symbiont populations exhibit a mixed mode of reproduction, wherein at least one sexual recombination event occurs in the genealogy between most genotypes, but clonal propagation predominates overall. (iii) Mutualistic Symbiodinium do not perpetually persist outside their hosts, instead undergoing turnover and replacement via the continuous shedding of viable clonal cells from host individuals. (iv) Symbiont populations living in the same host, but on different reefs, are often genetically subdivided, suggesting low connectivity, adaptation to local conditions, or prolific asexual reproduction and low effective population sizes leading to disproportionate success within and among hosts. Overall, this synthesis forms a basis for future investigations of coral symbiosis ecology and evolution as well as delimitation of species boundaries in Symbiodinium and other eukaryotic microorganisms.     

Cell Cycle Dynamics of Cultured Coral Endosymbiotic Microalgae (Symbiodinium) Across Different Types (Species) Under Alternate Light and Temperature Conditions

Dinoflagellates of the genus Symbiodinium live in symbiosis with many invertebrates, including reef‐building corals. Hosts maintain this symbiosis through continuous regulation of Symbiodinium cell density via expulsion and degradation (postmitotic) and/or constraining cell growth and division through manipulation of the symbiont cell cycle (premitotic). Importance of premitotic regulation is unknown since little data exists on cell cycles for the immense genetic diversity of Symbiodinium. We therefore examined cell cycle progression for several distinct SymbiodiniumITS2‐types (B1, C1, D1a). All types exhibited typical microalgal cell cycle progression, G₁ phase through to S phase during the light period, and S phase to G₂/M phase during the dark period. However, the proportion of cells in these phases differed between strains and reflected differences in growth rates. Undivided larger cells with 3n DNA content were observed especially in type D1a, which exhibited a distinct cell cycle pattern. We further compared cell cycle patterns under different growth light intensities and thermal regimes. Whilst light intensity did not affect cell cycle patterns, heat stress inhibited cell cycle progression and arrested all strains in G₁ phase. We discuss the importance of understanding Symbiodinium functional diversity and how our findings apply to clarify stability of host‐Symbiodinium symbioses.     

Molecular signatures of host specificity linked to habitat specialization in Exaiptasia sea anemones

Rising ocean temperatures associated with global climate change induce breakdown of the symbiosis between coelenterates and photosynthetic microalgae of the genus Symbiodinium. Association with more thermotolerant partners could contribute to resilience, but the genetic mechanisms controlling specificity of hosts for particular Symbiodinium types are poorly known. Here, we characterize wild populations of a sea anemone laboratory model system for anthozoan symbiosis, from contrasting environments in Caribbean Panama. Patterns of anemone abundance and symbiont diversity were consistent with specialization of holobionts for particular habitats, with Exaiptasia pallida/S. minutum (ITS2 type B1) abundant on vertical substrate in thermally stable, shaded environments but E. brasiliensis/Symbiodinium sp. (ITS2 clade A) more common in shallow areas subject to high temperature and irradiance. Population genomic sequencing revealed a novel E. pallida population from the Bocas del Toro Archipelago that only harbors S. minutum. Loci most strongly associated with divergence of the Bocas‐specific population were enriched in genes with putative roles in cnidarian symbiosis, including activators of the complement pathway of the innate immune system, thrombospondin‐type‐1 repeat domain proteins, and coordinators of endocytic recycling. Our findings underscore the importance of unmasking cryptic diversity in natural populations and the role of long‐term evolutionary history in mediating interactions with Symbiodinium.     

Latent virus‐like infections are present in a diverse range of Symbiodinium spp. (Dinophyta)

Coral reefs are increasingly threatened by disease outbreaks, which affect the coral animal and/or its algal symbionts (Symbiodinium spp.) and can cause mass mortalities. Currently around half of the recognized coral diseases have unknown causative agents. While many of the diseases are thought to be bacterial in origin, there is growing evidence that viruses may play a role. In particular, it appears that viruses may infect the algal symbionts, causing breakdown of the coral‐algal mutualism. In this study, we screened a wide range of Symbiodinium cultures in vitro for the presence of latent viral infections. Using flow cytometry and electron microscopy, we found that many types of Symbiodinium apparently harbor such infections, and that the type of putative virus varied within and among host types. Furthermore, the putative viral infections could be induced via abiotic stress and cause host cell lysis and population decline. If similar processes occur in Symbiodinium cells in hospite, they may provide an explanation for some of the diseases affecting corals and other organisms forming symbioses with these algae.     

Exploring Symbiodinium diversity and host specificity in Acropora corals from geographical extremes of Western Australia with 454 amplicon pyrosequencing

Scleractinian corals have demonstrated the ability to shuffle their endosymbiotic dinoflagellate communities (genus Symbiodinium) during periods of acute environmental stress. This has been proposed as a mechanism of acclimation, which would be increased by a diverse and flexible association with Symbiodinium. Conventional molecular techniques used to evaluate Symbiodinium diversity are unable to identify genetic lineages present at background levels below 10%. Next generation sequencing (NGS) offers a solution to this problem and can resolve microorganism diversity at much finer scales. Here we apply NGS to evaluate Symbiodinium diversity and host specificity in Acropora corals from contrasting regions of Western Australia. The application of 454 pyrosequencing allowed for detection of Symbiodinium operational taxonomic units (OTUs) occurring at frequencies as low as 0.001%, offering a 10 000‐fold increase in sensitivity compared to traditional methods. All coral species from both regions were overwhelmingly dominated by a single clade C OTU (accounting for 98% of all recovered sequences). Only 8.5% of colonies associated with multiple clades (clades C and D, or C and G), suggesting a high level of symbiont specificity in Acropora assemblages in Western Australia. While only 40% of the OTUs were shared between regions, the dominance of a single OTU resulted in no significant difference in Symbiodinium community structure, demonstrating that the coral‐algal symbiosis can remain stable across more than 15° of latitude and a range of sea surface temperature profiles. This study validates the use of NGS platforms as tools for providing fine‐scale estimates of Symbiodinium diversity and can offer critical insight into the flexibility of the coral‐algal symbiosis.     

Spatial and temporal genetic structure of Symbiodinium populations within a common reef‐building coral on the Great Barrier Reef

The dinoflagellate photosymbiont Symbiodinium plays a fundamental role in defining the physiological tolerances of coral holobionts, but little is known about the dynamics of these endosymbiotic populations on coral reefs. Sparse data indicate that Symbiodinium populations show limited spatial connectivity; however, no studies have investigated temporal dynamics for in hospite Symbiodinium populations following significant mortality and recruitment events in coral populations. We investigated the combined influences of spatial isolation and disturbance on the population dynamics of the generalist Symbiodinium type C2 (ITS1 rDNA) hosted by the scleractinian coral Acropora millepora in the central Great Barrier Reef. Using eight microsatellite markers, we genotyped Symbiodinium in a total of 401 coral colonies, which were sampled from seven sites across a 12‐year period including during flood plume–induced coral bleaching. Genetic differentiation of Symbiodinium was greatest within sites, explaining 70–86% of the total genetic variation. An additional 9–27% of variation was explained by significant differentiation of populations among sites separated by 0.4–13 km, which is consistent with low levels of dispersal via water movement and historical disturbance regimes. Sampling year accounted for 6–7% of total genetic variation and was related to significant coral mortality following severe bleaching in 1998 and a cyclone in 2006. Only 3% of the total genetic variation was related to coral bleaching status, reflecting generally small (8%) reductions in allelic diversity within bleached corals. This reduction probably reflected a loss of genotypes in hospite during bleaching, although no site‐wide changes in genetic diversity were observed. Combined, our results indicate the importance of disturbance regimes acting together with limited oceanographic transport to determine the genetic composition of Symbiodinium types within reefs.     

Vectored introductions of marine endosymbiotic dinoflagellates into Hawaii

Endosymbiotic dinoflagellates belonging to the genus Symbiodinium associate with a diverse range of marine invertebrate hosts and also exist free-living in the ocean. The genus is divided into eight lineages (clades A–H), which contain multiple subclade types that show geographic and host specificity. It is commonly known that free-living dinoflagellates can and have been introduced to new geographic locations, primarily through shipping ballast water. In this study we sequenced the ITS2 region of Symbiodinium found in symbiosis with the coral Acropora cytherea in the Northwestern Hawaiian Islands Marine National Monument and from shipping ballast water. Identification of an unusual symbiont in Acropora cytherea and an analysis of the distribution of this symbiont suggests an introduction to Hawaii vectored by the scyphozoan host, Cassiopea sp. Symbiodinium were also detected in shipping ballast water. This work confirms that marine invertebrate endosymbionts can be introduced to new geographic locations vectored by animal hosts or the ballast water of ships.     

Microsatellite allele sizes alone are insufficient to delineate species boundaries in Symbiodinium

Symbiodinium are a diverse group of unicellular dinoflagellates that are important nutritional symbionts of reef‐building corals. Symbiodinium putative species (‘types’) are commonly identified with genetic markers, mostly nuclear and chloroplast encoded ribosomal DNA regions. Population genetic analyses using microsatellite loci have provided insights into Symbiodinium biogeography, connectivity and phenotypic plasticity, but are complicated by: (i) a lack of consensus criteria used to delineate inter‐ vs. intragenomic variation within species; and (ii) the high density of Symbiodinium in host tissues, which results in single samples comprising thousands of individuals. To address this problem, Wham & LaJeunesse (2016) present a method for identifying cryptic Symbiodinium species from microsatellite data based on correlations between allele size distributions and nongeographic genetic structure. Multilocus genotypes that potentially do not recombine in sympatry are interpreted as secondary ‘species’ to be discarded from downstream population genetic analyses. However, Symbiodinium species delineations should ideally incorporate multiple physiological, ecological and molecular criteria. This is because recombination tests may be a poor indicator of species boundaries in Symbiodinium due to their predominantly asexual mode of reproduction. Furthermore, discontinuous microsatellite allele sizes in sympatry may be explained by secondary contact between previously isolated populations and by mutations that occur in a nonstepwise manner. Limitations of using microsatellites alone to delineate species are highlighted in earlier studies that demonstrate occasional bimodal distributions of allele sizes within Symbiodinium species and considerable allele size sharing among Symbiodinium species. We outline these issues and discuss the validity of reinterpretations of our previously published microsatellite data from Symbiodinium populations on the Great Barrier Reef (Howells et al. 2013).     

The Occurrence of Mixed Infections of Symbiodinium (Dinoflagellata) within Individual Hosts

Coral reef ecosystems depend on symbiosis between dinoflagellates of the genus Symbiodinium Freudenthal and their various hosts. The physiological characteristics associated with a particular lineage or species of Symbiodinium can determine a host's susceptibility to harmful bleaching. Therefore, the threat posed by global climate change on a host may be reduced if it can switch or shuffle its dominant algal symbiont type. An important prerequisite to this potential to switch or shuffle is the ability to host multiple alternative dominant symbiont genotypes. To examine the distribution of this trait, we review reports of mixed Symbiodinium infections in corals and nonscleractinian hosts from a phylogenetic perspective. Hosts showing evidence of mixed infection are broadly distributed across the most deeply divergent host lineages, including foraminifera, mollusks, sponges, and cnidarians. The occurrence of mixed infections is also broadly distributed across most clades of scleractinian corals. Individual colonies of certain well‐studied cosmopolitan coral genera, such as Acropora, Montastraea, and Pocillopora, yield many reports of mixed infection, while other genera, such as Porites, do not. We further discuss mixed Symbiodinium infections in the context of evolutionary ecology theory. Selection pressures that affect the prevalence of mixed infection may be exerted by variation in host environment, host ontogeny, symbiont transmission strategy, host regulation of symbiont populations, availability of free‐living symbiont lineages, competition between symbiont lineages, and niche partitioning of the internal host environment.     

Juvenile corals can acquire more carbon from high-performance algal symbionts

Algal endosymbionts of the genus Symbiodinium play a key role in the nutrition of reef building corals and strongly affect the thermal tolerance and growth rate of the animal host. This study reports that ¹⁴C photosynthate incorporation into juvenile coral tissues was doubled in Acropora millepora harbouring Symbiodinium C1 compared with juveniles from common parentage harbouring Symbiodinium D in a laboratory experiment. Rapid light curves performed on the same corals revealed that the relative electron transport rate of photosystem II (rETR_MAX) was 87% greater in Symbiodinium C1 than in Symbiodinium D in hospite. The greater relative electron transport through photosystem II of Symbiodinium C1 is positively correlated with increased carbon delivery to the host under the applied experimental conditions (r ² = 0.91). This may translate into a competitive advantage for juveniles harbouring Symbiodinium C1 under certain field conditions, since rapid early growth typically limits mortality. Both symbiont types exhibited severe reductions in ¹⁴C incorporation during a 10-h exposure to the electron transport blocking herbicide diuron (DCMU), confirming the link between electron transport through PSII and photosynthate incorporation within the host tissue. These findings advance the current understanding of symbiotic relationships between corals and their symbionts, providing evidence that enhanced growth rates of juvenile corals may result from greater translocation of photosynthates from Symbiodinium C1. Communicated by Biology Editor Dr. Ruth Gates     

One-year survey of a single Micronesian reef reveals extraordinarily rich diversity of <Emphasis Type="Italic">Symbiodinium</Emphasis> types in soritid foraminifera

Recent molecular studies of symbiotic dinoflagellates (genus Symbiodinium) from a wide array of invertebrate hosts have revealed exceptional fine-scale symbiont diversity whose distribution among hosts, regions and environments exhibits significant biogeographic, ecological and evolutionary patterns. Here, similar molecular approaches using the internal transcribed spacer-2 (ITS-2) region were applied to investigate cryptic diversity in Symbiodinium inhabiting soritid foraminifera. Approximately 1,000 soritid specimens were collected and examined during a 12-month period over a 40 m depth gradient from a single reef in Guam, Micronesia. Out of 61 ITS-2 types distinguished, 46 were novel. Most types found are specific for soritid hosts, except for three types (C1, C15 and C19) that are common in metazoan hosts. The distribution of these symbionts was compared with the phylotype of their foraminiferal hosts, based on soritid small subunit ribosomal DNA sequences, and three new phylotypes of soritid hosts were identified based on these sequences. Phylogenetic analyses of 645 host-symbiont pairings revealed that most Symbiodinium types associated specifically with a particular foraminiferal host genus or species, and that the genetic diversity of these symbiont types was positively correlated with the genetic diversity found within each of the three host genera. Compared to previous molecular studies of Symbiodinium from other locations worldwide, the diversity reported here is exceptional and suggests that Micronesian coral reefs are home to a remarkably large Symbiodinium assemblage.     

Seasonal Preservation Success of the Marine Dinoflagellate Coral Symbiont,Symbiodinium sp.

Coral reefs are some of the most diverse and productive ecosystems on the planet, but are threatened by global and local stressors, mandating the need for incorporating ex situ conservation practices. One approach that is highly protective is the development of genome resource banks that preserve the species and its genetic diversity. A critical component of the reef are the endosymbiotic algae, Symbiodinium sp., living within most coral that transfer energy-rich sugars to their hosts. Although Symbiodinium are maintained alive in culture collections around the world, the cryopreservation of these algae to prevent loss and genetic drift is not well-defined. This study examined the quantum yield physiology and freezing protocols that resulted in survival of Symbiodinium at 24 h post-thawing. Only the ultra-rapid procedure called vitrification resulted in success whereas conventional slow freezing protocols did not. We determined that success also depended on using a thin film of agar with embedded Symbiodinium on Cryotops, a process that yielded a post-thaw viability of >50% in extracted and vitrified Symbiodinium from Fungia scutaria, Pocillopora damicornis and Porites compressa. Additionally, there also was a seasonal influence on vitrification success as the best post-thaw survival of F. scutaria occurred in winter and spring compared to summer and fall (P < 0.05). These findings lay the foundation for developing a viable genome resource bank for the world’s Symbiodinium that, in turn, will not only protect this critical element of coral functionality but serve as a resource for understanding the complexities of symbiosis, support selective breeding experiments to develop more thermally resilient strains of coral, and provide a ‘gold-standard’ genomics collection, allowing for full genomic sequencing of unique Symbiodinium strains.     

Symbioses between salamander embryos and green algae

The symbiosis between Ambystoma maculatum (spotted salamander) embryos and green algae was initially described over 120 years ago. Algae populate the egg capsules that surround individual A. maculatum embryos, giving the intracapsular fluid a characteristic green hue. Early work established this symbiosis to be a mutualism, while subsequent studies sought to identify the material benefits of this association to both symbiont and host. These studies have shown that salamander embryos benefit from increased oxygen concentrations provided by their symbiotic algae. The algae, in turn, may benefit from ammonia excreted by the embryos. All of these early studies considered the association to be an ectosymbiotic mutualism. However our recent work has shown that algae invade both embryonic salamander cells and tissues during development. The unexpected invasion of algal cells into a salamander host changes our understanding of this symbiosis. This review will summarize the earlier research on this association in the context of these recent findings. It will also emphasize gaps in our understanding of this and other amphibian embryo-algal interactions and suggest various research avenues to address these unanswered questions.