Dinoflagellate Symbionts Research Articles

Multi-gene incongruence consistent with hybridisation in Cladocopium (Symbiodiniaceae), an ecologically important genus of coral reef symbionts.

Coral reefs rely on their intracellular dinoflagellate symbionts (family Symbiodiniaceae) for nutritional provision in nutrient-poor waters, yet this association is threatened by thermally stressful conditions. Despite this, the evolutionary potential of these symbionts remains poorly characterised. In this study, we tested the potential for divergent Symbiodiniaceae types to sexually reproduce (i.e. hybridise) within Cladocopium, the most ecologically prevalent genus in this family. With sequence data from three organelles (cob gene, mitochondrion; psbAncr region, chloroplast; and ITS2 region, nucleus), we utilised the Incongruence Length Difference test, Approximately Unbiased test, tree hybridisation analyses and visual inspection of raw data in stepwise fashion to highlight incongruences between organelles, and thus provide evidence of reticulate evolution. Using this approach, we identified three putative hybrid Cladocopium samples among the 158 analysed, at two of the seven sites sampled. These samples were identified as the common Cladocopium types C40 or C1 with respect to the mitochondria and chloroplasts, but the rarer types C3z, C3u and C1# with respect to their nuclear identity. These five Cladocopium types have previously been confirmed as evolutionarily distinct and were also recovered in non-incongruent samples multiple times, which is strongly suggestive that they sexually reproduced to produce the incongruent samples. A concomitant inspection of next generation sequencing data for these samples suggests that other plausible explanations, such as incomplete lineage sorting or the presence of co-dominance, are much less likely. The approach taken in this study allows incongruences between gene regions to be identified with confidence, and brings new light to the evolutionary potential within Symbiodiniaceae.

Transcriptome of Thalassicolla nucleata Holobiont Reveals Details of a Radiolarian Symbiotic Relationship

Radiolarians are a group of ubiquitous, yet poorly understood, large protists that often harbor photosymbionts. We studied the solitary radiolarian Thalassicolla nucleata by analyzing the transcriptome of its holobiont. We found that T. nucleata contained two dinoflagellate symbionts, one photosymbiont Brandtodinium sp., and one putative Peridiniales parasite. Through comparisons of gene expressions of Brandtodinium sp. and those of a close relative from a free-living culture, we found that the Brandtodinium sp. maintained its photosynthetic activities, but altered its carbon metabolism dramatically in hospite. Gene expression data also suggested carbon and nitrogen exchange between the host and photosymbiont and that lectin-glycan interaction might play an important role in host-symbiont recognition.

Sterol transfer by atypical cholesterol-binding NPC2 proteins in coral-algal symbiosis.

Reef-building corals depend on intracellular dinoflagellate symbionts that provide nutrients. Besides sugars, the transfer of sterols is essential for corals and other sterol-auxotrophic cnidarians. Sterols are important cell components, and variants of the conserved Niemann-Pick Type C2 (NPC2) sterol transporter are vastly up-regulated in symbiotic cnidarians. Types and proportions of transferred sterols and the mechanism of their transfer, however, remain unknown. Using different pairings of symbiont strains with lines of Aiptasia anemones or Acropora corals, we observe both symbiont- and host-driven patterns of sterol transfer, revealing plasticity of sterol use and functional substitution. We propose that sterol transfer is mediated by the symbiosis-specific, non-canonical NPC2 proteins, which gradually accumulate in the symbiosome. Our data suggest that non-canonical NPCs are adapted to the symbiosome environment, including low pH, and play an important role in allowing corals to dominate nutrient-poor shallow tropical seas worldwide.

Proteomics quantifies protein expression changes in a model cnidarian colonised by a thermally tolerant but suboptimal symbiont.

The acquisition of thermally tolerant algal symbionts by corals has been proposed as a natural or assisted mechanism of increasing coral reef resilience to anthropogenic climate change, but the cell-level processes determining the performance of new symbiotic associations are poorly understood. We used liquid chromatography-mass spectrometry to investigate the effects of an experimentally induced symbiosis on the host proteome of the model sea anemone Exaiptasia pallida. Aposymbiotic specimens were colonised by either the homologous dinoflagellate symbiont (Breviolum minutum) or a thermally tolerant, ecologically invasive heterologous symbiont (Durusdinium trenchii). Anemones containing D. trenchii exhibited minimal expression of Niemann-Pick C2 proteins, which have predicted biochemical roles in sterol transport and cell recognition, and glutamine synthetases, which are thought to be involved in nitrogen assimilation and recycling between partners. D. trenchii-colonised anemones had higher expression of methionine-synthesising betaine-homocysteine S-methyltransferases and proteins with predicted oxidative stress response functions. Multiple lysosome-associated proteins were less abundant in both symbiotic treatments compared with the aposymbiotic treatment. The differentially abundant proteins are predicted to represent pathways that may be involved in nutrient transport or resource allocation between partners. These results provide targets for specific experiments to elucidate the mechanisms underpinning compensatory physiology in the coral-dinoflagellate symbiosis.

Ocean acidification at a coastal CO2 vent induces expression of stress-related transcripts and transposable elements in the sea anemone Anemonia viridis.

Ocean acidification threatens to disrupt interactions between organisms throughout marine ecosystems. The diversity of reef-building organisms decreases as seawater CO2 increases along natural gradients, yet soft-bodied animals, such as sea anemones, are often resilient. We sequenced the polyA-enriched transcriptome of adult sea anemone Anemonia viridis and its dinoflagellate symbiont sampled along a natural CO2 gradient in Italy to assess stress levels in these organisms. We found that about 1.4% of the anemone transcripts, but only ~0.5% of the Symbiodinium sp. transcripts were differentially expressed. Processes enriched at high seawater CO2 were mainly linked to cellular stress, including significant up-regulation of protective cellular functions and deregulation of metabolic pathways. Transposable elements were differentially expressed at high seawater CO2, with an extreme up-regulation (> 100-fold) of the BEL-family of long terminal repeat retrotransposons. Seawater acidified by CO2 generated a significant stress reaction in A. viridis, but no bleaching was observed and Symbiodinium sp. appeared to be less affected. These observed changes indicate the mechanisms by which A. viridis acclimate to survive chronic exposure to ocean acidification conditions. We conclude that many organisms that are common in acidified conditions may nevertheless incur costs due to hypercapnia and/or lowered carbonate saturation states.

Host growth and survivorship varies with endosymbiotic algal partner in developing cnidarians

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 612:87-100 (2019) - DOI: https://doi.org/10.3354/meps12876 Host growth and survivorship varies with endosymbiotic algal partner in developing cnidarians Daniel M. Poland1, Mary Alice Coffroth2,* 1Department of Biological Sciences, University at Buffalo, Buffalo, NY 14260, USA 2Department of Geology, Graduate Program in Evolution, Ecology and Behavior, University at Buffalo, Buffalo, NY 14260, USA *Corresponding author: coffroth@buffalo.edu ABSTRACT: Juvenile cnidarians acquire diverse assemblages of dinoflagellate symbionts within the family Symbiodiniaceae from the environment, and the dominant symbionts often differ from those in conspecific adult hosts. Such non-homologous symbionts may confer benefits to their juvenile hosts that are not available from homologous symbionts (those found in adult symbioses). Benefits gained by harboring several heterologous symbiont types remain poorly understood. We investigated host growth and survivorship with several symbiont partners that naturally occur within juvenile Briareum asbestinum (Alcyonacea). We infected newly settled aposymbiotic juveniles with symbiont types within cultured Breviolum (formerly clade B) and Symbiodinium (formerly clade A) that naturally occur within juveniles of this host. The symbiont partner community significantly affected overall growth and mortality rates of juveniles during this 11 mo laboratory study. Heterologous symbionts found within juveniles may confer survival benefits, but they vary in the relative contributions to host fitness. Symbiont types within Breviolum provided the greatest growth and survivorship benefits to B. asbestinum hosts, and early acquisition of these symbionts increased survivorship as those juveniles initially harboring a more mixed population exhibited higher mortality rates. These findings demonstrate that symbiont types that naturally occur in developing octocoral hosts have significantly different effects on host growth and survival, and some types in particular play an important role in octocoral symbioses. KEY WORDS: Symbiodinium · Octocoral · Symbiosis · Ontogeny · Acquisition · Briareum asbestinum Full text in pdf format Supplementary material PreviousNextCite this article as: Poland DM, Coffroth MA (2019) Host growth and survivorship varies with endosymbiotic algal partner in developing cnidarians. Mar Ecol Prog Ser 612:87-100. https://doi.org/10.3354/meps12876 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 612. Online publication date: March 07, 2019 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2019 Inter-Research.

The genetic intractability of Symbiodinium microadriaticum to standard algal transformation methods.

Modern transformation and genome editing techniques have shown great success across a broad variety of organisms. However, no study of successfully applied genome editing has been reported in a dinoflagellate despite the first genetic transformation of Symbiodinium being published about 20 years ago. Using an array of different available transformation techniques, we attempted to transform Symbiodinium microadriaticum (CCMP2467), a dinoflagellate symbiont of reef-building corals, with the view to performing subsequent CRISPR-Cas9 mediated genome editing. Plasmid vectors designed for nuclear transformation containing the chloramphenicol resistance gene under the control of the CaMV p35S promoter as well as several putative endogenous promoters were used to test a variety of transformation techniques including biolistics, electroporation and agitation with silicon carbide whiskers. Chloroplast-targeted transformation was attempted using an engineered Symbiodinium chloroplast minicircle encoding a modified PsbA protein expected to confer atrazine resistance. We report that we have been unable to confer chloramphenicol or atrazine resistance on Symbiodinium microadriaticum strain CCMP2467.

Thermal and Herbicide Tolerances of Chromerid Algae and Their Ability to Form a Symbiosis With Corals.

Reef-building corals form an obligate symbiosis with photosynthetic microalgae in the family Symbiodiniaceae that meet most of their energy requirements. This symbiosis is under threat from the unprecedented rate of ocean warming as well as the simultaneous pressure of local stressors such as poor water quality. Only 1°C above mean summer sea surface temperatures (SSTs) on the Great Barrier Reef (GBR) can trigger the loss of Symbiodiniaceae from the host, and very low concentrations of the most common herbicide, diuron, can disrupt the photosynthetic activity of microalgae. In an era of rapid environmental change, investigation into the assisted evolution of the coral holobiont is underway in an effort to enhance the resilience of corals. Apicomplexan-like microalgae were discovered in 2008 and the Phylum Chromerida (chromerids) was created. Chromerids have been isolated from corals and contain a functional photosynthetic plastid. Their discovery therefore opens a new avenue of research into the use of alternative/additional photosymbionts of corals. However, only two studies to-date have investigated the symbiotic nature of Chromera velia with corals and thus little is known about the coral-chromerid relationship. Furthermore, the response of chromerids to environmental stressors has not been examined. Here we tested the performance of four chromerid strains and the common dinoflagellate symbiont Cladocopium goreaui (formerly Symbiodinium goreaui, ITS2 type C1) in response to elevated temperature, diuron and their combined exposure. Three of the four chromerid strains exhibited high thermal tolerances and two strains showed exceptional herbicide tolerances, greater than observed for any photosynthetic microalgae, including C. goreaui. We also investigated the onset of symbiosis between the chromerids and larvae of two common GBR coral species under ambient and stress conditions. Levels of colonization of coral larvae with the chromerid strains were low compared to colonization with C. goreaui. We did not observe any overall negative or positive larval fitness effects of the inoculation with chromerid algae vs. C. goreaui. However, we cannot exclude the possibility that chromerid algae may have more important roles in later coral life stages and recommend this be the focus of future studies.

Photochemical Efficiencies in Reef-Dwelling Anthozoans: Insights from “Survivor” Species

In recent decades, populations of many coral species have declined dramatically on reefs worldwide. A major factor in coral mortality has been photo-oxidative stress associated with both solar irradiance and elevated temperatures. While many studies have focused on species that have declined, fewer efforts have focused on the “survivor” species, those that have maintained relatively stable populations or even increased in abundance. The objective of this study was to assess temporal variability in photochemical efficiencies (Fv/Fm) as an indicator of potential photo-oxidative stress in the dinoflagellate symbionts in three species, the scleractinians Siderastrea siderea and Montastraea cavernosa, and the zoanthid Palythoa caribaeorum, whose populations have remained relatively stable along the Florida reef tract. Coral colonies with no visual indication of disease or bleaching were assessed quarterly in 2012 and 2013, at sites at 6 or 18 m depths. Colonies were dark-acclimated prior to measurements using pulse-amplitude modulated fluorometry. The mean Fv/Fm values for P. caribaeorum colonies assessed at 6 m depth were consistently the lowest (0.59, 0.02 SE). Siderastrea siderea assessed at both 6 and 18 m revealed significantly lower Fv/Fm values (p=0.0006) for those living at 6 m (0.64, 0.02 SE) than for those living at 18 m (0.68, 0.01 SE) depths. The Fv/Fm values for colonies of M. cavernosa assessed at 18 m also averaged 0.68 (0.01 SE). Thus, photochemical efficiencies were lower in colonies living at 6 m than in colonies living at 18 m, while no significant seasonal differences were found in the mean Fv/Fm values in the examined species. This study provides the first known report of photochemical efficiency in P. caribaeorum.

Microbial Associations of Four Species of Algal Symbiont-bearing Foraminifers from the Florida Reef Tract, Usa

Abstract While microbiome research is a rapidly expanding field of study, relatively little is known of the microbiomes associated with Foraminifera. This preliminary study investigated microbes associated with four species of Foraminifera, representing two taxonomic orders, which host three kinds of algal endosymbionts. A major objective was to explore potential influences on the microbiome composition, including phylogenetic relatedness among the host species, similarities in algal symbionts hosted, and environmental conditions from which the specimens were collected. Samples examined from two locations along the middle Florida Keys reef tract included 45 foraminiferal specimens and four environmental samples. Bacterial DNA extraction from individual specimens was followed by amplification and amplicon sequencing of the V4 variable region of the 16S rRNA gene; results were obtained from 21 specimens. The Order Miliolida, Family Soritidae, was represented by 5–8 specimens of each of three species: Archaias angulatus and Cyclorbiculina compressa, which both host chlorophyte symbionts, and Sorites orbiculus, which hosts dinoflagellate symbionts. Three Ar. angulatus specimens from which the microbiome was successfully sequenced shared 177 OTUs. Six C. compressa specimens successfully sequenced shared 58 OTUs, of which 31 were also shared by the three specimens of Ar. angulatus. Four successfully sequenced S. orbiculus specimens shared 717 unique OTUs. The 13 soritid specimens shared 26 OTUs, 23 of which represented Proteobacteria, predominantly of the bacterial family Rhodobacteraceae. The fourth foraminiferal species, Amphistegina gibbosa (Order Rotaliida) hosts diatom endosymbionts. Bacterial DNA extraction was attempted on 16 Am. gibbosa, including both normal-appearing and partly-bleached specimens. Only six OTUs, four of which represented Proteobacteria, were found in all eight specimens successfully sequenced. The partly bleached specimens shared nearly twice as many unique microbial OTUs (32) as the normal-appearing specimens (19). All Am. gibbosa specimens shared only four microbial OTUs with the soritid species, three of which may have been contaminants, indicating minimal commonality between the microbiomes of Am. gibbosa and the soritid taxa.

Patterns of Symbiodinium (Dinophyceae) diversity and assemblages among diverse hosts and the coral reef environment of Lizard Island, Australia.

Large-scale environmental disturbances may impact both partners in coral host-Symbiodinium systems. Elucidation of the assembly patterns in such complex and interdependent communities may enable better prediction of environmental impacts across coral reef ecosystems. In this study, we investigated how the community composition and diversity of dinoflagellate symbionts in the genus Symbiodinium were distributed among 12 host species from six taxonomic orders (Actinaria, Alcyonacea, Miliolida, Porifera, Rhizostoma, Scleractinia) and in the reef water and sediments at Lizard Island, Great Barrier Reef before the 3rd Global Coral Bleaching Event. 454 pyrosequencing of the ITS2 region of Symbiodinium yielded 83 operational taxonomic units (OTUs) at a 97% similarity cut-off. Approximately half of the Symbiodinium OTUs from reef water or sediments were also present in symbio. OTUs belonged to six clades (A-D, F-G), but community structure was uneven. The two most abundant OTUs (100% matches to types C1 and A3) comprised 91% of reads and OTU C1 was shared by all species. However, sequence-based analysis of these dominant OTUs revealed host species specificity, suggesting that genetic similarity cut-offs of Symbiodinium ITS2 data sets need careful evaluation. Of the less abundant OTUs, roughly half occurred at only one site or in one species and the background Symbiodinium communities were distinct between individual samples. We conclude that sampling multiple host taxa with differing life history traits will be critical to fully understand the symbiont diversity of a given system and to predict coral ecosystem responses to environmental change and disturbance considering the differential stress response of the taxa within.

Ground-truthing the planktic foraminifer-bound nitrogen isotope paleo-proxy in the Sargasso Sea

We report the nitrogen (N) isotope ratios (δ15N) of planktic foraminifera collected from upper-ocean net tows (surface to 200 m), moored sediment traps, and core-top sediments at the Bermuda Time-series Site in the northern Sargasso Sea between 2009 and 2013. Consistent with previous measurements from low-latitude core-top sediments, the annually-averaged δ15N of organic matter bound within the shells of euphotic zone-dwelling, dinoflagellate symbiont-bearing foraminifera collected in net tows (2.3‰ on average) approximates that of shallow thermocline (∼200 m) nitrate (2.6‰), the dominant source of new N to Sargasso Sea surface waters. Deeper-dwelling foraminifer species without dinoflagellate symbionts tend to have a higher δ15N (3.6‰ on average). We observe no systematic difference between the bulk tissue and shell-bound δ15N in net tow-collected foraminifera. A decline in shell N content is observed from net tows (6.8 nmol/mg) to sediment traps (5.4 nmol/mg) and surface sediment (3.0 nmol/mg). On average, shell-bound δ15N rises from net tows (3.1‰) to sediment traps (3.7‰) but does not change further upon incorporation into the sediments (3.7‰). Together, these observations are consistent with preferential loss of shells or shell portions with lower δ15N and higher N content during sinking through the upper 500 m, followed by a non-isotope fractionating decrease in N content between sinking and burial. Time-series data from sediment traps (and to a lesser extent, surface net tows) exhibit seasonal δ15N variations, with a minimum in early spring, a maximum in late spring and a decline from summer to fall. These variations appear to arise from seasonal changes in the δ15N of total upper-ocean biomass, which are, in turn, driven by early springtime nitrate supply, subsequent nitrate drawdown, and an increase in the relative importance of ammonium recycling into the late summer and early fall. The δ15N connection between total upper ocean biomass and foraminifera indicates that foraminifer-bound δ15N records the δ15N of the annual nitrate supply in oligotrophic (e.g., subtropical) environments but will also be sensitive to the degree of nitrate consumption in high-nutrient regions and possibly to changes in upper-ocean ammonium recycling under some conditions.

In situ shifts of predominance between autotrophic and heterotrophic feeding in the reef-building coral Mussismilia hispida: an approach using fatty acid trophic markers

Many species of reef-building corals are mixotrophic, relying on both photoautotrophy performed by their dinoflagellate symbionts and heterotrophy from consumption of zooplankton. Autotrophy and heterotrophy supply corals with specific ω3 fatty acids, which can be used as trophic markers and record the contribution of each feeding strategy. This study investigated whether the reef-building coral Mussismilia hispida, endemic to Brazil, is able to shift between predominantly autotrophic and predominantly heterotrophic by monitoring the concentration of fatty acids in the host tissue. We then examined whether shifts are related to changes in temperature and wind stress. For that purpose, M. hispida colonies were monitored for a year with monthly tissue sampling. Symbiont concentration was determined and lipid extraction performed. Four fatty acids were quantitatively analyzed by gas chromatography with flame ionization detector: the autotrophy markers: stearidonic acid (SDA), docosapentaenoic acid (DPA), docosahexaenoic acid (DHA), and a heterotrophy marker: cis-gondoic acid (CGA). Three preliminary experiments confirmed the specificity of SDA, DPA and CGA, but not of DHA. Shifts of predominance occurred multiple times during the year and were associated with minimal temperatures and wind stress. Colonies underwent mild bleaching during summer months, which they seemed to compensate with heterotrophic feeding. Our major findings include the validation of three FATM and a trophic index for coral reef ecology studies and also describing the in situ occurrences of shifts between feeding modes, while highlighting the role of temperature and meteorological events.

Comparative Metabolomics Approach Detects Stress-Specific Responses during Coral Bleaching in Soft Corals.

Chronic exposure to ocean acidification and elevated sea-surface temperatures pose significant stress to marine ecosystems. This in turn necessitates costly acclimation responses in corals in both the symbiont and host, with a reorganization of cell metabolism and structure. A large-scale untargeted metabolomics approach comprising gas chromatography mass spectrometry (GC-MS) and ultraperformance liquid chromatography coupled to high resolution mass spectrometry (UPLC-MS) was applied to profile the metabolite composition of the soft coral Sarcophyton ehrenbergi and its dinoflagellate symbiont. Metabolite profiling compared ambient conditions with response to simulated climate change stressors and with the sister species, S. glaucum. Among ∼300 monitored metabolites, 13 metabolites were modulated. Incubation experiments providing four selected upregulated metabolites (alanine, GABA, nicotinic acid, and proline) in the culturing water failed to subside the bleaching response at temperature-induced stress, despite their known ability to mitigate heat stress in plants or animals. Thus, the results hint to metabolite accumulation (marker) during heat stress. This study provides the first detailed map of metabolic pathways transition in corals in response to different environmental stresses, accounting for the superior thermal tolerance of S. ehrenbergi versus S. glaucum, which can ultimately help maintain a viable symbiosis and mitigate against coral bleaching.

How does an animal behave like a plant? Physiological and molecular adaptations of zooxanthellae and their hosts to symbiosis

Cnidarians (corals and sea anemones) harbouring photosynthetic microalgae derive several benefits from their association. To allow this association, numerous symbiotic-dependent adaptations in both partners, resulting from evolutionary pressures, have been selected. The dinoflagellate symbionts (zooxanthellae) are located inside a vesicle in the cnidarian host cell and are therefore exposed to a very different environment compared to the free-living state of these microalgae in terms of ion concentration and carbon content and speciation. In addition, this intracellular localization imposes that they rely completely upon the host for their nutrient supply (nitrogen, CO2). Symbiotic-dependent adaptations imposed to the animal host by phototrophic symbiosis are more relevant to photosynthetic organisms than to metazoans: indeed, the cnidarian host often harbours diurnal changes of morphology to adapt itself to the amount of light and possesses carbon-concentrating mechanisms, antioxidative defences and UV sunscreens similar to that present in phototrophs. These adaptations and the contrasting fragility of the association are discussed from both ecological and evolutionary points of view.

Sphingolipid Metabolism of a Sea Anemone Is Altered by the Presence of Dinoflagellate Symbionts.

In host-microbe interactions, signaling lipids function in interpartner communication during both the establishment and maintenance of associations. Previous evidence suggests that sphingolipids play a role in the mutualistic cnidarian-Symbiodinium symbiosis. Exogenously applied sphingolipids have been shown to alter this partnership, though endogenous host regulation of sphingolipids by the sphingosine rheostat under different symbiotic conditions has not been characterized. The rheostat regulates levels of pro-survival sphingosine-1-phosphate (S1P) and pro-apoptotic sphingosine (Sph) through catalytic activities of sphingosine kinase (SPHK) and S1P phosphatase (SGPP). The role of the rheostat in recognition and establishment of cnidarian-Symbiodinium symbiosis was investigated in the sea anemone Aiptasia pallida by measuring gene expression, protein levels, and sphingolipid metabolites in symbiotic, aposymbiotic, and newly recolonized anemones. Comparison of two host populations showed that symbiotic animals from one population had lower SGPP gene expression and Sph lipid concentrations compared to aposymbiotic animals, while the other population had higher S1P concentrations than their aposymbiotic counterparts. In both populations, the host rheostat trended toward host cell survival in the presence of symbionts. Furthermore, upregulation of both rheostat enzymes on the first day of host recolonization by symbionts suggests a role for the rheostat in host-symbiont recognition during symbiosis onset. Collectively, these data suggest a regulatory role of sphingolipid signaling in cnidarian-Symbiodinium symbiosis and symbiont uptake.

Coral larvae for restoration and research: a large-scale method for rearing Acropora millepora larvae, inducing settlement, and establishing symbiosis.

Here we describe an efficient and effective technique for rearing sexually-derived coral propagules from spawning through larval settlement and symbiont uptake with minimal impact on natural coral populations. We sought to maximize larval survival while minimizing expense and daily husbandry maintenance by experimentally determining optimized conditions and protocols for gamete fertilization, larval cultivation, induction of larval settlement by crustose coralline algae, and inoculation of newly settled juveniles with their dinoflagellate symbiont Symbiodinium. Larval rearing densities at or below 0.2 larvae mL−1 were found to maximize larval survival and settlement success in culture tanks while minimizing maintenance effort. Induction of larval settlement via the addition of a ground mixture of diverse crustose coralline algae (CCA) is recommended, given the challenging nature of in situ CCA identification and our finding that non settlement-inducing CCA assemblages do not inhibit larval settlement if suitable assemblages are present. Although order of magnitude differences in infectivity were found between common Great Barrier Reef Symbiodinium clades C and D, no significant differences in Symbiodinium uptake were observed between laboratory-cultured and wild-harvested symbionts in each case. The technique presented here for Acropora millepora can be adapted for research and restoration efforts in a wide range of broadcast spawning coral species.

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.

Condition-specific RNA editing in the coral symbiont Symbiodinium microadriaticum.

RNA editing is a rare post-transcriptional event that provides cells with an additional level of gene expression regulation. It has been implicated in various processes including adaptation, viral defence and RNA interference; however, its potential role as a mechanism in acclimatization has just recently been recognised. Here, we show that RNA editing occurs in 1.6% of all nuclear-encoded genes of Symbiodinium microadriaticum, a dinoflagellate symbiont of reef-building corals. All base-substitution edit types were present, and statistically significant motifs were associated with three edit types. Strikingly, a subset of genes exhibited condition-specific editing patterns in response to different stressors that resulted in significant increases of non-synonymous changes. We posit that this previously unrecognised mechanism extends this organism’s capability to respond to stress beyond what is encoded by the genome. This in turn may provide further acclimatization capacity to these organisms, and by extension, their coral hosts.

Body size and symbiotic status influence gonad development in Aiptasia pallida anemones

Pale anemones (Aiptasia pallida) coexist with dinoflagellates (primarily Symbiodinium minutum) in a mutualistic relationship. The purpose of this study was to investigate the role of these symbionts in gonad development of anemone hosts. Symbiotic and aposymbiotic anemones were subjected to light cycles that induced gametogenesis. These anemones were then sampled weekly for nine weeks, and gonad development was analyzed histologically. Anemone size was measured as mean body column diameter, and oocytes or sperm follicles were counted for each anemone. Generalized linear models were used to evaluate the influence of body size and symbiotic status on whether gonads were present and on the number of oocytes or sperm follicles produced. Body size predicted whether gonads were present, with larger anemones being more likely than smaller anemones to develop gonads. Both body size and symbiotic status predicted gonad size, such that larger and symbiotic anemones produced more oocytes and sperm follicles than smaller and aposymbiotic anemones. Overall, only 22 % of aposymbiotic females produced oocytes, whereas 63 % of symbiotic females produced oocytes. Similarly, 6 % of aposymbiotic males produced sperm follicles, whereas 60 % of symbiotic males produced sperm follicles. Thus, while gonads were present in 62 % of symbiotic anemones, they were present in only 11 % of aposymbiotic anemones. These results indicate that dinoflagellate symbionts influence gonad development and thus sexual maturation in both female and male Aiptasia pallida anemones. This finding substantiates and expands our current understanding of the importance of symbionts in the development and physiology of cnidarian hosts.