Symbiodinium (Dinophyceae) community patterns in invertebrate hosts from inshore marginal reefs of the southern Great Barrier Reef, Australia.

The broad range in physiological variation displayed by Symbiodinium spp. has proven imperative during periods of environmental change and contribute to the survival of their coral host. Characterizing how host and Symbiodinium community assemblages differ across environmentally distinct habitats provides useful information to predict how corals will respond to major environmental change. Despite the extensive characterizations of Symbiodinium diversity found amongst reef cnidarians on the Great Barrier Reef (GBR) substantial biogeographic gaps exist, especially across inshore habitats. Here, we investigate Symbiodinium community patterns in invertebrates from inshore and mid-shelf reefs on the southern GBR, Australia. Dominant Symbiodinium types were characterized using denaturing gradient gel electrophoresis fingerprinting and sequencing of the ITS2 region of the ribosomal DNA. Twenty one genetically distinct Symbiodinium types including four novel types were identified from 321 reef-invertebrate samples comprising three sub-generic clades (A, C, and D). A range of host genera harbored C22a, which is normally rare or absent from inshore or low latitude reefs in the GBR. Multivariate analysis showed that host identity and sea surface temperature best explained the variation in symbiont communities across sites. Patterns of changes in Symbiodinium community assemblage over small geographic distances (100s of kilometers or less) indicate the likelihood that shifts in Symbiodinium distributions and associated host populations, may occur in response to future climate change impacting the GBR.

Symbiodinium biogeography tracks environmental patterns rather than host genetics in a key Caribbean reef-builder, Orbicella annularis.

The physiological performance of a reef-building coral is a combined outcome of both the coral host and its algal endosymbionts, Symbiodinium While Orbicella annularis-a dominant reef-building coral in the Wider Caribbean-is known to be a flexible host in terms of the diversity of Symbiodinium types it can associate with, it is uncertain how this diversity varies across the Caribbean, and whether spatial variability in the symbiont community is related to either O. annularis genotype or environment. Here, we target the Symbiodinium-ITS2 gene to characterize and map dominant Symbiodinium hosted by O. annularis at an unprecedented spatial scale. We reveal northwest-southeast partitioning across the Caribbean, both in terms of the dominant symbiont taxa hosted and in assemblage diversity. Multivariate regression analyses incorporating a suite of environmental and genetic factors reveal that observed spatial patterns are predominantly explained by chronic thermal stress (summer temperatures) and are unrelated to host genotype. Furthermore, we were able to associate the presence of specific Symbiodinium types with local environmental drivers (for example, Symbiodinium C7 with areas experiencing cooler summers, B1j with nutrient loading and B17 with turbidity), associations that have not previously been described.

Prevalent endosymbiont zonation shapes the depth distributions of scleractinian coral species.

Bathymetric distributions of photosynthetic marine invertebrate species are relatively well studied, however the importance of symbiont zonation (i.e. hosting of distinct algal endosymbiont communities over depth) in determining these depth distributions still remains unclear. Here, we assess the prevalence of symbiont zonation in tropical scleractinian corals by genotyping the Symbiodinium of the 25 most common species over a large depth range (down to 60 m) on a Caribbean reef. Symbiont depth zonation was found to be common on a reef-wide scale (11 out of 25 coral species), and a dominant feature in species with the widest depth distributions. With regards to reproductive strategy, symbiont zonation was more common in broadcasting species, which also exhibited a higher level of polymorphism in the symbiont zonation (i.e. number of different Symbiodinium profiles involved). Species with symbiont zonation exhibited significantly broader depth distributions than those without, highlighting the role of symbiont zonation in shaping the vertical distributions of the coral host. Overall, the results demonstrate that coral reefs can consist of highly structured communities over depth when considering both the coral host and their obligate photosymbionts, which probably has strong implications for the extent of connectivity between shallow and mesophotic habitats.

A nanoscale secondary ion mass spectrometry study of dinoflagellate functional diversity in reef-building corals.

Nutritional interactions between corals and symbiotic dinoflagellate algae lie at the heart of the structural foundation of coral reefs. Whilst the genetic diversity of Symbiodinium has attracted particular interest because of its contribution to the sensitivity of corals to environmental changes and bleaching (i.e. disruption of coral-dinoflagellate symbiosis), very little is known about the in hospite metabolic capabilities of different Symbiodinium types. Using a combination of stable isotopic labelling and nanoscale secondary ion mass spectrometry (NanoSIMS), we investigated the ability of the intact symbiosis between the reef-building coral Isopora palifera, and Symbiodinium C or D types, to assimilate dissolved inorganic carbon (via photosynthesis) and nitrogen (as ammonium). Our results indicate that Symbiodinium types from two clades naturally associated with I. palifera possess different metabolic capabilities. The Symbiodinium C type fixed and passed significantly more carbon and nitrogen to its coral host than the D type. This study provides further insights into the metabolic plasticity among different Symbiodinium types in hospite and strengthens the evidence that the more temperature-tolerant Symbiodinium D type may be less metabolically beneficial for its coral host under non-stressful conditions.

Symbiodinium (Dinophyceae) diversity in reef-invertebrates along an offshore to inshore reef gradient near Lizard Island, Great Barrier Reef.

Despite extensive work on the genetic diversity of reef invertebrate-dinoflagellate symbioses on the Great Barrier Reef (GBR; Australia), large information gaps exist from northern and inshore regions. Therefore, a broad survey was done comparing the community of inshore, mid-shelf and outer reefs at the latitude of Lizard Island. Symbiodinium (Freudenthal) diversity was characterized using denaturing gradient gel electrophoresis fingerprinting and sequencing of the ITS2 region of the ribosomal DNA. Thirty-nine distinct Symbiodinium types were identified from four subgeneric clades (B, C, D, and G). Several Symbiodinium types originally characterized from the Indian Ocean were discovered as well as eight novel types (C1kk, C1LL, C3nn, C26b, C161a, C162, C165, C166). Multivariate analyses on the Symbiodinium species diversity data showed a strong link with host identity, consistent with previous findings. Of the four environmental variables tested, mean austral winter sea surface temperature (SST) influenced Symbiodinium distribution across shelves most significantly. A similar result was found when the analysis was performed on Symbiodinium diversity data of genera with an open symbiont transmission mode separately with chl a and PAR explaining additional variation. This study underscores the importance of SST and water quality related variables as factors driving Symbiodinium distribution on cross-shelf scales. Furthermore, this study expands our knowledge on Symbiodinium species diversity, ecological partitioning (including host-specificity) and geographic ranges across the GBR. The accelerating rate of environmental change experienced by coral reef ecosystems emphasizes the need to comprehend the full complexity of cnidarian symbioses, including the biotic and abiotic factors that shape their current distributions.

Host-specific interactions with environmental factors shape the distribution of symbiodinium across the Great Barrier Reef.

BACKGROUND: The endosymbiotic dinoflagellates (genus Symbiodinium) within coral reef invertebrates are critical to the survival of the holobiont. The genetic variability of Symbiodinium may contribute to the tolerance of the symbiotic association to elevated sea surface temperatures (SST). To assess the importance of factors such as the local environment, host identity and biogeography in driving Symbiodinium distributions on reef-wide scales, data from studies on reef invertebrate-Symbiodinium associations from the Great Barrier Reef (GBR) were compiled. METHODOLOGY/PRINCIPAL FINDINGS: The resulting database consisted of 3717 entries from 26 studies. It was used to explore ecological patterns such as host-specificity and environmental drivers structuring community complexity using a multi-scalar approach. The data was analyzed in several ways: (i) frequently sampled host species were analyzed independently to investigate the influence of the environment on symbiont distributions, thereby excluding the influence of host specificity, (ii) host species distributions across sites were added as an environmental variable to determine the contribution of host identity on symbiont distribution, and (iii) data were pooled based on clade (broad genetic groups dividing the genus Symbiodinium) to investigate factors driving Symbiodinium distributions using lower taxonomic resolution. The results indicated that host species identity plays a dominant role in determining the distribution of Symbiodinium and environmental variables shape distributions on a host species-specific level. SST derived variables (especially SSTstdev) most often contributed to the selection of the best model. Clade level comparisons decreased the power of the predictive model indicating that it fails to incorporate the main drivers behind Symbiodinium distributions. CONCLUSIONS/SIGNIFICANCE: Including the influence of different host species on Symbiodinium distributional patterns improves our understanding of the drivers behind the complexity of Symbiodinium-invertebrate symbioses. This will increase our ability to generate realistic models estimating the risk reefs are exposed to and their resilience in response to a changing climate.

Increased cell proliferation and mucocyte density in the sea anemone Aiptasia pallida recovering from bleaching.

Recovery of coral after bleaching episodes is a critical period for the health of the reef ecosystem. While events such as symbiont (genus Symbiodinium) shifting/shuffling or tissue apoptosis have been demonstrated to occur following bleaching, little is known concerning tissue recovery or cell proliferation. Here, we studied the sea anemone Aiptasia pallida exposed to a transient elevation of water temperature combined with high illumination (33°C and 1900 µmol photons x m(-2) x s(-1) for 30 h). Following such treatment bleached anemones showed a significant reduction of their Symbiodinium density. Cell proliferation in the ectodermis and gastrodermis was determined by assessing the densities of cells labeled with a thymidine analogue (EdU). Cell proliferation significantly increased during the first day following stress in both tissue types. This increased cell proliferation returned to pre-stress values after one week. Although cell proliferation was higher in the ectodermis in absence of stress, it was relatively more pronounced in the gastrodermis of stressed anemones. In addition, the ratio of ectodermal mucocytes significantly increased three weeks after induced stress. These results suggest that thermal/photic stress coupled with the loss of the symbionts is able to enhance cell proliferation in both gastrodermis and ectodermis of cnidarians. While new cells formed in the gastrodermis are likely to host new Symbiodinium, the fate of new cells in the ectodermis was only partially revealed. Some new ectodermal cells may, in part, contribute to the increased number of mucocytes which could eventually help strengthen the heterotrophic state until restoration of the symbiosis.

SymbioGBR: a web-based database of Symbiodinium associated with cnidarian hosts on the Great Barrier Reef.

BACKGROUND: The algal endosymbionts (genus Symbiodinium) associated with scleractinian corals (and other reef invertebrates) have received a lot of research attention in the past decade, particularly as certain host-symbiont associations appear more affected by increasing seawater temperatures than others. With the rapid accumulation of information on the diversity of Symbiodinium, it is becoming increasingly difficult to compare newly acquired Symbiodinium data with existing data to detect patterns of host-symbiont specificity on broader spatial scales. The lack of a general consensus on the classification of Symbiodinium species coupled with the variety of different markers used to identify the genus Symbiodinium (ITS1, ITS2, LSU D1/D2, chloroplast 23S rDNA and psbA minicircle) further complicate direct comparison. DESCRIPTION: The SymbioGBR database compiles all currently available Symbiodinium sequences and associated host information of data collected from the Great Barrier Reef into a single relational database that is accessible via a user-friendly, searchable web-based application (http://www.SymbioGBR.org). SymbioGBR allows users to query Symbiodinium types or sequences sourced from various genetic markers (e.g. ITS1, ITS2, LSU D1/D2 and chloroplast 23S) and invertebrate host species to explore their reported associations. In addition, as the database includes sequence information of multiple genetic markers, it allows cross-referencing between conventional (e.g. ITS2 region) and novel markers that exhibit low intragenomic variability (e.g. psbA region). Finally, the database is based on the collection details of individual specimens. Such host-symbiont associations can be assessed quantitatively and viewed in relation to their environmental and geographic context. CONCLUSIONS: The SymbioGBR database provides a comprehensive overview of Symbiodinium diversity and host-associations on the Great Barrier Reef. It provides a quick, user-friendly means to compare newly acquired data on Symbiodinium (e.g. raw sequences or characterized Symbiodinium types) with previous data on the diversity of invertebrate host-symbiont associations on the GBR. The inclusion of psbAncr sequence information allows for validation of widely used ITS1/ITS2 markers and their ability to accurately identify relevant sequences. Most importantly, centralization of sequence information from multiple genetic markers will aid the classification of Symbiodinium species diversity and allow researchers to easily compare patterns of host-Symbiodinium associations.

Reversal in competitive dominance of a toxic versus non-toxic cyanobacterium in response to rising CO2.

Climate change scenarios predict a doubling of the atmospheric CO(2) concentration by the end of this century. Yet, how rising CO(2) will affect the species composition of aquatic microbial communities is still largely an open question. In this study, we develop a resource competition model to investigate competition for dissolved inorganic carbon in dense algal blooms. The model predicts how dynamic changes in carbon chemistry, pH and light conditions during bloom development feed back on competing phytoplankton species. We test the model predictions in chemostat experiments with monocultures and mixtures of a toxic and non-toxic strain of the freshwater cyanobacterium Microcystis aeruginosa. The toxic strain was able to reduce dissolved CO(2) to lower concentrations than the non-toxic strain, and became dominant in competition at low CO(2) levels. Conversely, the non-toxic strain could grow at lower light levels, and became dominant in competition at high CO(2) levels but low light availability. The model captured the observed reversal in competitive dominance, and was quantitatively in good agreement with the results of the competition experiments. To assess whether microcystins might have a role in this reversal of competitive dominance, we performed further competition experiments with the wild-type strain M. aeruginosa PCC 7806 and its mcyB mutant impaired in microcystin production. The microcystin-producing wild type had a strong selective advantage at low CO(2) levels but not at high CO(2) levels. Our results thus demonstrate both in theory and experiment that rising CO(2) levels can alter the community composition and toxicity of harmful algal blooms.

Spatial divergence in the proportions of genes encoding toxic peptide synthesis among populations of the cyanobacterium Planktothrix in European lakes.

It has been frequently reported that seasonal changes in toxin production by cyanobacteria are due to changes in the proportion of toxic/nontoxic genotypes in parallel to increases or decreases in population density during the seasonal cycle of bloom formation. In order to find out whether there is a relationship between the proportion of genes encoding toxic peptide synthesis and population density of Planktothrix spp. we compared the proportion of three gene regions that are indicative of the synthesis of the toxic heptapeptide microcystin (mcyB), and the bioactive peptides aeruginoside (aerB) and anabaenopeptin (apnC) in samples from 23 lakes of five European countries (n=153). The mcyB, aerB, and apnC genes occurred in 99%, 99%, and 97% of the samples, respectively, and on average comprised 60 ± 3%, 22 ± 2%, and 54 ± 4% of the total population, respectively. Although the populations differed widely in abundance (10(-3)-10(3) mm(3) L(-1)) no dependence of the proportion of the mcyB, aerB, and apnC genes on the density of the total population was found. In contrast populations differed significantly in their average mcyB, aerB, and apnC gene proportions, with no change between prebloom and bloom conditions. These results emphasize stable population-specific differences in mcyB, aerB, and apnC proportions that are independent from seasonal influences.

Pulsed nitrogen supply induces dynamic changes in the amino acid composition and microcystin production of the harmful cyanobacterium Planktothrix agardhii.

Planktothrix agardhii is a widespread harmful cyanobacterium of eutrophic waters, and can produce the hepatotoxins [Asp(3)]microcystin-LR and [Asp(3)]microcystin-RR. These two microcystin variants differ in their first variable amino acid position, which is occupied by either leucine (L) or arginine (R). Although microcystins are extensively investigated, little is known about the mechanisms that determine the production of different microcystin variants. We hypothesize that enhanced nitrogen availability will increase the intracellular content of the nitrogen-rich amino acid arginine, and thereby promote the production of the variant [Asp(3)]microcystin-RR. To test this hypothesis, we transferred P. agardhii strain 126/3 from nitrogen-replete to nitrogen-deficient conditions, and after 2 weeks of growth under nitrogen deficiency, we added a nitrogen pulse. We found a rapid increase in the cellular nitrogen to carbon ratio and the amino acids aspartic acid and arginine, indicative of cyanophycin synthesis. This was followed by a more gradual increase of the total amino acid content connected to balanced growth. As expected, the [Asp(3)]microcystin-RR variant increased strongly after the nitrogen pulse, while the [Asp(3)]microcystin-LR increased to a much lesser extent. We conclude that sudden nitrogen enrichment affects the amino acid composition of harmful cyanobacteria, which, in turn, affects the production and composition of their microcystins.

Amino acid availability determines the ratio of microcystin variants in the cyanobacterium Planktothrix agardhii.

Cyanobacteria are capable of producing multiple microcystin variants simultaneously. The mechanisms that determine the composition of microcystin variants in cyanobacteria are still debated. [Asp(3)]microcystin-RR contains arginine at the position where the more toxic [Asp(3)]microcystin-LR incorporates leucine. We cultured the filamentous cyanobacterium Planktothrix agardhii strain 126/3 with and without external addition of leucine and arginine. Addition of leucine to the growth medium resulted in a strong increase of the [Asp(3)]microcystin LR/RR ratio, while addition of arginine resulted in a decrease. This demonstrates that amino acid availability plays a role in the synthesis of different microcystin variants. Environmental changes affecting cell metabolism may cause differences in the intracellular availability of leucine and arginine, which can thus affect the production of microcystin variants. Because leucine contains one nitrogen atom while arginine contains four nitrogen atoms, we hypothesized that low nitrogen availability might shift the amino acid composition in favor of leucine, which might explain seasonal increases in the [Asp(3)]microcystin LR/RR ratio in natural populations. However, when a continuous culture of P. agardhii was shifted from nitrogen-saturated to a nitrogen-limited mineral medium, leucine and arginine concentrations decreased, but the leucine/arginine ratio did not change. Accordingly, while the total microcystin concentration of the cells decreased, we did not observe changes in the [Asp(3)]microcystin LR/RR ratio in response to nitrogen limitation.

Competition for light between toxic and nontoxic strains of the harmful cyanobacterium Microcystis.

The cyanobacterium Microcystis can produce microcystins, a family of toxins that are of major concern in water management. In several lakes, the average microcystin content per cell gradually declines from high levels at the onset of Microcystis blooms to low levels at the height of the bloom. Such seasonal dynamics might result from a succession of toxic to nontoxic strains. To investigate this hypothesis, we ran competition experiments with two toxic and two nontoxic Microcystis strains using light-limited chemostats. The population dynamics of these closely related strains were monitored by means of characteristic changes in light absorbance spectra and by PCR amplification of the rRNA internal transcribed spacer region in combination with denaturing gradient gel electrophoresis, which allowed identification and semiquantification of the competing strains. In all experiments, the toxic strains lost competition for light from nontoxic strains. As a consequence, the total microcystin concentrations in the competition experiments gradually declined. We did not find evidence for allelopathic interactions, as nontoxic strains became dominant even when toxic strains were given a major initial advantage. These findings show that, in our experiments, nontoxic strains of Microcystis were better competitors for light than toxic strains. The generality of this finding deserves further investigation with other Microcystis strains. The competitive replacement of toxic by nontoxic strains offers a plausible explanation for the gradual decrease in average toxicity per cell during the development of dense Microcystis blooms.

The microcystin composition of the cyanobacterium Planktothrix agardhii changes toward a more toxic variant with increasing light intensity.

The cyanobacterium Planktothrix agardhii, which is dominant in many shallow eutrophic lakes, can produce hepatotoxic microcystins. Currently, more than 70 different microcystin variants have been described, which differ in toxicity. In this study, the effect of photon irradiance on the production of different microcystin variants by P. agardhii was investigated using light-limited turbidostats. Both the amount of the mRNA transcript of the mcyA gene and the total microcystin production rate increased with photon irradiance up to 60 micromol m(-2) s(-1), but they started to decrease with irradiance greater than 100 micromol m(-2) s(-1). The cellular content of total microcystin remained constant, independent of the irradiance. However, of the two main microcystin variants detected in P. agardhii, the microcystin-DeRR content decreased twofold with increased photon irradiance, whereas the microcystin-DeLR content increased threefold. Since microcystin-DeLR is considerably more toxic than microcystin-DeRR, this implies that P. agardhii becomes more toxic at high light intensities.