Symbiodinium genotypic and environmental controls on lipids in reef building corals

Background

Lipids in reef building corals can be divided into two classes; non-polar storage lipids, e.g. wax esters and triglycerides, and polar structural lipids, e.g. phospholipids and cholesterol. Differences among algal endosymbiont types are known to have important influences on processes including growth and the photobiology of scleractinian corals yet very little is known about the role of symbiont types on lipid energy reserves.

Methodology/Principal Findings

The ratio of storage lipid and structural lipid fractions of Scott Reef corals were determined by thin layer chromatography. The lipid fraction ratio varied with depth and depended on symbiont type harboured by two corals (Seriatopora hystrix and Pachyseris speciosa). S. hystrix colonies associated with Symbiodinium C1 or C1/C# at deep depths (>23 m) had lower lipid fraction ratios (i.e. approximately equal parts of storage and structural lipids) than those with Symbiodinium D1 in shallow depths (<23 m), which had higher lipid fraction ratios (i.e. approximately double amounts of storage relative to structural lipid). Further, there was a non-linear relationship between the lipid fraction ratio and depth for S. hystrix with a modal peak at ∼23 m coinciding with the same depth as the shift from clade D to C types. In contrast, the proportional relationship between the lipid fraction ratio and depth for P. speciosa, which exhibited high specificity for Symbiodinium C3 like across the depth gradient, was indicative of greater amounts of storage lipids contained in the deep colonies.

Conclusions/Significance

This study has demonstrated that Symbiodinium exert significant controls over the quality of coral energy reserves over a large-scale depth gradient. We conclude that the competitive advantages and metabolic costs that arise from flexible associations with divergent symbiont types are offset by energetic trade-offs for the coral host.     

FISH-Flow: a quantitative molecular approach for describing mixed clade communities of Symbiodinium

Our understanding of reef corals and their fate in a changing climate is limited by our ability to monitor the diversity and abundance of the dinoflagellate endosymbionts that sustain them. This study combined two well-known methods in tandem: fluorescent in situ hybridization (FISH) for genotype-specific labeling of Symbiodinium and flow cytometry to quantify the abundance of each symbiont clade in a sample. This technique (FISH-Flow) was developed with cultured Symbiodinium representing four distinct clades (based on large subunit rDNA) and was used to distinguish and quantify these types with high efficiency and few false positives. This technique was also applied to freshly isolated symbionts of Orbicella faveolata and Orbicella annularis. Isolates from acutely bleached coral tissues had significantly lower labeling efficiency; however, isolates from healthy tissue had efficiencies comparable to cultured Symbiodinium trials. RNA degradation in bleaching samples may have interfered with labeling of cells. Nevertheless, we were able to determine that, with and without thermal stress, experimental columns of the coral O. annularis hosted a majority of clade B and B/C symbionts on the top and side of the coral column, respectively. We demonstrated that, for cultured Symbiodinium and Symbiodinium freshly isolated from healthy host tissues, the relative ratio of clades could be accurately determined for clades present at as low as 7 % relative abundance. While this method does not improve upon PCR-based techniques in identifying clades at background levels, FISH-Flow provides a high precision, flexible system for targeting, quantifying and isolating Symbiodinium genotypes of interest.     

Flexible associations between Pocillopora corals and Symbiodinium limit utility of symbiosis ecology in defining species

Corals in the genus Pocillopora are the primary framework builders of eastern tropical Pacific (ETP) reefs. These corals typically associate with algal symbionts (genus Symbiodinium) in clade C and/or D, with clade D associations having greater thermal tolerance and resistance to bleaching. Recently, cryptic "species" delineations within both Pocillopora and Symbiodinium have been suggested, with host–symbiont specificity used as a supporting taxonomic character in both genera. In particular, it has been suggested that three lineages of Pocillopora (types 1–3) exist in the ETP, of which type 1 is the exclusive host of heat-tolerant Symbiodinium D1. This host specificity has been used to support the species name "Symbiodinium glynni" for this symbiont. To validate these host–symbiont relationships and their taxonomic utility, we identified Pocillopora types and their associated Symbiodinium at three sites in the ETP. We found greater flexibility in host–symbiont combinations than previously reported, with both Pocillopora types 1 and 3 able to host and be dominated by Symbiodinium in clade C or D. The prevalence of certain combinations did vary among sites, showing that a gradient of specificity exists which may be mediated by evolutionary relationships and environmental disturbance history. However, these results limit the utility of apparent host–symbiont specificity (which may have been a result of undersampling) in defining species boundaries in either corals or Symbiodinium. They also suggest that a greater diversity of corals may benefit from the thermal tolerance of clade D symbionts, affirming the need to conserve Pocillopora across its entire geographic and environmental range.     

Nitrate competition in a coral symbiosis varies with temperature among Symbiodinium clades

Many reef-building corals form symbioses with dinoflagellates from the diverse genus Symbiodinium. There is increasing evidence of functional significance to Symbiodinium diversity, which affects the coral holobiont''s response to changing environmental conditions. For example, corals hosting Symbiodinium from the clade D taxon exhibit greater resistance to heat-induced coral bleaching than conspecifics hosting the more common clade C. Yet, the relatively low prevalence of clade D suggests that this trait is not advantageous in non-stressful environments. Thus, clade D may only be able to out-compete other Symbiodinium types within the host habitat when conditions are chronically stressful. Previous studies have observed enhanced photosynthesis and fitness by clade C holobionts at non-stressful temperatures, relative to clade D. Yet, carbon-centered metrics cannot account for enhanced growth rates and patterns of symbiont succession to other genetic types when nitrogen often limits reef productivity. To investigate the metabolic costs of hosting thermally tolerant symbionts, we examined the assimilation and translocation of inorganic ¹⁵N and ¹³C in the coral Acropora tenuis experimentally infected with either clade C (sub-type C1) or D Symbiodinium at 28 and 30 °C. We show that at 28 °C, C1 holobionts acquired 22% more ¹⁵N than clade D. However, at 30 °C, C1 symbionts acquired equivalent nitrogen and 16% less carbon than D. We hypothesize that C1 competitively excludes clade D in hospite via enhanced nitrogen acquisition and thus dominates coral populations despite warming oceans.     

Lipid biomarkers in Symbiodinium dinoflagellates: new indicators of thermal stress

Lipid content and fatty acid profiles of corals and their dinoflagellate endosymbionts are known to vary in response to high-temperature stress. To better understand the heat-stress response in these symbionts, we investigated cultures of Symbiodinium goreauii type C1 and Symbiodinium sp. clade subtype D1 grown under a range of temperatures and durations. The predominant lipids produced by Symbiodinium are palmitic (C16) and stearic (C18) saturated fatty acids and their unsaturated analogs, the polyunsaturated fatty acid docosahexaenoic acid (C22:6, n-3; DHA), and a variety of sterols. Prolonged exposure to high temperature causes the relative amount of unsaturated acids within the C18 fatty acids in Symbiodinium tissue to decrease. Thermal stress also causes a decrease in abundance of fatty acids relative to sterols, as well as the more specific ratio of DHA to an algal 4-methyl sterol. These shifts in fatty acid unsaturation and fatty acid-to-sterol ratios are common to both types C1 and D1, but the apparent thermal threshold of lipid changes is lower for type C1. This work indicates that ratios among free fatty acids and sterols in Symbiodinium can be used as sensitive indicators of thermal stress. If the Symbiodinium lipid stress response is unchanged in hospite, the algal heat-stress biomarkers we have identified could be measured to detect thermal stress within the coral holobiont. These results provide new insights into the potential role of lipids in the overall Symbiodinium thermal stress response.     

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.     

Variability of Symbiodinium Communities in Waters,Sediments, and Corals of Thermally Distinct Reef Pools in American Samoa

Reef-building corals host assemblages of symbiotic algae (Symbiodinium spp.) whose diversity and abundance may fluctuate under different conditions, potentially facilitating acclimatization to environmental change. The composition of free-living Symbiodinium in reef waters and sediments may also be environmentally labile and may influence symbiotic assemblages by mediating supply and dispersal. The magnitude and spatial scales of environmental influence over Symbiodinium composition in different reef habitat compartments are, however, not well understood. We used pyrosequencing to compare Symbiodinium in sediments, water, and ten coral species between two backreef pools in American Samoa with contrasting thermal environments. We found distinct compartmental assemblages of clades A, C, D, F, and/or G Symbiodinium types, with strong differences between pools in water, sediments, and two coral species. In the pool with higher and more variable temperatures, abundance of various clade A and C types differed compared to the other pool, while abundance of D types was lower in sediments but higher in water and in Pavona venosa, revealing an altered habitat distribution and potential linkages among compartments. The lack of between-pool effects in other coral species was due to either low overall variability (in the case of Porites) or high within-pool variability. Symbiodinium communities in water and sediment also showed within-pool structure, indicating that environmental influences may operate over multiple, small spatial scales. This work suggests that Symbiodinium composition is highly labile in reef waters, sediments, and some corals, but the underlying drivers and functional consequences of this plasticity require further testing with high spatial resolution biological and environmental sampling.     

Symbiodinium Photosynthesis in Caribbean Octocorals

Symbioses with the dinoflagellate Symbiodinium form the foundation of tropical coral reef communities. Symbiodinium photosynthesis fuels the growth of an array of marine invertebrates, including cnidarians such as scleractinian corals and octocorals (e.g., gorgonian and soft corals). Studies examining the symbioses between Caribbean gorgonian corals and Symbiodinium are sparse, even though gorgonian corals blanket the landscape of Caribbean coral reefs. The objective of this study was to compare photosynthetic characteristics of Symbiodinium in four common Caribbean gorgonian species: Pterogorgia anceps, Eunicea tourneforti, Pseudoplexaura porosa, and Pseudoplexaura wagenaari. Symbiodinium associated with these four species exhibited differences in Symbiodinium density, chlorophyll a per cell, light absorption by chlorophyll a, and rates of photosynthetic oxygen production. The two Pseudoplexaura species had higher Symbiodinium densities and chlorophyll a per Symbiodinium cell but lower chlorophyll a specific absorption compared to P. anceps and E. tourneforti. Consequently, P. porosa and P. wagenaari had the highest average photosynthetic rates per cm² but the lowest average photosynthetic rates per Symbiodinium cell or chlorophyll a. With the exception of Symbiodinium from E. tourneforti, isolated Symbiodinium did not photosynthesize at the same rate as Symbiodinium in hospite. Differences in Symbiodinium photosynthetic performance could not be attributed to Symbiodinium type. All P. anceps (n = 9) and P. wagenaari (n = 6) colonies, in addition to one E. tourneforti and three P. porosa colonies, associated with Symbiodinium type B1. The B1 Symbiodinium from these four gorgonian species did not cluster with lineages of B1 Symbiodinium from scleractinian corals. The remaining eight E. tourneforti colonies harbored Symbiodinium type B1L, while six P. porosa colonies harbored type B1i. Understanding the symbioses between gorgonian corals and Symbiodinium will aid in deciphering why gorgonian corals dominate many Caribbean reefs.     

Understanding diversity in coral-algal symbiosis: a cluster-based approach to interpreting fine-scale genetic variation in the genus <Emphasis Type="Italic">Symbiodinium</Emphasis>

Reef corals associate with an extraordinary diversity of dinoflagellate endosymbionts (genus Symbiodinium), and this diversity has become critical to understanding how corals respond to environmental changes. A popular molecular marker for Symbiodinium diversity, the Internal Transcribed Spacer-2 (ITS-2) region of ribosomal DNA, has revealed hundreds of distinct variants that are generally interpreted as representing different species, even though many have not been systematically tested for functional or ecological differentiation. Many of these variants are only minimally divergent from one another (1 bp or less), and others occupy basal nodes of traditional species phylogenies (“living ancestors”), indicating that some Symbiodinium ITS-2 diversity may represent intraspecific sequence variation. This hypothesis was tested for Symbiodinium clades A–D (the dominant symbionts of reef corals) through the construction of statistical parsimony networks of ITS-2 sequence diversity, and identification of clusters of closely related sequences within these networks. Initial assessments indicated that ecological differentiation exists between, but not within, these clusters. This approach, although imperfect in its ability to identify species boundaries in all cases, nevertheless dramatically reduces “species” diversity in Symbiodinium (from ~175 to 35). This testable alternative hypothesis indicates that, in Symbiodinium, “species” consist of clusters of closely related ITS-2 sequences diverging from ancestral variants that are typically ecologically dominant. A cluster-based view of Symbiodinium ITS-2 diversity improves our ability to: (1) construct well-supported symbiont phylogenies; (2) establish functional niches for symbiont species; and (3) understand flexibility and specificity within coral-algal symbioses. This cluster-based approach can ultimately be integrated with emerging population-level datasets (microsatellites and microsatellite flanking regions) to improve understanding of species diversity in Symbiodinium. 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     

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.     

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.     

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.     

Presence of Symbiodinium spp. in macroalgal microhabitats from the southern Great Barrier Reef

Coral reefs are highly dependent on the mutualistic symbiosis between reef-building corals and dinoflagellates from the genus Symbiodinium. These dinoflagellates spend part of their life cycle outside the coral host and in the majority of the cases have to re-infect corals each generation. While considerable insight has been gained about Symbiodinium in corals, little is known about the ecology and biology of Symbiodinium in other reef microhabitats. This study documents Symbiodinium associating with benthic macroalgae on the southern Great Barrier Reef, including some Symbiodinium that are genetically close to the symbiotic strains from reef-building corals. It is possible that some of these Symbiodinium were in hospite, associated to soritid foraminifera or ciliates; nevertheless, the presence of Symbiodinium C3 and C15 in macroalgal microhabitats may also suggest a potential link between communities of Symbiodinium associating with both coral hosts and macroalgae.     

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.     

Acropora recruits harbor “rare” Symbiodinium in the environmental pool

Coral–algal symbioses are essential for the survival of corals. Algal endosymbionts, specifically the dinoflagellate genus Symbiodinium, are divided into several genetic clades. The composition of Symbiodinium within corals plays an important role in the tolerance and/or sensitivity of host corals to local environments, due to individual Symbiodinium-specific physiological characteristics. While the majority of gamete-spawning corals acquire Symbiodinium from the surrounding environment, little is known about whether corals specifically select or randomly acquire Symbiodinium from the environmental population. In the present study, we compared the Symbiodinium clade composition of newly recruited Acropora corals with that of the environmental pool (water column, sediments, and adult colonies). More than 90 % of recruits harbored clades A and/or D until 6 months after settlement, despite the Symbiodinium environmental pool being mainly composed of clade C (mainly ITS1 type C2), and to a lesser extent clades A and D. In addition, the environmentally dominant type C2 Symbiodinium was not detected in Acropora recruits, while a few recruits harbored ITS1 types C1 or C15. Therefore, the clade composition of recruits may not reflect the abundance/density of Symbiodinium populations in the environment. Some members of clades A and D are known to exhibit tolerance to a wide range of environments. ITS1 type C1 also exhibits greater tolerance to thermal stress compared to ITS1 type C2. These tolerance characteristics of certain Symbiodinium may be vital for the initial survival of Acropora recruits, even if these Symbiodinium are rare in the environment.     

The endosymbiotic dinoflagellates (Symbiodinium sp.) of corals are parasites and mutualists

The evolutionary success and continued survival of reef-building corals under increasing environmental change will, in part, be determined by the composition of their endosymbiotic dinoflagellate communities (Symbiodinium sp.). Recent research suggests that differences in the phylotype composition of Symbiodinium in the same host can lead to different outcomes for the host when exposed to similar environmental conditions. One explanation for these observations is that symbioses between corals and Symbiodinium represent a continuum of interaction states that encompass mutualisms and parasitisms consistent with current evolutionary theory developed for other symbiotic systems. Here, we discuss the evidence supporting the existence of a parasitic to mutualistic continuum in Symbiodinium interactions and propose that a consideration of the evolutionary ecology of these associations will advance our understanding of how environmental change will influence the ecological outcomes in these important symbioses. We advocate that a robust taxonomic structure for Symbiodinium sp. and empirical studies on sexual reproduction in Symbiodinium, the stability of interaction states among Symbiodinium symbioses spatially and temporally and how interaction states change as the environment changes will generate data for models that accurately forecast how climate change will influence the persistence of corals and the reefs they structure.     

Symbiont diversity in scleractinian corals from tropical reefs and subtropical non-reef communities in Taiwan

We examined zooxanthellae diversity in scleractinian corals from southern Taiwan and the Penghu Archipelago, a tropical coral reef and a subtropical non-reefal community, respectively. Zooxanthellae diversity was investigated in 52 species of scleractinian corals from 26 genera and 13 families, using restriction fragment length polymorphism (RFLP), and phylogenetic analyses of the nuclear small-subunit ribosomal DNA (nssrDNA) and large-subunit ribosomal DNA (nlsrDNA). RFLP and phylogenetic analyses of nuclear-encoded ribosomal RNA genes showed that Symbiodinium clade C was the dominant zooxanthellae in scleractinian corals in the seas around Taiwan; Symbiodinium clade D was also found in some species. Both Symbiodinium clade C and D were found in colonies of seven species of scleractinian corals. Symbiodinium clade D was associated with corals that inhabit either shallow water or the reef edge in deep water, supporting the hypothesis that Symbiodinium clade D is a relatively stress-tolerant zooxanthellae found in marginal habitats.Communicated by Biological Editor H.R. Lasker     

Variations in Reactive Oxygen Release and Antioxidant Activity in Multiple Symbiodinium Types in Response to Elevated Temperature

As ocean temperatures rise, investigations into what the physiological effects will be on the symbiotic microalga Symbiodinium, and how these may play into the cnidarian bleaching response, have highlighted the contribution of reactive oxygen species (ROS). Previous studies have laid this groundwork using a limited number of Symbiodinium phylotypes, and so this study aims to expand this understanding by exploring the effects of sub-lethal elevated temperatures on the physiological response of seven genetically distinct types of Symbiodinium, including A1, B1, B2, C1, D, E1, and F2. The production of ROS (at 26?°C, 29?°C, 30?°C, and 31?°C) and activity of the antioxidants catalase (CAT) and superoxide dismutase (SOD) (at 26?°C and 31?°C) were measured as indicators of sensitivity or tolerance to heat stress. Symbiodinium types B1 and C1 were the most thermally sensitive, with C1 producing the highest amount of ROS at elevated temperatures. Types A1 and F2 were tolerant, having no increase in ROS production, and were the only types to increase both CAT and SOD activity with temperature stress. Type B2 had decreased ROS production and elevation of CAT activity, while type E1 had decreased levels of ROS production at elevated temperatures. Type D was the only Symbiodinium type to remain unaffected by elevated temperatures. These results are consistent with previous findings of relative sensitivity or tolerance to elevated temperatures, specifically with regards to types A1, B1, and F2. The inclusion of types B2, C1, D, and E1 provides further new evidence of how types differ in their thermal responses, suggesting differing mechanisms exist in the Symbiodnium response to higher temperature and highlighting the importance of establishing symbiont identity when exploring the response of intact associations to this type of stress.