Intraspecific and interspecific variation in thermotolerance and photoacclimation in Symbiodinium dinoflagellates

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1 rspb.royalsocietypublishing.org Research Cite this article: Díaz-Almeyda EM, Prada C, Ohdera AH, Moran H, Civitello DJ, Iglesias- Prieto R, Carlo TA, LaJeunesse TC, Medina M. 7 Intraspecific and interspecific variation in thermotolerance and photoacclimation in Symbiodinium dinoflagellates. Proc. R. Soc. B 8: Received: 7 August 7 Accepted: November 7 Subject Category: Ecology Subject Areas: ecology, microbiology, plant science Keywords: Symbiodinium, thermotolerance, photoacclimation, F v /F m, intraspecific variation Authors for correspondence: Erika M. Díaz-Almeyda erika.diaz@emory.edu M. Medina mum@psu.edu Electronic supplementary material is available online at figshare.c.99. Intraspecific and interspecific variation in thermotolerance and photoacclimation in Symbiodinium dinoflagellates Erika M. Díaz-Almeyda,, C. Prada, A. H. Ohdera, H. Moran, D. J. Civitello, R. Iglesias-Prieto, T. A. Carlo, T. C. LaJeunesse and M. Medina Department of Biology, The Pennsylvania State University, State College, PA 8, USA Department of Biology, Emory University, Atlanta, GA 7, USA Department of Biological Sciences, University of Rhode Island, Kingston, RI 88, USA EMD-A, --9-7; DJC, Light and temperature are major drivers in the ecology and biogeography of symbiotic dinoflagellates living in corals and other cnidarians. We examined variations in physiology among strains comprising five species of clade A Symbiodinium. We grew cultures at 8C (control) and 8C (high temperature) over a duration of 8 days while measuring growth and photochemical efficiency (F v /F m ). Responses to thermal stress ranged from susceptible to tolerant across species and strains. Most strains exhibited a decrease in cell densities and F v /F m when grown at 8C. Tolerance to high temperature (T ) was calculated for all strains, ranging from (unable to survive at high temperature) to (able survive at high temperature). There was substantial variation in thermotolerance across species and among strains. One strain had a T close to, indicating that growth was not reduced at 8C for only this one strain. To evaluate the combined effect of temperature and light on physiological stress, we selected three strains with different levels of thermotolerance (tolerant, intermediate and susceptible) and grew them under five different light intensities (, 8,, and mmol quanta m s ) at and 8C. High irradiance exacerbated the effect of high temperature, particularly in strains from thermally sensitive species. This work further supports the recognition that broad physiological differences exist not only among species within Symbiodinium clades, but also among strains within species demonstrating that thermotolerance varies widely between species and among strains within species.. Introduction The ongoing burning of fossil fuels has substantially increased atmospheric concentrations of carbon dioxide [], causing global temperatures to rise at unprecedented rates [,]. These changes are negatively impacting the planet s more sensitive biota. Episodes of high sea surface temperature (SST) have increased in frequency and intensity, affecting the health and viability of reefbuilding corals, thus degrading the ecosystems they build and sustain [,]. Temperatures that are..8c above normal are physiologically stressful and can disrupt the mutualisms between corals and their dinoflagellate endosymbionts, Symbiodinium. This leads to the whitening, or bleaching, of corals. While severe events cause significant mortality, not all symbiotic corals are equally affected [ 8], even among conspecifics from the same population [9]. Certain populations, as well as particular species and colonies of the same species, exhibit differential responses to heat stress. Often, these differences in coral mortality are in large part due to physiological differences & 7 The Author(s) Published by the Royal Society. All rights reserved.

2 Table. Cultures used in this study, culture collection code, ITS type, animal species from which the culture was obtained, and geographical location where the culture was collected. code species name strain name type collected from origin thermotolerance Smic S. microadriaticum CassKB8 A a Cassiopeia xamachana North Pacific tolerant Smic S. microadriaticum RT- A b Cassiopeia xamachana Caribbean tolerant Smic S. microadriaticum RT- A b Cassiopeia andromeda Red Sea sensitive Snec S. necroappetens RT-8 A b Condylactis gigantea Caribbean tolerant Snec S. necroappetens MAC- A a Porites astreoides Caribbean sensitive Slin S. linucheae RT-79 A b Plexaura homamalla Caribbean tolerant Spil S. pilosum RT- A b Bartholomea annulata Caribbean sensitive Spil S. pilosum RT- A b Heliopora sp. West Pacific tolerant Spil S. pilosum RT- A b Meandrina sp. Caribbean tolerant Stri S. tridacnidorum RT-9 A b Tridacna maxima West Pacific sensitive Stri S. tridacnidorum CassEl- A a Cassiopeia sp. North Pacific sensitive a BURR collection at SUNNY Buffalo. b Trench collection at The Pennsylvania State University. rspb.royalsocietypublishing.org Proc. R. Soc. B 8: 777 among the species of Symbiodinium they harbour [ 7] and possibly genotypic differences among individual strains. Physiological and genetic diversity within Symbiodinium taxa is vast [,8,9]. Numerous divergent clades (currently designated A through I) comprise the genus Symbiodinium [] and it is apparent that each clade contains a diversity of species with distinct ecological attributes and physiologies [ ]. For example, clade C probably contains hundreds of undescribed species that are mutualistic with a wide array of cnidarian hosts []. Similarly, there are tens of species within clade B, many of them endemic to the western tropical and subtropical Atlantic [, 9]. While the diversity within Symbiodinium one of the most species-rich dinoflagellate genera is well recognized, the physiological variability within and between species remains poorly characterized. Stress tolerance among certain Symbiodinium may help coral populations cope with rising temperatures [9, ]. Tolerance of high temperatures was first observed and described in Symbiodinium glynnii, a species in clade D [ ]. Subsequent studies indicate that Symbiodinium spp. from other clades also tolerate higher temperatures [,,7,8]. In the Persian Gulf where SSTs can reach a maximum of 8C, several coral species harbour Symbiodinium thermophilum, a clade C strain. In the Red Sea, coral reefs experience the most extensive seasonal and latitudinal thermal gradients ( 8C) worldwide. Along this gradient, the coral species Pocillopora verrucosa also exhibit thermotolerant symbionts belonging to clade A (ITS type A and A) at higher temperatures (7 8C) [8]. Beyond interspecies variation, the degree of physiological thermotolerance among strains within populations or among populations across a species range of distribution remains largely unknown. Individual strains within species of Symbiodinium can exhibit different photophysiologies [8], and such variation will factor into the adaptation of Symbiodinium to rapid climate change. Thus, it is critical to also assess inter-individual physiological variation. An important aspect of Symbiodinium spp. physiology is their ability to acclimate under physiological stress. For example, members of clade A acclimate differently from one another to various light levels [ ]. The range of thermal acclimation also differs among Symbiodinium spp. [,,,7]. Still, the tempo and mode of acclimatory responses within and between most species under the dual effects of light and temperature requires further study. We conducted experiments to study the acclimation potential to temperature and light within and among species of Symbiodinium clade A a phylogenetically well-characterized group [,]. We used cultures from different populations corresponding to five species: Symbiodinium microadriaticum (¼type A), Symbiodinium pilosum (¼type A), Symbiodinium tridacnidorum (¼type A Pacific ), Symbiodinium linucheae (¼type A) and Symbiodinium necroappetens (¼type A). We specifically tested the occurrence of thermotolerance among species, as well as if intraspecific variation is comparable to that found among species. We also evaluated the effects of temperature and light on Symbiodinium growth and photochemistry. Our results suggest that thermotolerance is not restricted to individual species, but instead it is widespread throughout clade A, and that species often adjust to variations in light. We also observed that species surviving under stressful light and thermal conditions often incur physiological costs that slow down growth and accelerate photodamage.. Methods (a) Symbiodinium cultures: growth conditions To quantify physiological responses, we cultured Symbiodinium in ASP-8A medium [9] at 8C with full-spectrum fluorescent lights at mmol quanta m s with a : (light : dark) photoperiod. Light intensities were measured inside the culture flasks with a p sensor (Biospherical, USA). Cultures were maintained at logarithmic phase by replacing the culture media every. weeks. Symbiodinium strains were obtained from the Trench collection maintained by Todd LaJeunesse at The Pennsylvania State University and the BURR collection at SUNY Buffalo maintained by Mary Alice Coffroth. Herein, we follow a binomial nomenclature for Symbiodinium species [,,] but table for culture collection and strain designations. (b) High-temperature acclimation: experimental design To test for acclimation to high temperature, we used cultures from clade A that included five species: three cultures of S.

3 microadriaticum (ITS type A), two cultures of S. necroappetens (ITS type A), three cultures of S. pilosum (ITS type A), two cultures of S. tridacnidorum (ITS type A Pacific ) and one culture of S. linucheae (ITS type A) (table ). We inoculated cultures at an initial concentration of cells ml in ml tubes containing 8 ml of culture (day ). To elevate the temperature to 8C, we increased from 8.8C and then gradually increased the temperature by.8c every h until 8C was reached (day ). We counted cell densities in triplicate every days, and measured maximum quantum yield of photochemistry (F v /F m ) daily (see below) for 7 days in four replicates per strain per temperature (control: 8C and high temperature: 8C). (c) High-temperature acclimation and photoacclimation: experimental design To test for the effects of interaction between temperature and light on Symbiodinium health, we selected three cultures with different levels of known physiological thermotolerance: a tolerant S. microadriaticum (Smic) with no significant cell densities changes at 8C, an intermediate tolerant S. microadriaticum (Smic) with significant cell densities changes at 8C and a sensitive strain from S. tridacnidorum (Stri). We inoculated cultures at cells ml in ml tubes, containing 8 ml of culture, initially grown at mmol quanta m s at 8C. We set the high-temperature treatment at 8C (without ramping) and the control temperature treatment at 8C. Light levels were adjusted to five different intensities ranging from under-saturating levels to over-saturating light levels:, 8,, and mmol quanta m s, using mesh fabric to produce shade. We incubated replicates per culture at each of the treatments. (d) Photochemical efficiency To quantify photochemical activity, we measured the maximum quantum yield of charge separation of photosystem II (F v /F m ) with a MiniPAM fluorometer (Walz, USA) after min of dark acclimation at the end of the light cycle. We measured daily F v /F m in five independent tubes without disturbing the sample. For the species comparisons of strains, these five measurements were performed in at least two independent runs, and in three independent runs for four of these cultures (Snec, Smic, Stri and Spil). (e) Population growth To evaluate the performance of strains during high-temperature acclimation, we measured cell density by fixing ml aliquots of culture in % Lugol s iodine and determined cell densities using a haematocytometer. At least nine replicate aliquots were counted for each sample at each time point. Cell densities were measured for days,, 9, and 7. We measured them at least once for all strains; Smic, Snec, Spil and Stri were estimated twice (these strains were chosen randomly). We used these cell density estimates to estimate relative population growth rates (r, units: d ) at and 8C, as well as tolerance to 8C for each strain. We estimated r as the slope of the relationship between log-transformed cell densities and time, a standard technique for growth experiments with photosynthetic microbes and algae that exhibit exponential growth from low initial densities []. To avoid taking the logarithm of zero, we added cells ml to each observed count, which is % of the smallest observed cell density. We then estimated tolerance to 8C for each strain, T, as the ratio of the relative growth rate at 8C, r, to the relative growth rate at 8C, r. In some cases, strains had negative growth rates at 8C (i.e. they declined towards extinction). In these cases, we set the tolerance value to a defined minimum value of zero. A tolerance photochemical efficiency (F v /F m ) at C. a a Smic Spil Slin Snec Stri Figure. Average photochemical efficiency (F v /F m )at8c of five Symbiodinium spp. over a 7-day period. Slin, S. linucheae (one strain); Smic, S. microadriaticum (three pooled strains); Snec, S. necroappetens (two pooled strains), Spil, S. pilosum (three pooled strains) and Stri, S. tridacnidorum (two pooled strains). Bars depict one standard deviation of the mean. (One-way ANOVA, F,8 ¼ 9., p,., Tukey Kramer HSD.) Grouping from post hoc comparisons are depicted with letter above each bar. value equal to one indicates that growth is not reduced at 8C, whereas a value of zero indicates that the isolate cannot grow at this high temperature. (f) Statistical analysis We examined how cell densities and photochemical efficiency (F v /F m ) were affected by temperature, comparing Symbiodinium strains (), temperatures (8., 9, 9.,,.,,. and 8C) and time (days,, 9, and 7). We also analysed the effect of light and temperature, comparing Symbiodinium strain (), temperature ( and 8C) and light intensity (, 8,, and mmol quanta m s ). One-way ANOVAs and ANCOVAs (fixed-effects, LS) were used to analyse the effect of temperature and/or light; all two-way interactions were included. We used JMP Pro for all analyses (SAS Institute, Cary, NC, USA, ). We tested for significantly negative effects of high temperature on strain growth using bootstrapping tests ( iterations, alternative hypothesis: T, ) in which we randomly resampled data with replacement stratified by each strain, temperature and sampling date []. Strains with no thermotolerance are shown in red, while strains with some thermotolerance are shown in a gradient from purple (T. ) to blue (T ¼ ).. Results (a) Variation in temperature sensitivity within and among species Significant differences in F v /F m were measured among species (figure ) and within species (figure ). Our linear models identified all factors (strain, day, temperature) and two-way interactions as having a significant effect on Symbiodinium cell densities and F v /F m (table and figures and ). Temperatureday explained.% of variance in cell densities responses, and 8.% variance in F v /F m values (see F ratios, table ). The relative population growth rate at 8C was significantly different among strains (figure ). Grouping strains by species (figure ), we could conclude that there are three groups of species with different thermotolerance: S. microadriaticum and S. pilosum (group A), S. linucheae and S. necroappetens (group B) and S. tridacnidorum [group C Tukey Kramer honestly significant difference (HSD)]. However, fine scale analysis at the level of strains within species showed higher thermotolerance diversity (figures ). b b c rspb.royalsocietypublishing.org Proc. R. Soc. B 8: 777

4 Table. ANCOVA analysis examining cell density response (log transformed) and photochemical efficiency (F v /F m ) of strains of dinoflagellates to high temperature. Cell densities and F v /F m showed similar results where temperature was the major source of variability. Cell densities were measured on days,, 9, and 7 in three replicates per culture per treatment (n ¼ 8). F v /F m was measured every day for 7 days with six technical replicates. F v /F m was measured at least two times for all strains and three times for four of them in independent repetitions of the experiment. Cultures were grown at mmol quanta m s under two temperatures: control (8C), and high temperature (8C). d.f., degrees of freedom. response source d.f. F ratio Prob > F cell densities (log transformed) strain.87 less than. temperature 8. less than. day.78 less than. straintemperature 8. less than. strainday.8 less than. temperatureday 9.77 less than. whole model test: F,78 ¼ 7.7, p,., R ¼.77 photochemical efficiency strain.79 less than. temperature.9 less than. day 98.9 less than. straintemperature 9.87 less than. strainday.9 less than. temperatureday less than. whole model test: F,78 ¼ 78., p,., R ¼.8 rspb.royalsocietypublishing.org Proc. R. Soc. B 8: 777 Using our tolerance estimate, T, we categorize strains into sensitive (T ¼ ) or tolerant (T. ) (table and figure ). All strains were significantly affected by high temperature (Spil p ¼.7, Smic p ¼., p ¼ for the rest of the strains). An exception was Spil ( p ¼.7), which was not affected by high temperature (T ¼.9). Our thermotolerant strains, in order of tolerance, are Spil, Spil, Smic, Snec, Smic and Slin. The most tolerant strains (Spil, Spil and Smic) showed no significant difference in cell densities (log transformed) at high temperature when compared with controls (figures and ). Only the two most tolerant strains, Spil and Spil, showed no significant difference on F v /F m at high temperature compared with control temperature (figure ). The relative population growth at high temperature was positive for all tolerant strains (figure ), indicating the ability to grow at high temperature. For the thermotolerant strains Snec, Smic and Slin, cell densities and F v /F m were significantly different among treatments indicating a cost in fitness when acclimated to high temperature (figures and ). The sensitive strains, in order of thermosensitivity, are: Stri, Stri, Snec, Smic and Spil (figures ). Thermosensitive strains showed negative values of relative population growth (figure ) indicating the inability to survive at high temperature (T ¼, figure ). The most thermosensitive strain, Stri, decreased its population size faster than Stri and so on. Sensitive strains Stri, Stri and Snec showed higher sensitivity because cultures died during the experiment. Although cultures of Smic and Spil contained available cells by the end of the experiment ( figure ), the negative trend indicates the culture would likely die if the experiment was conducted for a longer duration (figure ). All these strains failed to thrive under high temperature, indicating an inability to cope with high temperature. Species thermotolerance cannot be estimated by most species in this study. This is particularly true for S. microadriaticum, S. necroappetens and S. pilosum. For example, for S. microadriaticum, the two strains Smic and Smic decreased in cell density, with Smic showing greater decrease in density than Smic (average.9 cells ml, versus. cells ml at day 7 of the experiment, figure ). In both cases, F v /F m also decreased (figure ). While cell densities were low for both, these cell cultures were still physiological active (mean F v /F m of 89 for Smic and. for Smic). However, the relative population growth rate at high temperature indicates that Smic was negatively affected by high temperature, becoming unable to thrive. By contrast, cell densities of Smic did not change significantly with temperature, although there was a significant decrease in F v /F m indicating that it had acclimated better to the heat stress. For S. necroappetens, the tolerant Snec strain decreased significantly in cell density and showed a significant decrease in F v /F m under high temperature (figures and ). By contrast, the sensitive strain Snec died under high-temperature conditions. For S. pilosum, the cell densities for Spil remained constant until day, when growth rates began to decrease (figure ). Photochemical efficiency for Spil was significantly lower than controls at high temperature (figure ). Thus, the cells remained physiologically active (F v /F m ¼. versus.9), but this activity was insufficient to allow the culture to grow (figure ). The F v /F m values for thermotolerant strains Spil and Spil were not significantly different under high-temperature treatment, and both maintained similar cell densities when grown under control and treatment conditions (electronic supplementary material, table S; figure ). Strains of S. tridacnidorum (Stri and Stri) werethermo- sensitive and both died when exposed to high temperature (figures, and ). Cell densities and F v /F m in these cultures decreased soon after the high-temperature treatment was

5 log cell density (cells ml ) Smic Smic Smic Snec Snec Slin Spil Spil Spil rspb.royalsocietypublishing.org Proc. R. Soc. B 8: 777 Stri 8 8 days Stri 8 8 days Figure. Cell density (log transformed) of strains of clade A under different growth temperatures, at mmol quanta m s. Blue line indicates linear regression of data under control temperature (8C) and red line indicates linear regression of data under high temperature (8C). Dotted lines indicate confidence intervals. To avoid taking the logarithm of zero, we added, which is % of the lowest observed non-zero density, to each value before transformation. A value closer to indicates zero cells. We found that significant differences at the p,. level between the control and the high-temperature treatments for all species except Smic, Spil and Spil, which showed no statistical difference. initiated (figure ). Symbiodinium linucheae (Slin) exhibited lower growth rates under the control temperature, compared to other clade A species. The relative population growth rate was positive at high temperature (figure ), demonstrating its ability to survive under the temperature treatment (figures and ). For these two species, we cannot categorize them as thermosensitive or thermotolerant, given the limited number of strains, we were able to include in this study. (b) The synergistic effect of light and temperature in photochemistry and growth Three strains with different thermal tolerances were chosen to evaluate the synergistic effects of light and high temperature (i.e. Smic, Smic and Stri). Under five different light levels (from low light to high light), all three strains had a linear photoacclimation response independent of temperature (table and figure ). F v /F m was highest at low light and decreased at high light. This linear response had similar slopes for both control and high temperature for all strains. The levels of photochemical efficiency were variable among strains. For the high tolerant strain Smic (T ¼ ), F v /F m changed equally under both temperatures and different light intensities. The intermediate tolerant strain Smic (T ¼.) and sensitive strain Stri (T ¼ ) decreased F v /F m under high temperature but maintained the same slope at both control and high-temperature conditions. The values measured for F v /F m decreased at a faster rate for Stri than for Smic. After 7 days of experiment, F v /F m remained constant for both tolerant strains. The sensitive

6 photchemical efficiency (F v /F m ) Smic Smic Snec. Snec Slin Spil Spil Spil Smic rspb.royalsocietypublishing.org Proc. R. Soc. B 8: Stri 8 8 days days Figure. Photochemical efficiency (F v /F m ) of strains of clade A under different growth temperatures, at mmol quanta m s. Blue line indicates linear regression of data under control temperature (8C) and red line indicates linear regression of data under high temperature (8C). Dotted lines indicate confidence intervals. We found that significant differences at the p,. level between the control and the high-temperature treatments for all species except Spil and Spil, which showed no statistical difference. Stri strain under high temperature vastly decreased F v /F m and no live cells remained at the end of the experiment (electronic supplementary material, figure S).. Discussion This work substantiates the growing realization for the existence of broad physiological variation in thermal tolerance among and within species of Symbiodinium. Physiologies of strains that are sensitive, intermediate or tolerant appear to be independent of the species under study. There are species comprised strains that are generally thermotolerant (S. linucheae), while strains from other species (e.g. S. tridacnidorum) are sensitive. Other species within clade A (S. microadriaticum, S. necroappetens and S. pilosum) contain strains that diverge significantly in thermal tolerance. Thus, acclimation to high temperature is dependent on the species identity and the physiological attributes possessed by a particular genotype within a species, i.e. strain. This work also supports previous findings that high light and temperature interact to accelerate physiological breakdown [], especially among strains with limited or no tolerance to high temperature. (a) Widespread thermotolerance variation within species from clade A lineage These findings indicate that at least some strains from each of the clade A species examined here can persist under 8C, with the possible exception of S. tridacnidorum. It appears that thermotolerance is not phylogenetically restricted to certain species, but may indeed be extensive across members of clade A (figure ). While earlier studies reported limited within

7 i...8 g h f e a b relative population growth rate at C (d ) d c c b Smic Smic Smic Snec Snec Slin Stri Stri Spil Spil Spil changes Figure. Relative population growth rate at 8C with simplified phylogeny of Symbiodinium clade A inferred from ITS, ITS and.8s rdna sequences [,,]. (One-way ANOVA, F, ¼.7, p,., Tukey Kramer HSD.) Error bars denote standard error. 7 rspb.royalsocietypublishing.org Proc. R. Soc. B 8: 777 thermotolerance...8. Snec Stri Stri Spil Smic Slin Spil Snec Smic Spil Smic.... photochemical efficiency at C (F v /F m ) Figure. Thermotolerance (T ) versus photochemical efficiency (F v /F m )at high temperature (8C). Error bars denote standard error. clade variation in thermal stress [,,,8], our work with additional strains provides evidence for more widespread variation at intraspecific taxonomic levels (i.e. individual strains within species) than previously expected []. Extensive variation to thermal tolerance within species hints at genetically encoded variation among populations. Comparing molecular mechanisms at supraspecific taxonomic ranks (i.e. across species and across clades) may be less fruitful than infraspecific comparisons (i.e. within species), given the amount of genetic divergence within the genus. Major Symbiodinium clades diverged between and Ma [,]; a divergence time similar to bird [] and primate families [7]. It is thus not surprising that contradictory results have been reported on the molecular mechanisms underlying thermotolerance [8,9]. Comparative transcriptomics of sensitive (Symbiodinium C K) versus thermotolerant (clade D) species (i.e. from separate clades) could not postulate a clear mechanism for thermal acclimation due to the long divergence time between clade C and D ( Ma according to []). Similarly, Rosic et al.[]wereunabletoseeconvergingmolecularpatterns of thermotolerance between S. pilosum (¼type A), Symbiodinium psygmophilum (¼type B), Symbiodinium goreaui (¼type C) and Symbiodinium trenchii (¼type Da), representing species from highly divergent lineages. In this study, we demonstrate that comparative analysis at infraspecific taxonomic ranks (i.e. genotypes within and between closely related species) will be necessary if we want to understand underlying genetic attributes responsible for variation in thermotolerance. In all clade A species examined, we find that 8C is closer to the optimal growth temperature than 8C. Growth rate appears to be much more affected than F v /F m by high temperature. The adverse effects of high temperature (above 8C) on photosynthesis have been extensively characterized for Symbiodinium grown in culture [,,7]. For instance, in a strain of S. microadriaticum, photosynthesis impairment occurs at temperatures above 8C, stopping completely at higher temperatures of 8C []. Our results show that for strains comprising five species of Symbiodinium clade A, a temperature of 8C was high enough to distinguish between sensitive and thermotolerant strains. Notably, thermotolerance is not phylogenetically constrained and instead, there are sensitive, intermediate and tolerant populations within species. Although we were able to statistically categorize thermotolerance into these groups, a gradient in thermotolerance can be observed (figure ). Some strains might fall into the tolerantsideofthegradient,suchasmembersofs. pilosum and S. microadriaticum, while others such as strains from S. tridacnidorum fall into the sensitive side of the tolerance gradient. Nevertheless, the thermotolerance of more strains from each species should be evaluated in order to make broader generalizations. (b) Combined effect of light and temperature in physiological acclimation Most investigators recognize the importance of considering the role of multiple stressors in order to improve predictions regarding physiological responses to climate change []. Acclimation to high temperature in Symbiodinium often

8 (a) (b) (c). relative F v /F m... T = T =. T = relative light relative light relative light Figure. Relative photochemical efficiency of three strains with different thermotolerances under different light conditions. Relative light was calculated by subtracting the total light dose of the experimental conditions (, 8,, and mmol quanta m s ) from the initial light conditions ( mmol quanta m s ). Relative photochemical efficiency was calculated with the initial F v /F m minus the value after h at the light/temperature conditions. (a) Tolerant strain (Smic), (b) intermediate tolerant strain (Smic) and (c) sensitive strain (Stri). T, thermotolerance value. Blue line indicates linear regression of data under control temperature (8C) and red line indicates linear regression of data high temperature (8C). Dotted lines indicate confidence intervals. Table. ANCOVA analysis examining the combined effect of temperature and light on Symbiodinium in photochemical efficiency (F v /F m ). F v /F m was measured after h of exposure to light/temperature treatments with six technical replicates. Cultures were grown under control (8C), and high temperature (8C), and five light intensities, 8,,, mmol quanta m s. d.f., degrees of freedom. Whole model test: F 9, ¼.987, p,., R ¼ rspb.royalsocietypublishing.org Proc. R. Soc. B 8: 777 response source d.f. F ratio Prob > F photochemical efficiency strain 7.8 less than. temperature 8.8 less than. relative light 9. less than. straintemperature. less than. strainrelative light temprelative light.9.78 comes with physiological trade-offs, including decreased photochemical activity and reduced growth []. It has been observed that during coral bleaching, high temperatures are usually interacting with high levels of light [] and variation in available nutrients []. Combinations of various environmental factors will elicit different physiological responses by different genotypes and species. Having the ability to acclimate to multiple stressors may come with a cost to the organism s physiology. Studying these environmental factors separately and in combination aids the identification of possible trade-offs and any consequences for the functional performance of a particular host symbiont combination. Our results showed that some tolerant strains had the ability to acclimate well to both high light and high temperature. The strain with intermediate tolerance tested here was affected by high temperature during photoacclimation. By contrast, thermal stress in the sensitive strain Stri was compounded by exposure to high light. Thus, the combined effect of light and temperature exacerbates stress, particularly in thermosensitive species and genotypes. The ability to acclimate to multiple interacting factors including temperature and light should be further studied, especially among closely related Symbiodinium lineages. These physiological attributes will likely determine the future ecological success of certain Symbiodinium taxa to changing climatic conditions []. Understanding further the synergistic effects of temperature and light on Symbiodinium physiology is crucial for predicting the response of coral algal symbioses to climate change.. Conclusion A comparative approach examining closely related species and individual genotypes within species revealed broad physiological diversity among and within Symbiodinium spp. Thermotolerance is not species- or clade-specific, but rather widespread and varied among members of the genus. Moreover, the further delineation of species within the genus should improve research on mechanisms of thermotolerance, especially when comparing the performance of certain host symbiont combinations under various environmental conditions. Data accessibility. Data are available from the Dryad Digital Repository at: []. Competing interests. We declare we have no competing interests. Funding. This project was funded by CONACYT PhD award, Institutional Research and Career Development Award NIH/NIGMS K GM8, NSF grants nos OCE and OCE and Penn State startup funds. Acknowledgements. We thank Mark Warner for technical advice, Mary Alice Coffroth (SUNY Buffalo) for providing algal cultures and Dr John E. Parkinson and students Ashley Porter and Victoria Wu for technical assistance. We thank our anonymous reviewers for their feedback that helped improved this manuscript.

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