<i>Symbiodinium</i> Community Composition in Scleractinian Corals Is Not Affected by Life-Long Exposure to Elevated Carbon Dioxide

<div><p>Ocean acidification (OA) is expected to negatively affect coral reefs, however little is known about how OA will change the coral-algal symbiosis on which reefs ultimately depend. This study investigated whether there would be differences in coral <i>Symbiodinium</i> types in response to OA, potentially improving coral performance. We used denaturing gradient gel electrophoresis (DGGE) of the internal transcribed spacer 2 (ITS2) region of ribosomal DNA to investigate the dominant types of <i>Symbiodinium</i> associating with six species of scleractinian coral that were exposed to elevated partial pressures of carbon dioxide (pCO₂) <i>in situ</i> from settlement and throughout their lives. The study was conducted at three naturally occurring volcanic CO₂ seeps (pCO₂ ∼500 to 900 ppm, pH_Total 7.8 – 7.9) and adjacent control areas (pCO₂ ∼390 ppm, pH_Total ∼8.0 – 8.05) in Papua New Guinea. The <i>Symbiodinium</i> associated with corals living in an extreme seep site (pCO₂ >1000 ppm) were also examined. Ten clade C types and three clade D types dominated the 443 coral samples. <i>Symbiodinium</i> types strongly contrasted between coral species, however, no differences were observed due to CO₂ exposure. Within five species, 85 – 95% of samples exhibited the same <i>Symbiodinium</i> type across all sites, with remaining rare types having no patterns attributable to CO₂ exposure. The sixth species of coral displayed site specific differences in <i>Symbiodinium</i> types, unrelated to CO₂ exposure. Symbiodinium types from the coral inhabiting the extreme CO₂ seep site were found commonly throughout the moderate seeps and control areas. Our finding that symbiotic associations did not change in response to CO₂ exposure suggest that, within the six coral hosts, none of the investigated 13 clade C and D <i>Symbiodinium</i> types had a selective advantage at high pCO₂. Acclimatisation through changing symbiotic association therefore does not seem to be an option for Indo-Pacific corals to deal with future OA.</p></div

The number of samples and the <i>Symbiodinium</i> ITS2 DGGE profiles for each coral species from each site used in this study.

a<p>The coral species Acropora millepora, Pocillopora damicornis, Seriatopora hystrix, Porites cylindrica, massive Porites sp., Galaxea fascicularis and Favites pentagona used in this study. ^b The modes of Symbiodinium acquisition (Symb. Acqu.) employed by each coral species. Horizontal being from the environment (post-settlement) and vertical from maternal sources. ^cThe reproductive strategy of each coral species with either broadcast spawning gametes, larvae brooded in the parental colony or a combination of the two. ^dThe assigned Symbiodinium ITS2 DGGE profiles (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0063985#pone-0063985-g001" target="_blank">Figures 1</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0063985#pone-0063985-g003" target="_blank">3</a>). ^eThe Seep and control (Ctr) sites at Upa-Upasina (Upa-U), Dobu and Esa’ Ala (Esa’ A).</p

<i>Symbiodinium</i> ITS2 haplotype networks of the 13 dominant bands identified in this study.

<p>Parsimony networks of Clade D (A) and Clade C (B) <i>Symbiodinium</i> ITS2 haplotypes from dominant bands identified in this study. Coral species are shown in different colours and Roman numerals indicate dominant band numbers. Previously published sequences are also indicated (1–4) along with their Genbank accession number. Each node represents a base pair change and indel lengths are shown by the boxed numbers along branches. Bands III to XI and XIII represent clade C types, while bands I, II and XII represent clade D type <i>Symbiodinium.</i> Pies indicate the presence of <i>Symbiodinium</i> types in multiple coral species and are not indicative of frequency.</p

The frequency of different <i>Symbiodinium</i> ITS2 profiles between sites and CO₂ exposures.

<p>The different <i>Symbiodinium</i> ITS2 profiles from the corals <i>Acropora millepora, Pocillopora damicornis, Seriatopora hystrix</i>, <i>Porites cylindrica</i>, massive <i>Porites sp.</i> and <i>Galaxea fascicularis</i> found at the three sites, each with High (Hi) and control (Lo) CO₂ exposures. Each colour corresponds to one of the 14 unique <i>Symbiodinium</i> ITS2 profiles found in these coral species. Colours common between sites or coral species indicate synonymous <i>Symbiodinium</i> profiles (See also <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0063985#pone.0063985.s002" target="_blank">Table S1</a>). The different colour schemes represent clade C (shades of green, yellow and brown), D (shade of grey and blue) and mixed (pink) <i>Symbiodinium.</i></p

The effects of macroalgae on maximum quantum yield and RNA/DNA ratio of corals in the laboratory experiment.

<p>(A) Maximum quantum yield (Fv/Fm) and (B) RNA/DNA ratio in nubbins of the coral <i>Acropora millepora</i> after 10 days of exposure to microenvironments simulating conditions underneath macroalgal assemblages. Data are untransformed means ± SE (n = 9; and n = 6 in the S100 treatment).</p

Physical and chemical conditions underneath and above macroalgal assemblages in the field on inshore coral reefs.

<p>Physical and chemical conditions underneath and 0.3 m above mat- (N = 8) and canopy-forming (N = 11) macroalgal assemblages on inshore coral reefs of the Great Barrier Reef, and results of paired t-tests. Abbreviations and units: pH, dissolved oxygen (DO, mg L⁻¹), alkalinity (ALK, µEq/kg), dissolved inorganic carbon (DIC, µM) dissolved organic carbon (DOC, mg L⁻¹), silicic acid (Si, µM), soluble reactive phosphorus (SRP, µM) and Irradiance (µmol photons m⁻² s⁻¹).</p

Effects of organic enrichment and nitrate on the fluorescent yields of heat-stressed and control <i>M. tuberculosa</i>.

<p>Time series of the fluorescence yields in <i>Montipora tuberculosa.</i> For details see the legend for <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0054399#pone-0054399-g002" target="_blank">Fig. 2</a>. In this species, heat stress (31.2°C) was applied between Days 10 and 59.</p

The effects of macroalgal biomass and assemblage type on the physical and chemical conditions in the field on inshore coral reefs.

<p>Results of two way ANOVA comparing the effects of macroalgal assemblage type (mat- or canopy forming) and macroalgal biomass on the ratios (underneath vs above) of pH, dissolved oxygen (DO, %) alkalinity (ALK, µEq/kg), dissolved inorganic carbon (DIC, µM), dissolved organic carbon (DOC, mg L⁻¹), silicic acid (Si, µM), soluble reactive phosphorus (SRP, µM) and irradiance (µmol photons m⁻² s⁻¹) (see also <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0012685#pone-0012685-g002" target="_blank">Fig. 2</a>).</p

A representative of each <i>Symbiodinium</i> ITS2 DGGE profile from the seven coral species investigated.

<p>The <i>Symbiodinium</i> profiles from the species <i>Acropora millepora</i> (Am), <i>Pocillopora damicornis</i> (Pd), <i>Seriatopora hystrix</i> (Sh), <i>Porites cylindrica</i> (Pc), massive <i>Porites</i> sp. (Pm), <i>Galaxea fascicularis</i> (Gf) and <i>Favites pentagona</i> (Fp) are shown at the top of each DGGE column. Each of the 13 dominant bands (I-XIII), which characterise the profiles, are also indicated. Bands VI and VII are labelled twice as they appear to co-migrate, however sequence data differentiates them.</p

Effects of organic enrichment and nitrate on the fluorescent yields of heat-stressed and control <i>A. millepora</i>.

<p>Time series of the fluorescence yields in <i>Acropora millepora.</i> Two treatments (Controls and +OE +NO3) remained at ambient temperature (mean: 25°C) throughout the experimental period (a, b). Tanks in the other treatments were exposed to heat stress (31.2°C) between Days 10 and 33, followed by a recovery period at ambient temperature (c-f). The nutrient treatments applied to these tanks were (c) Controls, (d) +NO3, (e) +OE and (f) +OE +NO3. Points represent means across colonies for each tank; solid lines are estimated temporal trends and dashed lines are 95% confidence intervals.</p