Transcriptional Response of Two Core Photosystem Genes in Symbiodinium spp. Exposed to Thermal Stress

Mutualistic symbioses between scleractinian corals and endosymbiotic dinoflagellates (Symbiodinium spp.) are the foundation of coral reef ecosystems. For many coral-algal symbioses, prolonged episodes of thermal stress damage the symbiont\u27s photosynthetic capability, resulting in its expulsion from the host. Despite the link between photosynthetic competency and symbiont expulsion, little is known about the effect of thermal stress on the expression of photosystem genes in Symbiodinium. This study used real-time PCR to monitor the transcript abundance of two important photosynthetic reaction center genes, psbA(encoding the D1 protein of photosystem II) and psaA (encoding the P700 protein of photosystem I), in four cultured isolates (representing ITS2-types A13, A20, B1, and F2) and two in hospite Symbiodinium spp. within the coral Pocillopora spp. (ITS2-types C1b-c and D1). Both cultured and in hospite Symbiodinium samples were exposed to elevated temperatures (32°C) over a 7-day period and examined for changes in photochemistry and transcript abundance. Symbiodinium A13 and C1b-c (both thermally sensitive) demonstrated significant declines in both psbA and psaA during the thermal stress treatment, whereas the transcript levels of the other Symbiodinium types remained stable. The downregulation of both core photosystem genes could be the result of several different physiological mechanisms, but may ultimately limit repair rates of photosynthetic proteins, rendering some Symbiodinium spp. especially susceptible to thermal stress

Annual coral bleaching and the long-term recovery capacity of coral

Mass bleaching events are predicted to occur annually later this century. Nevertheless, it remains unknown whether corals will be able to recover between annual bleaching events. Using a combined tank and field experiment, we simulated annual bleaching by exposing three Caribbean coral species (Porites divaricata, Porites astreoides and Orbicella faveolata) to elevated temperatures for 2.5 weeks in 2 consecutive years. The impact of annual bleaching stress on chlorophyll a, energy reserves, calcification, and tissue C and N isotopes was assessed immediately after the second bleaching and after both short- and long-term recovery on the reef (1.5 and 11 months, respectively). While P. divaricata and O. faveolata were able to recover from repeat bleaching within 1 year, P. astreoides experienced cumulative damage that prevented full recovery within this time frame, suggesting that repeat bleaching had diminished its recovery capacity. Specifically, P. astreoides was not able to recover protein and carbohydrate concentrations. As energy reserves promote bleaching resistance, failure to recover from annual bleaching within 1 year will likely result in the future demise of heat-sensitive coral species

Short-term coral bleaching is not recorded by skeletal boron isotopes.

Coral skeletal boron isotopes have been established as a proxy for seawater pH, yet it remains unclear if and how this proxy is affected by seawater temperature. Specifically, it has never been directly tested whether coral bleaching caused by high water temperatures influences coral boron isotopes. Here we report the results from a controlled bleaching experiment conducted on the Caribbean corals Porites divaricata, Porites astreoides, and Orbicella faveolata. Stable boron (δ11B), carbon (δ13C), oxygen (δ18O) isotopes, Sr/Ca, Mg/Ca, U/Ca, and Ba/Ca ratios, as well as chlorophyll a concentrations and calcification rates were measured on coral skeletal material corresponding to the period during and immediately after the elevated temperature treatment and again after 6 weeks of recovery on the reef. We show that under these conditions, coral bleaching did not affect the boron isotopic signature in any coral species tested, despite significant changes in coral physiology. This contradicts published findings from coral cores, where significant decreases in boron isotopes were interpreted as corresponding to times of known mass bleaching events. In contrast, δ13C and δ18O exhibited major enrichment corresponding to decreases in calcification rates associated with bleaching. Sr/Ca of bleached corals did not consistently record the 1.2°C difference in seawater temperature during the bleaching treatment, or alternatively show a consistent increase due to impaired photosynthesis and calcification. Mg/Ca, U/Ca, and Ba/Ca were affected by coral bleaching in some of the coral species, but the observed patterns could not be satisfactorily explained by temperature dependence or changes in coral physiology. This demonstrates that coral boron isotopes do not record short-term bleaching events, and therefore cannot be used as a proxy for past bleaching events. The robustness of coral boron isotopes to changes in coral physiology, however, suggests that reconstruction of seawater pH using boron isotopes should be uncompromised by short-term bleaching events

Annual coral bleaching and the long-term recovery capacity of coral

Mass bleaching events are predicted to occur annually later this century. Nevertheless, it remains unknown whether corals will be able to recover between annual bleaching events. Using a combined tank and field experiment, we simulated annual bleaching by exposing three Caribbean coral species (Porites divaricata, Porites astreoides and Orbicella faveolata) to elevated temperatures for 2.5 weeks in 2 consecutive years. The impact of annual bleaching stress on chlorophyll a, energy reserves, calcification, and tissue C and N isotopes was assessed immediately after the second bleaching and after both short- and long-term recovery on the reef (1.5 and 11 months, respectively). While P. divaricata and O. faveolata were able to recover from repeat bleaching within 1 year, P. astreoides experienced cumulative damage that prevented full recovery within this time frame, suggesting that repeat bleaching had diminished its recovery capacity. Specifically, P. astreoides was not able to recover protein and carbohydrate concentrations. As energy reserves promote bleaching resistance, failure to recover from annual bleaching within 1 year will likely result in the future demise of heat-sensitive coral species

Maximum quantum yield of PSII (F_v/F_m) and the effective absorption cross-section of PSII (σ_PSII) during the simulated bleaching event.

<p>(a.) Maximum quantum yield of PSII (F_v/F_m) and (b.) the effective absorption cross-section of PSII (<b>σ</b>_PSII) for <i>Symbiodinium</i> C1b-c and D1 after Day 1 and 7 of thermal stress (32<b>°</b>C). Asterisks (*) represent statistically significant differences between controls and treatments for each <i>Symbiodinium</i> type at a time point (Mann-Whitney U test; p<b><</b>0.05). For each point, n = 3–6 <b>±</b>SD.</p

Chlorophyll <i>a</i> fluorescence measurements of cultured <i>Symbiodinium</i> during the thermal stress treatment.

<p>(a.) Maximum quantum yield of PSII (F_v/F_m) and (b.) the effective absorption cross-section of PSII (<b>σ</b>_PSII) across four <i>Symbiodinium</i> phylotypes, prior to (Day 0) and during (Day 1–7) thermal stress (32<b>°</b>C). Asterisks (*) represent statistically significant values that differed from the untreated controls (One-way ANOVA; p<0.05). For each point, n = 4 ±SD.</p

Relative expression of <i>psbA</i> and <i>psaA</i> for <i>Symbiodinium</i> C1b-c and D1 during the simulated bleaching event.

<p>Each bar represents the mean expression value (logarithmic scale; <b>±</b>SE) relative to the untreated controls (28<b>°</b>C) for four biological replicates. Asterisks (*) represent values that statistically differed from the untreated controls sampled the same day (REST; p<b><</b>0.05).</p

Target gene, encoded protein, and primer sequence used for real-time PCR.

<p>F designates the forward primer; R designates the reverse primer.</p

Results of two-way ANOVAs for Mg/Ca, Sr/Ca, Ba/Ca, and U/Ca of <i>P. divaricata, P. astreoides,</i> and <i>O. faveolata</i>.

<p>The effect of temperature (Temp.) was fixed with two levels (CO  =  control 30.4°C, TR  =  treatment 31.6°C), and time was fixed with 2 levels (0, 6 weeks). When main effects (but no interaction terms) were significant, Tukey post hoc results are shown. Significant <i>p</i>-values (<i>p</i>≤0.05) are highlighted in bold. df  =  degrees of freedom, SS  =  sum of squares of the effect.</p><p>Results of two-way ANOVAs for Mg/Ca, Sr/Ca, Ba/Ca, and U/Ca of <i>P. divaricata, P. astreoides,</i> and <i>O. faveolata</i>.</p