Thermal stress affects organism performance differently depending on the ambient temperature to which they are acclimatized, which varies along latitudinal gradients. This study investigated whether differences in physiological responses to temperature are consistent with regional differences in temperature regimes for the stony coral Oculina patagonica. To resolve this question, we experimentally assessed how colonies originating from four different locations characterized by >3 °C variation in mean maximum annual temperature responded to warming from 20 to 32 °C. We assessed plasticity in symbiont identity, density, and photosynthetic properties, together with changes in host tissue biomass. Results show that, without changes in the type of symbiont hosted by coral colonies, O. patagonica has limited capacity to acclimatize to future warming. We found little evidence of variation in overall thermal tolerance, or in thermal optima, in response to spatial variation in ambient temperature. Given that the invader O. patagonica is a relatively new member of the Mediterranean coral fauna, our results also suggest that coral populations may need to remain isolated for a long period of time for thermal adaptation to potentially take place. Our study indicates that for O. patagonica, mortality associated with thermal stress manifests primarily through tissue breakdown under moderate but prolonged warming (which does not impair symbiont photosynthesis and, therefore, does not lead to bleaching). Consequently, projected global warming is likely to cause repeat incidents of partial and whole colony mortality and might drive a gradual range contraction of Mediterranean corals.
The physiological response of the scleractinian coral Turbinaria reniformis to ammonium enrichment (3 ?mol l(-1)) was examined at 26°C as well as during a 7 day increase in temperature to 31°C (thermal stress). At 26°C, ammonium supplementation had little effect on the coral physiology. It induced a decrease in symbiont density, compensated by an increase in chlorophyll content per symbiont cell. Organic carbon release was reduced, likely because of a better utilization of the photosynthesized carbon (i.e. incorporation into proteins, kept in the coral tissue). The ?(15)N signatures of the ammonium-enriched symbionts and host tissue were also significantly decreased, by 4 and 2‰, respectively, compared with the non-enriched conditions, suggesting a significant uptake of inorganic nitrogen by the holobiont. Under thermal stress, coral colonies that were not nitrogen enriched experienced a drastic decrease in photosynthetic and photoprotective pigments (chlorophyll a, ?-carotene, diadinoxanthin, diatoxanthin and peridinin), followed by a decrease in the rates of photosynthesis and calcification. Organic carbon release was not affected by this thermal stress. Conversely, nitrogen-enriched corals showed an increase in their pigment concentrations, and maintained rates of photosynthesis and calcification at ca. 60% and 100% of those measured under control conditions, respectively. However, these corals lost more organic carbon into the environment. Overall, these results indicate that inorganic nitrogen availability can be important to determining the resilience of some scleractinian coral species to thermal stress, and can have a function equivalent to that of heterotrophic feeding concerning the maintenance of coral metabolism under stress conditions.
Temperate symbiotic corals, such as the Mediterranean species Cladocora caespitosa, live in seasonally changing environments, where irradiance can be ten times higher in summer than winter. These corals shift from autotrophy in summer to heterotrophy in winter in response to light limitation of the symbionts photosynthesis. In this study, we determined the autotrophic carbon budget under different conditions of irradiance (20 and 120 µmol photons m(-2) s(-1)) and feeding (fed three times a week with Artemia salina nauplii, and unfed). Corals were incubated in H(13)CO(3) (-)-enriched seawater, and the fate of (13)C was followed in the symbionts and the host tissue. The total amount of carbon fixed by photosynthesis and translocated was significantly higher at high than low irradiance (ca. 13 versus 2.5-4.5 µg cm(-2) h(-1)), because the rates of photosynthesis and carbon fixation were also higher. However, the percent of carbon translocation was similar under the two irradiances, and reached more than 70% of the total fixed carbon. Host feeding induced a decrease in the percentage of carbon translocated under low irradiance (from 70 to 53%), and also a decrease in the rates of carbon translocation per symbiont cell under both irradiances. The fate of autotrophic and heterotrophic carbon differed according to irradiance. At low irradiance, autotrophic carbon was mostly respired by the host and the symbionts, and heterotrophic feeding led to an increase in host biomass. Under high irradiance, autotrophic carbon was both respired and released as particulate and dissolved organic carbon, and heterotrophic feeding led to an increase in host biomass and symbiont concentration. Overall, the maintenance of high symbiont concentration and high percentage of carbon translocation under low irradiance allow this coral species to optimize its autotrophic carbon acquisition, when irradiance conditions are not favourable to photosynthesis.
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