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In shallow tropical waters, ultraviolet radiation (UVR) occurs at high intensity simultaneously with high water temperatures, and both stressors are predicted to increase in the future and to have a major impact on reef coral survival. The poor knowledge of the interactive effects of those two factors, however, prevents a good estimation of the coral resistance to climate change pressure. The results obtained in this study on two genetically distant scleractinian coral species, Pocillopora damicornis and Turbinaria reniformis, highlight an interaction of temperature and UVR on the corals' physiology as well as a species-specific response. Overall, increase in UVR level exacerbated the negative impact of thermal stress. Whereas stressors did not reduce the autotrophic capacity (symbiont density, net photosynthetic rates) of T. reniformis, they significantly lowered it in P. damicornis, which initially contained twice more symbionts in its tissue. Only grazing rates (heterotrophy) were significantly decreased in T. reniformis under thermal stress. For both corals, calcification was slowed by the combination of thermal and UVR stress. Calcification was impaired likely due to a decrease in autotrophic energy supply in P. damicornis, and in heterotrophic energy supply in T. reniformis. This study confirms that the response of corals to global change needs to be studied using multifactorial approaches and a combination of different environmental factors.

Carbon and nitrogen isotope ratios (d13C and d15N) were determined in the zooxanthellae, host tissue, and whole symbiotic association of the symbiotic temperate coral Cladocora caespitosa, as well as in the different components of the coral’s potential food sources (plankton, particulate organic matter in seawater and in the sediment). Data were collected both in winter and summer at three different locations, to assess the extent of auto and heterotrophy in this species and get a better understanding of the functioning of temperate symbioses. There was a marked seasonal difference in the signature of the zooxanthellae and host tissue, highlighting two clear feeding patterns. In summer, d13C signatures of the coral host and the zooxanthellae were similar (217%) and very different from the signature of the food sources (from 221% to 225%), suggesting that corals were relying on autotrophy for the acquisition of carbon. d15N values also suggested that nitrogen was not acquired through feeding. Conversely, in winter, the d13C signature of the host decreased by ca. 223% to 228%, and was more comparable to the signature of the external food sources (between 224% and 225%), suggesting a substantial reliance of C. caespitosa on external food sources during this season. There was also a 3%enrichment between the d15N signatures of the food (4–5%) and the signature of the symbiotic association (7–8%), suggesting that nitrogen was also acquired through feeding. Overall, these results give evidence that C. caespitosa and temperate corals in general derive a large fraction of their energy from heterotrophic feeding in winter.

A general decline in coral calcification has been observed in response to ocean acidification. It has recently been shown that heterotrophic feeding (the acquisition of organic nutrients) enables one massive coral species to maintain its calcification rates while exposed to ocean acidification but the consequences of higher seawater partial pressure of CO2 (P-CO2) on coral feeding capacities themselves have never been investigated. Exposing long-term acclimated starved and fed colonies of the major reef builder Stylophora pistillata to either ambient pH(T) (8.06 +/- 0.05; 417 Pa P-CO2) or low pH(T) (7.51 +/- 0.06; 1697 Pa P-CO2) showed that higher feeding rates can counterbalance the negative effects of seawater acidification on coral calcification. Indeed, relative to unfed corals under ambient pH(T) conditions, calcification rates in unfed corals decreased by 53.6% but remained constant in fed corals under high P-CO2 conditions. Results also clearly show that acidification affects the feeding capacity and feeding effort of the branching coral S. pistillata. Organic nutrient acquisition was severely reduced under high P-CO2, with a simultaneous decrease in the dissolved free amino acid uptake rates, the alkaline phosphatase activities, and the feeding rates on Artemia salina nauplii. Considering that all these processes are needed to fulfill the nutrient needs of the symbiotic association, on a long-term basis, lower feeding rates will likely aggravate the already fragile physiological state of many corals under warmer, more acidic conditions.

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We show here that CO2 partial pressure (pCO(2)) and temperature significantly interact on coral physiology. The effects of increased pCO(2) and temperature on photosynthesis, respiration and calcification rates were investigated in the scleractinian coral Stylophora pistillata. Cuttings were exposed to temperatures of 25degreesC or 28degreesC and to pCO(2) values of ca. 460 or 760 muatm for 5 weeks. The contents of chlorophyll c(2) and protein remained constant throughout the experiment, while the chlorophyll a content was significantly affected by temperature, and was higher under the `high-temperature-high-pCO(2)' condition. The cell-specific density was higher at `high pCO(2)' than at `normal pCO(2)' (1.7 vs. 1.4). The net photosynthesis normalized per unit protein was affected by both temperature and pCO(2), whereas respiration was not affected by the treatments. Calcification decreased by 50% when temperature and pCO(2) were both elevated. Calcification under normal temperature did not change in response to an increased pCO(2). This is not in agreement with numerous published papers that describe a negative relationship between marine calcification and CO2. The confounding effect of temperature has the potential to explain a large portion of the variability of the relationship between calcification and pCO(2) reported in the literature, and warrants a re-evaluation of the projected decrease of marine calcification by the year 2100.

This paper presents the results of an effort to label calcium carbonates formed by marine organisms with stable isotopes to obtain information about the biomineralization processes. The growing skeleton of the scleractinian coral Porites porites was labeled three times with enhanced abundances of Sr-86. The distribution of Sr-86 in the skeleton was imaged with the NanoSIMS ion microprobe with a spatial resolution of similar to 200 nm and combined with images of the skeletal ultra-structure. Importantly, the distribution of the Sr-86 label in the P. porites skeleton was found to be strongly heterogeneous. This constrains the physical dimensions of the hypothetical Extracellular Calcifying Fluid (ECF) reservoir at the surface of the growing skeleton, which is implicit in most geochemical models for coral biomineralization. These new experimental capabilities allow for a much more detailed view of the growth dynamics for a wide range of marine organisms that biomineralize carbonate structures. Citation: Houlbreque, F., A. Meibom, J.-P. Cuif, J. Stolarski, Y. Marrocchi, C. Ferrier-Pages, I. Domart-Coulon, and R. B. Dunbar ( 2009), Strontium-86 labeling experiments show spatially heterogeneous skeletal formation in the scleractinian coral Porites porites, Geophys. Res. Lett., 36, L04604, doi: 10.1029/2008GL036782.

The intensity and frequency of coral bleaching events have increased worldwide especially due to thermal stress and seawater pollution. Although it has been observed that metal concentration in seawater can affect the coral’s ability to adopt cellular defensive mechanisms to counteract bleaching, more investigations are needed to better understand the role of metals in coral physiology. In this study, we analyzed the individual and combined effects of prolonged heat stress and manganese (Mn) and iron (Fe) enrichments at the cellular level on the coralStylophora pistillata. Thermal stress caused an up-regulation in the expression of the host Heat shock proteins (Hsps) 60 and 70, which showed a parallel pattern of modulation in all treatments, as well as an increase of lipid peroxidation (LPO) in the holobiont. Fe enrichment did not induce any change in Hsp expression or in the oxidative status of the corals both at the ambient temperature of 26°C or at increased temperature, suggesting that Fe didn’t seem to play a role in mitigating the cellular damages and the coral bleaching. Mn or MnFe enrichment at 26°C seemed to increase the oxidative stress in zooxanthellae, since high LPO and glutathione reductase (GR) levels were recorded, but it did not cause any effect on polyp Hsp expression, probably due to the antioxidant action of GR. With the temperature increase, Mn enrichments prevented any increase in Hsp levels and caused a significant decrease of LPO and GR activity, strengthening a previous hypothesis suggesting that Mn could mitigate the negative cellular effects produced by the thermal stress.