Abstract
The effects of ocean acidification and elevated seawater temperature on coral calcification and photosynthesis have been extensively investigated over the last two decades, whereas they are still unknown on nutrient uptake, despite their importance for coral energetics. We therefore studied the separate and combined impacts of increases in temperature and pCO2 on phosphate, ammonium, and nitrate uptake rates by the scleractinian coral S. pistillata. Three experiments were performed, during 10 days i) at three pHT conditions (8.1, 7.8, and 7.5) and normal temperature (26°C), ii) at three temperature conditions (26°, 29°C, and 33°C) and normal pHT (8.1), and iii) at three pHT conditions (8.1, 7.8, and 7.5) and elevated temperature (33°C). After 10 days of incubation, corals had not bleached, as protein, chlorophyll, and zooxanthellae contents were the same in all treatments. However, photosynthetic rates significantly decreased at 33°C, and were further reduced for the pHT 7.5. The photosynthetic efficiency of PSII was only decreased by elevated temperature. Nutrient uptake rates were not affected by a change in pH alone. Conversely, elevated temperature (33°C) alone induced an increase in phosphate uptake but a severe decrease in nitrate and ammonium uptake rates, even leading to a release of nitrogen into seawater. Combination of high temperature (33°C) and low pHT (7.5) resulted in a significant decrease in phosphate and nitrate uptake rates compared to control corals (26°C, pHT = 8.1). These results indicate that both inorganic nitrogen and phosphorus metabolism may be negatively affected by the cumulative effects of ocean warming and acidification.
Highlights
Ocean acidification is the result of anthropogenic carbon dioxide (CO2) emissions partially dissolving into seawater and progressively declining its pH [1]: over the 20th century, the oceans’ average pHT has decreased by 0.1 unit from 8.21 to 8.10 [2,3], and it is predicted to further decrease by 0.3-0.5 unit by the end of this century [2,4]
Colonies were cultured at the Centre Scientifique de Monaco (CSM), under controlled conditions (26uC, salinity of 38). 1.5 month before the start of the experiment, the ca. 220 nubbins used during the experiment were prepared by cutting branches (2.561.0 cm long and 0.660.3 cm in diameter) of about twenty parent colonies with pliers after Tambutteet al. [49]
After 10 days of incubation, there was no effect of either pH, temperature, or both on the areal content of zooxanthellae (Fig. 1a,d,g), chlorophyll (Fig. 1b,e,h), and protein (Fig. 1c,f,i), nor on the rates of respiration normalized to the chlorophyll content (Fig. 2; 1-way and 2-ways ANOVA, all p.0.05)
Summary
Ocean acidification is the result of anthropogenic carbon dioxide (CO2) emissions partially dissolving into seawater and progressively declining its pH [1]: over the 20th century, the oceans’ average pHT (total scale) has decreased by 0.1 unit from 8.21 to 8.10 [2,3], and it is predicted to further decrease by 0.3-0.5 unit by the end of this century [2,4]. Corals have for instance adapted to their oligotrophic environment by developing a symbiosis with dinoflagellates of the genus Symbiodinium, commonly called zooxanthellae These symbionts largely contribute to the nutrition of their animal host by providing 1) photosynthesis-derived carbon to the animal tissue [16]; and 2) essential nutrients, such as nitrogen and phosphorus, either directly taken up from the external environment or recycled from the animal wastes [17]. It has been calculated that uptake of inorganic nitrogen, at natural concentrations, contributes approximately 30% to the daily nitrogen requirement of the species Acropora palmata for gamete and mucus production, growth, and tissue repair [19] In another coral species, Pocillopora damicornis, uptake of ammonium could even completely satisfy the nitrogen demand of this coral at field concentrations [20]. Provision of nutrient to corals is important for the entire reef, because healthy corals sustain a high reef biomass production
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