Date Published: April 7, 2014
Publisher: Public Library of Science
Author(s): Yasmin Gabay, Maoz Fine, Zahava Barkay, Yehuda Benayahu, John Murray Roberts.
Increase in anthropogenic pCO2 alters seawater chemistry and could lead to reduced calcification or skeleton dissolution of calcifiers and thereby weaken coral-reef structure. Studies have suggested that the complex and diverse responses in stony coral growth and calcification, as a result of elevated pCO2, can be explained by the extent to which their soft tissues cover the underlying skeleton. This study compared the effects of decreased pH on the microstructural features of both in hospite (within the colony) and isolated sclerites (in the absence of tissue protection) of the zooxanthellate reef-dwelling octocoral Ovabunda macrospiculata. Colonies and isolated sclerites were maintained under normal (8.2) and reduced (7.6 and 7.3) pH conditions for up to 42 days. Both in hospite and isolated sclerites were then examined under SEM and ESEM microscopy in order to detect any microstructural changes. No differences were found in the microstructure of the in hospite sclerites between the control and the pH treatments. In stark contrast, the isolated sclerites revealed dissolution damage related to the acidity of the water. These findings suggest a protective role of the octocoral tissue against adverse pH conditions, thus maintaining them unharmed at high pCO2. In light of the competition for space with the less resilient reef calcifiers, octocorals may thus have a significant advantage under greater than normal acidic conditions.
Ocean acidification, the continuing global decline in oceanic pH resulting from rising atmospheric carbon dioxide (CO2), also reduces carbonate ion concentration ([CO32−]) and saturation state (Ω), which are essential components of the CaCO3 mineral from which marine calcifiers build their shells and skeletons –. The effect of ocean acidification on coral reefs, in particular on stony corals (Scleractinia), the main reef framework-builders, has been extensively studied. Several species exhibited a significantly decreased skeletal growth under high CO2 concentrations, including Stylophora pistillata–, Acropora sp. –, Montipora capitata and Porites compressa–, Oculina patagonica and Madracis pharencis, as well as M. auretenra. A species-specific response of stony corals to lower pH has often been explained by the extent to which the living tissue covers the underlying skeleton. For example, under lower pH Cladocora caespitosa, which features large areas of exposed skeleton, underwent skeletal dissolution, while the fully tissue-covered Balanophyllia europea did not exhibit any signs of dissolution . Rodolfo-Metalpa et al.  highlighted the role played by an external organic layer as a protective barrier against the harmful effects of lower pH, and demonstrated that dead colonies of the bryozoan Myriapora truncata and mollusk shells underwent dissolution at high CO2 levels, as opposed to the respective live specimens, which maintained the same net calcification rate as that occurring under normal conditions.
The current study examined the effects of reduced pH on the microstructure of in hospite sclerites vs. isolated ones of the reef-dwelling octocoral O. macrospiculata. It was found that the former had remained intact (Fig. 1), while the latter had undergone a remarkable dissolution (Figs. 2–5). These findings thus suggest a possible protective role of the octocoral tissue against corrosion of in hospite sclerites under acidic conditions. Research conducted to date has revealed various effects, mostly negative, of reduced pH on external calcite-composed skeleton of marine organisms . The responses of scleractinian corals to ocean acidification would seem to range from total skeleton dissolution to increased calcification rate. For example, Oculina patagonica and primary polyps of Favia fragum exhibited substantial skeleton dissolution under such conditions. Coccolithophores  and crustaceans  exhibited increased calcification. Rodolfo-Metalpa et al.  demonstrated that the skeleton of dead colonies of the bryozoan Myriapora truncata dissolved at pH 7.66, whereas its living colonies continued to maintain the same net calcification rate under similar conditions. Recent studies on the Mediterranean red coral Corallium rubrum have contended that the shape of in hospite sclerites, grown under low pH conditions, significantly differed from that of the control , . These results are in stark contrast to our findings and might be explained by the relatively thin tissue of C. rubrum, compared to the fleshy colonies of O. macrospiculata. The sclerites of C. rubrum are located near the colony surface, in contrast to those of O. macrospiculata, which are mostly embedded in the thick fleshy coenenchyme. It should be noted that the O. macroscpicuta sclerites demonstrated a repetitive pattern of damage, which was also quantified (this study: Figs. 3–5), whereas the studies on C. rubrum lack such data (: Fig. S3; : Figs. 3, 4). Nonetheless, a comparison between different octocoral species may further demonstrate the protective role played by the tissue as a barrier against such damage, and highlights the difference between organisms in the extant of their tissue protection. The results of our earlier study had revealed that several biological features of O. macrospiculata were not affected by the reduced seawater pH, thus suggesting the protective role of the living tissue . Those findings are in congruence with the current ones, and further imply that the fleshy tissues of octocorals may act as a barrier that maintains a stable internal pH condition, thus preventing adverse effects on their internally-located sclerites. Undoubtedly, further studies are needed in order to elucidate the mechanisms of acid-based regulation in octocorals.