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[AHW24-P09] Effects of the pumice pebbles on the survival rate of juvenile corals especially focusing on pH and concentrations of metal ions dissolved into seawater
Keywords:pumice, coral
The culture seawater was changed every 3-4 days during the experimental period of 12 days. Dry pumice pebbles weighing 0.5, 1, 5 and 10 g were prepared and floated in 200 mL polypropylene containers. The culture medium for coral polyps was seawater, which mimics the ambient conditions. The pH of the seawater and concentrations of phosphate phosphorus (PO4-P) and dissolved metal ions were measured. An F-72S (Horiba) pH meter was used and PO4-P was measured using the molybdenum blue method. The dissolved metal ions were estimated using an inductively coupled plasma mass spectrometer (ICP-MS; iQAP-Q, Thermo Scientific) after pretreatment with a solid-phase extraction kit (3MTM MporeTM Disk Chelate, GL Science).
To compare the solubilities of substrates from the pumice pebbles between seawater and freshwater, 30 g of the pumice pebbles were treated with either of the two waters. The survival rate decreased with increasing amounts of the pumice pebbles. Notably, none of the polyps survived for three days when 10 g of the pumice pebbles was floated in 200 mL of seawater. Seawater acidification was observed in the seawater containing the pumice pebbles in a dose-dependent manner, While the pH of the seawater (200 mL) did not change significantly when 0.5 or 1 g of the pumice pebbles was added, it dropped to 7.8 and 7.7 when 5 g and 10 g were added, respectively. Moreover, Cu, Zn, Al, Mn ions were detected in the seawater containing the pumice pebbles. These results indicate that some metal ions (e.g., Mn2+) dissolved from the pumice pebbles decreased the seawater pH. Moreover, it is well known that such concentrations of Cu and Zn ions are toxic to aquatic organisms, suggesting the reason why the coral polyps could not survive in the seawater containing the pumice pebbles. It is likely that when the pumice pebbles sink into the seawater, their air-filled spaces on float are replaced by the seawater, thus releasing metal ions originally on the surface of the pumice pebbles into the seawater. Because the pumice pebbles are expected to be deposited on the seafloor over a long period of time, it is necessary to monitor the composition of the seawater containing the pumice pebbles as well as to examine their biological impact.