11:00 〜 13:00
[AHW24-P09] 2021年海底火山福徳岡ノ場火山噴火で発生した軽石によるpH変化と海水中の溶存金属イオンの濃度が稚サンゴの生存率に及ぼす影響
キーワード：軽石、サンゴ
An unexpectedly large submarine eruption occurred between 13-15 August 2021 at the Fukutoku-Oka-no-Ba submarine volcano, located approximately 60 km south of the Iwo Jima Island in the Ogasawara Islands, and large amounts of pumice were generated. Pumice rafts were carried by ocean currents and drifted to the southwestern islands in the Japanese Archipelago, approximately 1300 km away from the original place, after a period of two months. The pumice rafts covered the seashore and affected the fishing industry and ferry services on the southwestern islands. However, the effect of the pumice pebbles on local, tropical and subtropical ecosystems has not yet been investigated. In this study, we report whether or not pumice pebbles affect the marine organisms in the region where the pumice rafts drifted using our laboratory-based coral culture system. Pumice pebbles were collected from Yoron Island, Kagoshima Prefecture, and the main island of Okinawa in November 2021 where a large size of pumice rafts had drifted. Juvenile coral polyps (Acropora digitifera) were used in this study. As an experimental set-up, 10–13 coral polyps from bundles spawned in August 2021 were settled to start to grow on glass-based dishes and planulae were cultured for a couple of months in our laboratory.
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.
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.