[B-PT02] バイオミネラリゼーションと古環境プロキシー

[BPT02-08] What is a “Black-box” in Mg/Ca temperature proxy in Foraminifera?

*氏家 由利香¹、吉村 寿紘²、石谷 佳之³、三浦 功暉¹、ウラノバ ダナ⁴、遠藤 博寿⁵ (1.高知大学海洋コア国際研究所、2.海洋研究開発機構　海洋機能利用部門、3.海洋研究開発機構　超先鋭研究開発部門、4.高知大学農林海洋科学部、5.鶴岡工業高等専門学校)

キーワード：カルシウムイオン、膜輸送体、チャンバー別測定、水温

Magnesium/calcium (Mg/Ca) ratio derived from foraminiferal tests has been largely used to estimate seawater temperature in geological time. The tests of the order Rotaliida, as the most common organisms for Mg/Ca temperature estimation, contain few amount of Mg in their calcium carbonate (CaCO₃) component, though Mg²⁺ is highly contained in seawater as well as inorganic carbonate. Therefore, foraminifers had been thought to have a Mg²⁺ exclusion system during calcification. However, our latest transcriptome study has found the exact opposite mechanism that is active excretion of Ca²⁺ through the membrane transporter toward the calcification site as a part of the Ca²⁺ trafficking system (Ujiie et al., 2023). Thus, foraminifers take less Mg²⁺ from seawater and do not expel them during calcification. These findings suggest that Mg/Ca ratio of foraminiferal tests are largely associated with excretion of Ca²⁺, that depends on “denominator effect”. Here, we focused on the relationship between the expression level of a Ca²⁺ membrane transporter gene and Mg/Ca ratio.
In the present study, we cultured benthic foraminiferal strain, Ammonia veneta, at three different temperatures: 18, 23, and 26 °C. The specimens, which were clearly forming a new chamber, meaning calcification, were used to measure the expression level of P-type Ca²⁺ transporter (PMCA) gene based on single-cell qPCR method. We also measured Mg and Ca of newly formed chambers via femtosecond LA-ICP-MS. The expression level of PMCA varied due to individual variations. Such expression level was normalized with other genes, which are regularly expressed both in calcification and non-calcification states. The ratio of expression level between PMCA and regularly expressed genes clearly demonstrated that the PMCA expression level fluctuated according to temperature: it decreased with a temperature rise. On the other hand, Mg/Ca ratio based on precise measurement of trace elements at chamber showed temperature differences with increasing temperature. Our findings indicate, when the expression level of PMCA decreases at high temperature, Ca²⁺excretion can be regulated leading to increase of Mg/Ca ratio in test. Thus, the expression level of PMCA is associated to amounts of calcium component in CaCO₃ test. Foraminiferal Mg/Ca ratio is likely attributed to the activity of calcium-related genes depending on temperature.

Ujiie, Y., Ishitani, Y., Nagai, Y., et al. (2023) Unique evolution of foraminiferal calcification to survive global changes. Science Advances, 9(25). DOI:10.1126/sciadv.add3584