[B-CG07] 微化石生物学の最前線

[BCG07-P09] The gene expression of plasma membrane Ca²⁺ ATPase represents a “vital effect” on foraminiferal Mg/Ca temperature

*氏家 由利香¹、吉村 寿紘²、石谷 佳之³、ウラノバ ダナ⁴、遠藤 博寿⁵、牛久保 孝行⁶ (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、6.海洋研究開発機構　高知コア研究所)

キーワード：Mg/Ca水温計、底生および浮遊性有孔虫、カルシウムイオン関連遺伝子、遺伝子発現

Ocean temperature is the primary driver for the global environment and its change provides an important clue to understand the mechanism of global climate change for its future prediction. The past ocean temperature has been examined by geochemical analyses of calcium carbonate (CaCO₃) tests of foraminifers, because these biominerals are precipitated under the effect of physicochemical conditions, such as the temperature and salinity of ambient seawater. Rotaliida foraminifers have the wide distribution in both sea-surface and deep oceans due to two lifestyles: benthic and planktic. With such aspect, the magnesium/calcium (Mg/Ca) ratio of their tests has been frequently used to reconstruct major variations of ocean temperature through geological time. However, this indicator involves the fundamental problem that the mechanism of Mg incorporation into foraminiferal carbonate and its relationship with temperature remains unclear. Mg concentration in foraminiferal calcite is much lower than that in inorganic calcites precipitated in seawater. Such nonequilibrium state of Mg/Ca ratio between foraminiferal calcites and the external environment has hypothesized a biological control called as “vital effect” that foraminifers actively exclude Mg during calcification after its uptake from the ambient seawater. The latest transcriptome study on benthic species (Ammonia) has found the exact opposite mechanism that is active excretion of Ca²⁺ through the membrane transporter (PMCA: P-type Ca²⁺transporter) toward the calcification site as a part of the Ca²⁺ trafficking system (Ujiie et al., 2023). Given that the expression level of PMCA is associated with temperature, the exclusion of Ca²⁺ could be altered resulting in changes of the denominator calcium for Mg/Ca ratio of foraminiferal tests. Here we first confirmed the calcification process of a planktic foraminifer via single-cell transcriptome analyses. We further examined the temperature dependences of the PMCA gene expression in both a clonal-cultured benthic foraminifer (Ammonia veneta) and a wild planktic foraminifer (Trilobatus sacculifer) and compared it with Mg/Ca ratio of tests measured by a femtosecond LA-ICP-MS. Most genes, in particular Ca²⁺-related genes, were commonly expressed in a planktic foraminifer as same as in a benthic foraminifer Ammonia. The real-time qPCR (RT qPCR) analysis of PMCA with single-cell samples showed a negative correlation between the expression level of PMCA gene and temperature. Contrary, Mg/Ca ratio based on chamber-by-chamber measurement had a positive correlation with temperature. The expression level of PMCA gene acts to control the exclusion of Ca²⁺, which is used to secrete foraminiferal CaCO₃ tests, as the key for “vital effect”.

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