[B-PT01] Biomineralization and Geochemistry of Proxies

[BPT01-09] Evolution of Mg transporters and their possible effects on coral skeletal proxies in geochemistry

*Tomoko Bell¹, Akira Iguchi², Kazuhiko Sakai³, Yusuke Yokoyama⁴ (1.WERI, Univ. of Guam, 2.AIST, 3.TBRC, Univ. of Ryukyus, 4.AORI, Univ. of Tokyo)

Keywords:Coral, Evolution, Skeletal proxy, Mg transporter

Corals have been used as geochemical proxies since the 1970's and they play prominent roles in paleoclimatology. However, it has not been well elucidated how minerals in seawater are transported and precipitated in aragonite coral skeleton. There are no foundational methods to differentiate and quantify biogenic and abiogenic effects during skeletal formation, thus it hampers corals from being fully reliable environmental proxies. To address these uncertainties, in this study, we attempt to evaluate biological complexity by investigating how coral genes evolved over geologic time scales (e.g. million years). In coral skeletons, Mg is an abundant element of coral skeleton (1000-2100ppm), and Mg can be concentrated by almost double that of modern seawater. Therefore, we decided to focus on coral Mg transporters of five coral species (Acropora digitifera, Acropora hyacinthus, Acropora millepora, Acropora palmate and Acropora tenuis) and investigated how Mg transporters have been evolving. Our preliminary result indicated that Mg transporter of Acropora digitifera, Acropora hyacinthus and Acropora millepora showed high similarity to Mg transporter of Japanese rice fish. On the other hand, Acropora palmate and Acropora tenuis did not show such a trend. According to the Coral Trait database, the three types of corals that showed high similarity to rice fish appeared on the earth less than 1 million years ago while the other two species showed ages older than 3 million years ago. Such difference could be possibly triggered by an environmental change such as the availability of Mg ion in seawater (seawater [Mg]). There have been continuous increases in seawater [Mg] from 5 Ma to present: from 50 mmol/L to 55 mmol/L. This increase (about 10%) of seawater [Mg] could potentially facilitate coral Mg transport systems to respond to the seawater chemistry change and evolve to develop more Mg specific transporters. Our result indicated a strong biogenic response and control (Mg transporter) to abiotic factors (seawater [Mg]). That is to say, using Mg ratio in coral skeletons might not be ideal due to vital effect. We suggest our approach to investigate gene evolution may guide us to identify the geochemical proxies with possible strong vital effects thus help us identify robust proxies.