[B-BG01] Earth and Planetary Science Frontiers for Life and Global Environment

[BBG01-03] New calcification model unveiled by comparative transcriptome analysis of Foraminifera

★Invited Papers

*Yurika Ujiie¹, Yoshiyuki Ishitani², Yukiko Nagai², Yoshihiro Takaki², Takashi Toyofuku², Shunichi Ishii² (1.Marine Core Research Institute, Kochi University, 2.Japan Agency for Marine-Earth Science and Technology)

Keywords:Calcification, transcriptome, calcium ion, carbonic anhydrase, foraminifera

Calcification has important aspects for both biology and Earth sciences. Foraminiferal calcification has been diligently worked mainly in the field of Earth sciences because they are crucial players in biogeochemical cycles and useful environmental indicators in paleoceanography. However, the molecular mechanism: transport and function of ions and enzymes, was almost shrouded in mist except for the studies with fluorescent probes showing the presence of calcium-ion (Ca²⁺) and high pH vesicles in the foraminiferal cytoplasm during calcification. The other molecular analyses of multicellular organisms (e.g., mollusks and corals) have revealed distinct calcification-related genes, though it is how Ca²⁺ and inorganic carbon (CO₂ and bicarbonate: HCO₃⁻) may be rerouted from other metabolic functions toward calcification. Here we paid attention to the calcification process of foraminifers that regulate all metabolisms within a single cell and have periodic interval between calcification and non-calcification (intermediate) stages. The single-cell transcriptomic comparison between two stages is capable to identify high expression genes, which are predominantly working during calcification. We found Ca²⁺ transport/secretion genes and α-carbonic anhydrases as main controller for foraminiferal calcification. Foraminifers have the well-coordinated Ca²⁺ trafficking system to boost mitochondrial adenosine triphosphate synthesis for maintain of their large sized cell. The secretion of Ca²⁺ toward calcification site has a role to take a balance of intracellular concentration of Ca²⁺. Moreover, foraminiferal unique α-carbonic anhydrase genes induce the generation of bicarbonate and proton from multiple CO₂ sources. In the process of bicarbonate generation, proton vesicles are released via exocytosis, lowing the pH of ambient seawater. These mechanisms have evolved independently since the Precambrian. Eukaryotic calcification is observed in many marine clades, and it is generally thought that the primary function of calcification is to provide structural complexity, protection from predators, light refraction, and/or assistance during grazing. The independent evolution of both Ca²⁺-related and carbonic anhydrase genes indicates that foraminiferal calcification appeared essential for sustaining the growth of outsized cells as a different calcification model from others. This organism can consequently survive global changes along with lowering Ca²⁺concentration and pH in seawater. Future studies will be expected to examine the expression changes of the key genes, which regulate calcification, associating with environmental conditions.