11:15 〜 11:30
[BCG05-03] Carbonic anhydrase in Foraminifera: Perspective on one of major role of ‘vital effect’
キーワード：炭酸脱水酵素、石灰化、安定酸素・炭素同位体
Foraminifera account for ~25% of calcium carbonate (CaCO3) production in global carbon cycle, owing to their abundance in both seafloor and pelagic environments. Rotaliida, the major foraminiferal order in the class Globothalamea, has been widely used in geological studies as a proxy for modern and paleo-environments because the chemical and isotopic composition of its CaCO3 tests (i.e., stable oxygen and carbon isotopes: d18O, and d13C) is influenced by temperature and salinity of ambient seawater. However, the non-equilibrium state of chemical compounds between foraminiferal tests and the external environment poses a problem for such measurements due to ‘vital effect’: physiology and diet of organisms dictat the uptake of inorganic carbon along with environments. To date, the mechanism of this vital effect has been masked because it is unclear how inorganic carbon may be rerouted from other metabolic functions toward calcification. Here we conducted comparative transcriptomics between calcification and non-calcifying stages of Rotaliida foraminifer to identify calcification-related genes and reconstructed their metabolic pathway concerning calcification.
Carbonic anhydrase (CA) and aquaporin 4 (AQP4) genes showed significant expression during calcification in the transcriptomics. The enzyme CA catalyzes the interconversion of H2O + CO2 to HCO3- + H+. The integral membrane protein AQP4 allows the passage of water molecules through the cell membrane and supplies H2O for further HCO3-production around the cell membrane. CO2 can be diffused from seawater as well as calcification sites and produced during metabolic processes. The CA and AQP4 expressions imply the intracellular generation of HCO3-. The phylogenetic analysis of CAs indicated that Rotaliida foraminifer has two out of five CA families: α-CA and b-CA members. α-CAs are involved in CO2/HCO3- interconversion and calcification in mollusks, sea urchins, and corals. In mollusks, calcification-related α-CAs contain acidic low-complexity regions (LCRs), which could bind Ca2+ leading to CaCO3 nucleation. While, Rotaliida’s α-CAs formed two clades, one of which (Rotaliida-αCA1) is a novel independent clade from metazoans and some has acidic LCRs. Rotaliida-αCA1 seems to catalyze CO2/HCO3- interconversion which is related to foraminiferal calcification. Moreover, our in silico protein motif predictions indicated that Rotaliida-αCA1 comprised both secretory α-CAs and intracellular α-CAs. The former is likely involved in HCO3- generation and calcium-binding at the extracellular calcification site. The latter does not interact with Ca2+ as they lack acidic LCRs; hence, they likely participate in HCO3- production in the cytosol. These different distributions of α-CA enable the utilization of CO2 derived from both extracellular and intracellular origins. The multiple sources, in particular those of CO2, serve as a physiological control mechanism for d18O and d13C in foraminiferal tests via HCO3- production. Such physiology could be partly responsible for the non-equilibrium state of d18O and d13C between foraminiferal tests and the environment observed in geochemical studies.
Carbonic anhydrase (CA) and aquaporin 4 (AQP4) genes showed significant expression during calcification in the transcriptomics. The enzyme CA catalyzes the interconversion of H2O + CO2 to HCO3- + H+. The integral membrane protein AQP4 allows the passage of water molecules through the cell membrane and supplies H2O for further HCO3-production around the cell membrane. CO2 can be diffused from seawater as well as calcification sites and produced during metabolic processes. The CA and AQP4 expressions imply the intracellular generation of HCO3-. The phylogenetic analysis of CAs indicated that Rotaliida foraminifer has two out of five CA families: α-CA and b-CA members. α-CAs are involved in CO2/HCO3- interconversion and calcification in mollusks, sea urchins, and corals. In mollusks, calcification-related α-CAs contain acidic low-complexity regions (LCRs), which could bind Ca2+ leading to CaCO3 nucleation. While, Rotaliida’s α-CAs formed two clades, one of which (Rotaliida-αCA1) is a novel independent clade from metazoans and some has acidic LCRs. Rotaliida-αCA1 seems to catalyze CO2/HCO3- interconversion which is related to foraminiferal calcification. Moreover, our in silico protein motif predictions indicated that Rotaliida-αCA1 comprised both secretory α-CAs and intracellular α-CAs. The former is likely involved in HCO3- generation and calcium-binding at the extracellular calcification site. The latter does not interact with Ca2+ as they lack acidic LCRs; hence, they likely participate in HCO3- production in the cytosol. These different distributions of α-CA enable the utilization of CO2 derived from both extracellular and intracellular origins. The multiple sources, in particular those of CO2, serve as a physiological control mechanism for d18O and d13C in foraminiferal tests via HCO3- production. Such physiology could be partly responsible for the non-equilibrium state of d18O and d13C between foraminiferal tests and the environment observed in geochemical studies.