Japan Geoscience Union Meeting 2022

Presentation information

[E] Oral

B (Biogeosciences ) » B-PT Paleontology

[B-PT03] Biomineralization and Geochemistry of Proxies

Wed. May 25, 2022 3:30 PM - 5:00 PM 303 (International Conference Hall, Makuhari Messe)

convener:Takashi Toyofuku(Japan Agency for Marine-Earth Science and Technology (JAMSTEC)), convener:Hiroshi Kitazato(Tokyo University of Marine Science and Technology (TUMSAT)), Jelle Bijma(Alfred-Wegener-Institut Helmholtz-Zentrum für Polar- und Meeresforschung), convener:Kotaro Hirose(Faculty of Science and Engineering, Waseda University), Chairperson:Takashi Toyofuku(Japan Agency for Marine-Earth Science and Technology (JAMSTEC)), Kotaro Hirose(Faculty of Science and Engineering, Waseda University), Hiroshi Kitazato(Tokyo University of Marine Science and Technology (TUMSAT)), Jelle Bijma(Alfred-Wegener-Institut Helmholtz-Zentrum für Polar- und Meeresforschung)

4:08 PM - 4:23 PM

[BPT03-03] Photosynthetic rates in planktonic foraminiferal symbiosis: Comparison of two measures and its implications for carbon sources for photosynthesis

*Haruka TAKAGI1, Katsunori KIMOTO2, Tetsuichi FUJIKI2 (1.Chiba University, 2.JAMSTEC)

Keywords:Planktonic foraminifera, Photosymbiosis, Photosynthesis, Fast repetition rate fluorometry, Carbon assimilation rate

Photosymbiosis is one of the conspicuous features in modern planktonic foraminifera. Since the number of symbiont cells in a single host has been reported to be several thousand or more, photosynthesis by photosymbiosis can be regarded as a hotspot for primary production, especially in oligotrophic oceans. The microenvironment surrounding photosymbiotic foraminifera, where calcification takes place, is greatly affected by rapid biological activities such as photosynthesis and respiration. Therefore, information on the photosynthetic activities of the symbionts is essential for interpreting geochemical proxies, such as δ13C, recorded in foraminiferal tests. Recently, active chlorophyll fluorometry has been adopted as a useful tool for immediate estimation of photosynthetic activity. However, the only direct indicator to understand photosynthetic carbon flux is the carbon assimilation rate. Therefore, the relationship between photosynthetic rate based on active chlorophyll fluorescence (electron transport rate, ETR) and carbon assimilation rate (P) needs to be confirmed before using the fluorescence methods to understand carbon dynamics in foraminiferal symbiosis. Here, we compared these two rates for two species, Trilobatus sacculifer and Globigerinella siphonifera Type II, using 14C tracer experiments and active fluorescence measurements using the fast repetition rate fluorometry. The results showed a significant positive correlation between P and ETR of the two species, indicating that carbon assimilation can be estimated by the fluorescence method. However, the regression slope representing the apparent electron demand for carbon assimilation (e-/C) was significantly different between the two species, estimated to be 26.2 for T. sacculifer and 96.5 for G. siphonifera. These are surprisingly high values considering the theoretically and empirically realistic values of e-/C. We hypothesized that this high e-/C may also be due to the use of unlabeled respired carbon (underestimation of P). Simple mass-balance calculations suggested that a significant amount of carbon should be derived from host respired CO2, and that this contribution was higher in G. siphonifera than in T. sacculifer. The attempts to couple ETR and P in this study could comprehensively reveal interesting perspectives on the close interactions that exist within photosymbiotic systems. Moreover, our results suggests that when using geochemical parameters such as δ13C as paleoceanographic proxies, it is important to note that the potential magnitude of the photosynthetic effect varies among species.