[B-PT03] Biomineralization and Geochemistry of Proxies

[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.