[B-PT02_1AM2] Biocalcification and the Geochemistry of Proxies

[BPT02-13] クロロフィル蛍光法を用いた浮遊性有孔虫の光共生生態の探究：生態指標確立に向けて

*高木 悠花¹、木元 克典²、藤木 徹一²、倉沢 篤史²、平野 弘道³ (1.早稲田大学・院・ 創造理工学、2.独立行政法人海洋研究開発機構、3.早稲田大学・教育・地球科学)

キーワード：浮遊性有孔虫, 光共生生態, 高速フラッシュ励起蛍光法

Endosymbiosis of planktic foraminifers with photosynthetic algae (photosymbiosis) is established especially among species which dominate in warm, low-nutrient surface water. Here, photosymbiosis probably plays an important role for host foraminifers, and can be considered as an adaptive ecology to live in such oligotrophic oceans. Therefore, back in geologic time, photosymbiosis could have been involved with species adaptive radiation as well. In such viewpoint, stable isotopic change of foraminiferal test through ontogeny, attributed to change of symbiont photosynthetic effect, has been used as an indicator to detect fossil photosymbiosis. However, how host-symbiont association change through ontogeny, if any, is practically unknown and has never been quantified. Here, we offer new insights for photosymbiosis based on photosynthetic characteristics of symbionts, obtained by in vivo fluorometric analysis (Fast Repetition Rate Fluorometry, FRRF).We cultured two symbiont-bearing species, Globigerinoides sacculifer and Globigerinella siphonifera, and conducted FRRF measurement on individual host-algal consortium during the culture period. FRRF can identify photosymbiosis of individual foraminifer instantly in a non-destructive manner, and gives us various photosynthetic characteristics of symbionts, i.e., maximum fluorescence yield (Fm, index of chlorophyll content), photochemical efficiency (Fv/Fm, index of potential photosynthetic activity), and effective absorption cross-section of photosystem II (σ_PSII, capability of the absorbed energy to promote a photochemical reaction).Sequential FRRF analyses on single individuals revealed that Fm increases with growth, and then decrease drastically at the end of their life, which means that the algal biomass per individual foraminifer increases through ontogeny, but the symbionts are rapidly digested at the end. Fv/Fm and σ_PSII values were constant through ontogeny, though Fv/Fm drops in correspondence with the decrease of Fm. Compared between the two species, average values of both Fv/Fm and σ_PSII showed statistically significant differences. Fv/Fm was significantly higher in Gs. sacculifer, which means that symbionts are more actively photosynthesizing in Gs. sacculifer. Because Fv/Fm is mainly depends on nutrient availability, it is a direct evidence of nutrient (metabolite) flow from host to symbionts. On the other hand, σ_PSII was higher in Gn. siphonifera, indicating that this species can utilize low light energy more efficiently, i.e., more “low-light-adapted” than Gs. sacculifer. Actually, it is consistent with inferred habitat preference of Gn. siphonifera, which is relatively deeper than Gs. sacculifer.These FRRF results provide us information of foraminiferal photosymbiosis both quantitatively and qualitatively. When the information is combined with test geochemistry mentioned above, it will presumably enable us to quantify the photosynthetic activity from foraminiferal tests. Then, it can be applied to fossil specimens as a validated ecological proxy of photosymbiosis.