海洋浮游植物的元素鋇攝取和同位素分化

Abstract

元素鋇（Ba）和其穩定同位素常作為重建「海洋生產力」的示蹤劑，但元素鋇與海洋浮游植物攝取的關係不明確，其應用常受到限制。本論文通過一系列浮游植物培養實驗，探討生物鋇攝取，在遠洋重晶石（硫酸鋇，BaSO4）形成過程中扮演的角色，及其對海洋鋇和碳循環的影響。研究結果包括：（1）取得各種浮游植物（包括矽藻Thalassiosira weissflogii、Thalassiosira pseudonana和Chaetoceros muelleri，以及鈣板藻Emiliania huxleyi和Gephyrocapsa oceanica）的元素鋇磷比（Ba/P）。（2）首次探討矽藻Thalassiosira weissflogii的碳鋇比（C/Ba），發現顯著高於海洋顆粒中觀察到的比率（至少1000倍）。由此顯著高的比率推論，為維持海洋中的顆粒態鋇通量，需要有額外的鋇來源。（3）首次分析矽藻Thalassiosira weissflogii和鈣板藻Emiliania huxleyi兩種浮游植物的鋇同位素組成。結果表明，浮游植物從培養海水中，優先攝取較輕的鋇同位素。當培養液中的鋇濃度從90 nM調升到200 nM時，Thalassiosira weissflogii的同位素分化因子（isotope fractionation factors, Δ138/134Babio-sw）從−0.47‰變化到−0.20‰，然而此變化與生長速率無關。從培養實驗中得知，與表層海水相近的鋇濃度範圍內，該藻中的同位素分化因子（−0.33 ~ −0.47‰），與遠洋重晶石之觀測值近似（−0.4 ~ −0.5‰），進而推測大多數浮游植物所攝取的鋇，在完全轉化成遠洋重晶石的過程中，沒有產生進一步的同位素分化。本研究為海洋浮游植物鋇磷比、及鋇同位素分化因子提供了新見解，增強了我們對海洋環境中鋇循環和碳循環的理解。

Barium (Ba) and it stable isotopes have been commonly used as tracers for reconstructing marine productivity, but such applications are often limited by unrevealed relationships between Ba and marine phytoplankton uptake. This thesis sets up a series of phytoplankton culture experiments to explore the biological Ba uptake that contributes to pelagic barite (BaSO4) formation, and its impact on the marine Ba and C cycles. The results reveal Ba cell quotas (Ba/P) in various phytoplankton, including diatoms (Thalassiosira weissflogii, Thalassiosira psueudonana, and Chaetoceros muelleri) and coccolithophores (Emiliania huxleyi and Gephyrocapsa oceanica), ranging from 5 to 30 𝜇mol/mol. Culture experiments of Thalassiosira weissflogii show a positive relationship between Ba uptake and Ba concentration ([Ba]) in the culture medium. However, the slopes of this relationship may result from variations in phytoplankton growth rates. This study also investigates the C/Ba ratios of cultured Thalassiosira weissflogii for the first time, and these ratios are significantly higher (> 1000-fold) than the ratios observed in field marine particles, suggesting that additional Ba supply is necessary to sustain particulate Ba flux along the export production. The Ba isotope compositions of phytoplankton are reported for the first time for Thalassiosira weissflogii and Emiliania huxleyi. The results indicate a preferential uptake of lighter Ba isotopes from ambient seawater, with isotope fractionation factors (138/134Babio-sw) varying from −0.47‰ to −0.20‰ as the culture medium [Ba] changes from 90 nM to 200 nM, independently of growth rates. The consistent isotope fractionation factors (−0.33 ~ −0.47‰) between the culture experiments and pelagic barite within the range of surface seawater [Ba] (−0.4 ~ −0.5‰) imply that phytoplankton Ba uptake is largely transformed into pelagic barite without further isotope fractionation. This study provides better constraints on marine phytoplankton Ba cell quotas and their isotope fractionation factors, enhancing our understanding of Ba and C cycles in marine environments.