Sr in coccoliths of Scyphosphaera apsteinii: partitioning behavior and role in coccolith morphogenesis

Coccolithophores are important contributors to global calcium carbonate through their species-specific production of calcite coccoliths. Nannofossil coccolith calcite remains an important tool for paleoreconstructions through geochemical analysis of isotopic and trace element incorporation, including Sr, which is a potential indicator of past surface ocean temperature and productivity. Scyphosphaera apsteinii exhibits an unusually high Sr/Ca ratio and correspondingly high partitioning coefficient (DSr) in their two morphologically distinct types of coccoliths. Whether or not this reflects mechanistic differences in calcification compared to other coccolithophores is unknown. We therefore examined the possible role of Sr in S. apsteinii calcification by growing cells in deplete, ambient, and higher than ambient Sr conditions (between 0.33 - 140 mmol/mol Sr/Ca). The effects on growth, quantum efficiency of photosystem II (Fv/Fm), coccolith morphology, and calcite DSr were evaluated. Reducing the Sr/Ca from ambient (9 mmol/mol) did not significantly alter the frequency of malformed and aberrant muroliths and lopadoliths, but at higher than ambient Sr/Ca conditions coccolith morphology was significantly disrupted. This implies that Sr is not a critical determining factor in normal coccolith calcite morphology in this dimorphic species. Interestingly, muroliths had significantly lower Sr/Ca than lopadoliths at ambient and elevated [Sr], and lopadolith tips had lower Sr than bases in ambient conditions. In summary, the Sr fractionation behavior of S. apsteinii is unusual because of an overall high DSr, and an inter- and intra-coccolith variability in Sr/Ca. We hypothesize that differential Sr-and Ca-binding capacity of coccolith associated polysaccharides may account for the unusual Sr fractionation of this species which can explain all observations made in this study.颗石藻（Coccolithophores）通过物种特异性合成方解石颗石（calcite coccoliths），是全球碳酸钙沉积的重要贡献者。纳米化石颗石方解石（Nannofossil coccolith calcite）凭借其同位素与微量元素掺入特征的地球化学分析，仍是古环境重建的重要工具，其中锶（Sr）是重建过去表层海水温度与海洋生产力的潜在指标。斯氏平颗石藻（Scyphosphaera apsteinii）的两种形态迥异的颗石类型均表现出异常高的锶钙比（Sr/Ca ratio），以及相应较高的锶分配系数（DSr）。目前尚不清楚这是否反映了该物种与其他颗石藻在钙化机制上的差异。为此，我们通过将细胞培养在低锶、本底以及高于本底的锶钙比条件（0.33~140 mmol/mol Sr/Ca）下，探究了锶在斯氏平颗石藻钙化过程中的潜在作用。本研究评估了不同锶钙比条件对细胞生长、光系统II量子效率（Fv/Fm）、颗石形态以及方解石锶分配系数的影响。研究发现，将锶钙比从本底水平（9 mmol/mol）降低，并未显著改变畸形与异常的穆罗颗石（muroliths）与洛帕德颗石（lopadoliths）的出现频率；但当锶钙比高于本底水平时，颗石形态会受到显著破坏。这表明，对于这种双形态颗石藻而言，锶并非决定正常颗石方解石形态的关键因素。有趣的是，在本底与高锶钙比条件下，穆罗颗石的锶钙比均显著低于洛帕德颗石；且在本底条件下，洛帕德颗石尖端的锶含量显著低于其基部。综上，斯氏平颗石藻的锶分馏行为较为特殊，不仅表现为整体较高的锶分配系数，还存在颗石间与颗石内的锶钙比差异。我们推测，颗石相关多糖（coccolith associated polysaccharides）对锶与钙的结合能力存在差异，这或许可以解释该物种特殊的锶分馏行为，同时也能合理解释本研究中的所有观测结果。