Coccolithophores are an important group of marine phytoplankton which cover themselves with the coccosphere – a shell composed of numerous calcium carbonate (CaCO3) platelets. Despite more than a century of coccolithophore research, it remains speculative why coccolithophores calcify. Resolving this question is essential to assess the competitive fitness of coccolithophores in the future ocean where changes in calcification are expected. Here, we used the Emiliania huxleyi – Emiliania huxleyi Virus 86 host-virus model system to test the hypothesis that the coccosphere serves as a physical barrier reducing viral infection. Therefore, we removed the coccosphere from living E. huxleyi cells and compared the infection progress relative to calcified cells in a series of 6 experiments under different growth conditions. We found that the coccosphere does not constitute an effective physical barrier against viral penetration, since non-growing calcified cells were susceptible to viral infection and lysis (growth stopped by light limitation). However, we also found that protection against the virus may depend on the daily growth cycle. E. huxleyi reached higher peak abundances when decalcified cells were allowed to rebuild their coccosphere before entering cell division phase and being exposed to the virus, thereby suggesting that rates of viral infection could be reduced by the coccosphere during the critical phase in the cell cycle. However, the benefit of this potential protection is arguably of limited ecological significance since the concentrations of both, calcified and decalcified E. huxleyi approached similar values until the end of the bloom. We conclude that the coccosphere provides at best a limited protection against infection with the EhV86.

中文翻译：

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Emiliania huxleyi 的碳酸钙壳提供有限的病毒感染保护

球石藻是一类重要的海洋浮游植物，它们覆盖着球球——一个由无数碳酸钙 (CaCO3) 血小板组成的壳。尽管对颗石藻进行了一个多世纪的研究，但对于颗石藻钙化的原因仍然是推测性的。解决这个问题对于评估未来海洋中钙化会发生变化的球石藻的竞争适应性至关重要。在这里，我们使用 Emiliania huxleyi – Emiliania huxleyi Virus 86 宿主病毒模型系统来测试球球作为减少病毒感染的物理屏障的假设。因此，我们从活的 E. huxleyi 细胞中去除了球球，并在不同生长条件下的一系列 6 次实验中比较了相对于钙化细胞的感染进程。我们发现球状体不构成抵抗病毒渗透的有效物理屏障，因为未生长的钙化细胞容易受到病毒感染和裂解（生长受光限制而停止）。然而，我们还发现对病毒的保护可能取决于每日的生长周期。E. huxleyi 在进入细胞分裂阶段并暴露于病毒之前允许脱钙细胞重建其球状体时达到更高的峰值丰度，从而表明在细胞周期的关键阶段，球状体可以降低病毒感染率。然而，这种潜在保护的好处可以说是有限的生态意义，因为钙化和脱钙 E. huxleyi 的浓度在开花结束前接近相似的值。