The silica cell wall of diatoms, a widespread group of unicellular microalgae, is an exquisite example for the ability of organisms to finely sculpt minerals under strict biological control. The prevailing paradigm for diatom silicification is that this is invariably an intracellular process, occurring inside specialized silica deposition vesicles that are responsible for silica precipitation and morphogenesis. Here, we study the formation of long silicified extensions that characterize many diatom species. We use cryo-electron tomography to image silica formation in situ, in 3D, and at a nanometer-scale resolution. Remarkably, our data suggest that, contradictory to the ruling paradigm, these intricate structures form outside the cytoplasm. In addition, the formation of these silica extensions is halted at low silicon concentrations that still support the formation of other cell wall elements, further alluding to a different silicification mechanism. The identification of this unconventional strategy expands the suite of mechanisms that diatoms use for silicification.

硅藻的二氧化硅细胞壁是一种广泛存在的单细胞微藻群，是生物体在严格的生物控制下精细雕刻矿物质的能力的一个绝妙例子。硅藻硅化的普遍范式是，这总是一个细胞内过程，发生在专门的二氧化硅沉积囊泡内，这些囊泡负责二氧化硅沉淀和形态发生。在这里，我们研究了表征许多硅藻物种的长硅化延伸物的形成。我们使用冷冻电子断层扫描仪以 3D 和纳米级分辨率对二氧化硅形成进行原位成像。值得注意的是，我们的数据表明，与统治范式相矛盾的是，这些复杂的结构是在细胞质外形成的。此外，这些二氧化硅延伸的形成在低硅浓度下停止，仍然支持其他细胞壁元素的形成，进一步暗示了不同的硅化机制。这种非常规策略的识别扩展了硅藻用于硅化的机制套件。