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Abstract
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.
Highlights
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
This process is more susceptible to low-Si medium than other silica elements that grow inside silica deposition vesicles (SDVs)
Two long setae grow at an angle of ~45° to the apical axis from opposite locations on the rim of each valve[17]
Summary
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. Several diatom species have elaborate silica appendages that can be much longer than the cell body Such extensions challenge the concept of intracellular silicification, with some resemblance to the extremely long spicules formed by sponges in a process that is proposed to start intracellularly but is predominantly extracellular[14]. The seta silica is always present outside the cell membrane, suggesting a formation process that is not mediated by an SDV This process is more susceptible to low-Si medium than other silica elements that grow inside SDVs. In addition, this process is more susceptible to low-Si medium than other silica elements that grow inside SDVs Such a highly controlled extracellular silicification process in diatoms raises many interesting questions regarding the fundamentals of biological silicification
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