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Diatoms are unicellular photosynthetic micro-algae that live in both fresh and salt water. There are over 250 genera and more than 200,000 species. Diatoms have a shell called a frustule, made of amorphous silica. These frustules feature complex 3D architectures obtained by genetically controlled self-assembly of regularly arranged nanometric elements (pores, channels, protuberances).

These highly aesthetic and functional structures have inspired nano-artists as well as architects and engineers in the design of buildings and aircraft. Using photosynthesis as their energy source (photo-assisted conversion of carbon dioxide and water into sugars and oxygen), diatoms live where sunlight can penetrate. In other words, in the euphotic zone between the surface of the sea or lake and depths of around 200m. They are essential to life on earth, producing around 20% of the oxygen we breathe. When they die, they settle to the bottom of the ocean, producing an immense carbon dioxide sink. Their role is therefore essential in the earth's carbon cycle.

These microalgae are also rich models for the study of silica biomineralization processes. The fact that the genomes of several diatoms have been sequenced in the last decade, combined with advances in physico-chemical characterization methods, has clarified in part the relationships between the biomacromolecules present, self-assembly processes and the construction of complex silica architectures. Three groups of biomolecules are strongly associated with the biosilification process: silaffins, silacidins and long-chain polyamines. Silaffins and silacidins are peptides or proteins bearing numerous phosphate residues attached to amino acids (serine and threonine), while long-chain polyamines are essentially linear non-protein components made up of oligo-propyleneimine chains. Through self-assembly processes, mainly under electrostatic control, these biological macromolecules form bio-aggregates. At first order, the size of the bio-aggregates and the amount of silicon oligomers bound to them determine the final size of the silica spheres in the diatom's silica architecture. In vitro, these biomolecules also have the capacity to considerably influence the kinetics and structure of the silicas formed. Understanding these mechanisms is a source of inspiration for developing new synthetic processes and original organic-inorganic hybrid structures. From the point of view of fundamental research, diatoms are likely to contribute to the resolution of one of the still unresolved questions in biology: What is the detailed involvement of the genome in the creation of forms, and what are the levers of control over their evolution? Although a few competing hypotheses have recently been put forward for a small group of diatoms, we must be wary of jumping to hasty conclusions. More observations and intracellular physico-chemical analyses are needed to better understand the nature and dynamics of the phenomena involved.