Although the emergence of complex biomineralized forms has been investigated for over a century, still little is known on how single cells control morphology of skeletal structures, such as frustules, shells, spicules, or scales. We have run experiments on the shell formation in foraminifera, unicellular, mainly marine organisms that can build shells by successive additions of chambers. We used live imaging to discover that all stages of chamber/shell formation are controlled by dedicated actin-driven pseudopodial structures. Successive reorganization of an F-actin meshwork, associated with microtubular structures, is actively involved in formation of protective envelope, followed by dynamic scaffolding of chamber morphology. Then lamellar dynamic templates create a confined space and control mineralization separated from seawater. These observations exclude extracellular calcification assumed in selected foraminiferal clades, and instead suggest a semiintracellular biomineralization pattern known from other unicellular calcifying and silicifying organisms. These results give a challenging prospect to decipher the vital effect on geochemical proxies applied to paleoceanographic reconstructions. They have further implications for understanding multiscale complexity of biomineralization and show a prospect for material science applications.

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