Diatoms are considered unicellular eukaryotic organisms exclusively depositing biogenic silica. Heretofore there has been no report of calcifi cation by these algae. Here it is shown that calcium carbonate within the stalks of Didymosphenia geminata , a nuisance species that has prolifi cally colonized streams and rivers globally, is biogenic in origin and occurs as a network of calcite nanofi bers. The nanofi brous framework in the mineralized polysaccharide matrix imparts mechanical support to the stalks, providing stability in variable fl ow conditions. The results demonstrate that D. geminata possesses cellular and periplasmic carbonic-anhydrases that contribute to carbon fi xation and biomineralization, respectively. The activity of external carbonic-anhydrase was more than 50% of the total activity, which points to its role in anchoring this bioeroding diatom on hard surfaces. The fi rst evidence of multiphase biomineralization by diatoms that deposit both biogenic silica and crystalline biogenic calcite which are imparting distinct functional advantage to the organism is provided.

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For the first time, the three-dimensional (3D) internal structure of naturally produced Didymosphenia geminata frustules were nondestructively visualized at sub-100 nm resolution. The well-optimized hierarchical structures of these natural organisms provide insight that is needed to design novel, environmentally friendly functional materials. Diatoms, which are widely distributed in freshwater, seawater and wet soils, are well known for their intricate, siliceous cell walls called ‘frustules’. Each type of diatom has a specific morphology with various pores, ribs, minute spines, marginal ridges and elevations. In this paper, the visualization is performed using nondestructive nano X-ray computed tomography (nano-XCT). Arbitrary cross-sections through the frustules, which can be extracted from the nano-XCT 3D data set for each direction, are validated via the destructive focused ion beam (FIB) cross-sectioning of regions of interest (ROIs) and subsequent observation by scanning electron microscopy (SEM). These 3D data are essential for understanding the functionality and potential applications of diatom cells.

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