1. Diatoms and their sea-ice habitat Diatoms are unicellular microalgae that contribute to 20% of the global carbon fixation. This is as much as the carbon fixed by all tropical rainforests combined (Armbrust et al., 2004). Diatoms are found all over the globe, in fresh-and seawater, in hot and cold habitats. Their most distinctive feature is a silicified cell wall (termed frustule) made of hydrated amorphous silica and a small amount of organic material (sugar). The architecture of the frustule is based on silica patterns that are structured on a nano-to-micrometer scale. These nano-patterns can vary from species to species, creating unique morphotypes that are used as taxonomic keys (fig.1). Figure 1. SEM picture of the psychrophilic pennate diatom Fragilariopsis cylindrus (Bacillariophyceae). Valve view of two cells. SEM is taken by Henrik Lange and Gerhard Dieckmann.