Diatoms live in both seawater and freshwater. They can dominate aquatic environments, yet they have no active method of propulsion to seek and trap nutrients. At the same time, they must use some kind of mechanism to exclude deleterious particles such as viruses and bacteria. Thus, the way they interact with their fluid environment is critical for their success. What distinguishes diatoms from their competitors is their rigid nanostructured silica frustule. This exists as an exoskeleton and forms a porous barrier between the cell membrane and the ambient environment. In this chapter, we examine the environment that diatoms live in, and how diatoms interact with their fluid environment specifically in relation to the unique frustule properties.

Diatoms can manipulate their interactions with their fluid environment in a range of ways that can affect their uptake of nutrients. These include changes in buoyancy, rotation and, on the nanoscale, local manipulation of streamlines over their surface, rotation, and filtration through the silica pores. We cover all of these scenarios and link these to the cell nutrient uptake rate of diatoms. Finally, we suggest some mechanisms that help describe how the unique frustule structure can act as a selective filter.