Diatoms are autotrophic siliceous unicellular eukaryotes believed to contribute ~40% of global depth-integrated marine primary production and export of organic carbon from the surface ocean. In addition to providing carbon to sustain ocean food webs they thus contribute significantly to its transfer to the deep ocean, i.e. to the biological carbon pump. Beyond the classical view of diatoms being abundant in nutrient-rich turbulent waters, observations and models both indicate their dominance in meso/submesoscale structures such as fronts and filaments, and as shade flora within the deep chlorophyll maximum. High-resolution observations and simulations, together with inferences from genomics, are changing our understanding of processes regulating and regulated by diatom diversity and abundance. Diatoms display wide variations in size, morphology, and elemental composition, all of which control the quality, quantity, and sinking speed of biogenic matter to depth. As regards carbon export diatoms are unique among the phytoplankton because of their silica shells which provide ballast to marine snow and faecal pellets. Evidence is growing that diatoms are not only efficient transporters of organic carbon to the mesopelagic layer, but can also transport it to the deep ocean. Herein we show that all diatoms are not equal, in that varying Si/C ratios and life strategies will modulate the transfer of carbon to the deep ocean. Except for the Southern Ocean, models predict a decline in the contribution of diatoms to primary production in the future ocean. However, we argue that to better predict the changes of the biological carbon pump in a warmer and acidified ocean we need to address the impacts of physical and chemical changes on diatom diversity, their interactions with other planktonic components, and their complex life histories.