Ocean fertilisation for CO2 storage by microalgae: Anthropogenic greenhouse gas emissions are the dominant cause of the observed global warming (IPCC 2021). In 2021, anthropogenic emissions of carbon dioxide were estimated at 11.0 GtC yr-1 (Friedlingsstein et al. 2022). In spite of this, to prevent warming beyond 1.5°C, greenhouse gas emissions need to reach net zero by 2050. However, currently all projected scenarios capable of reaching this goal rely on ‚negative emissions‘ (Rogelj et al. 2018). Negative emissions describe carbon dioxide removal methods that enhance the uptake and sequestration of anthropogenic carbon dioxide. This chapter focuses on ocean fertilisation, a specific concept of carbon sequestration where nutrients are added to the ocean surface with the intent to increase algae productivity and sequester newly generated biomass in the deep sea.