Oceanic phytoplankton help regulate the planetary cycling of major elements and compounds. The smallest and most abundant phytoplankton in the ocean is the unicellular cyanobacterium of the genus Prochlorococcus. It has been estimated that Prochlorococcus contribute between 10-50% of oceanic primary production, and thus play a critical role in the ocean carbon cycle. Though best known to thrive in a high light, high oxygen, and low nutrient environment, such as in the subtropical gyres, they are also known occupy anoxic regions in parts of the Pacific and Indian Oceans, characteristic of low light, low oxygen, and high nutrients. Less is known about the role these low-oxygen, low-light adapted Prochlorococcus play in planetary biogeochemical cycles, owing to difficulties observing them at synoptic scales. In recent years, however, there has been a proliferation of autonomous platforms capable of observing phytoplankton and oxygen dynamics at the base of the euphotic zone. Making use of autonomous platforms, such as Biogeochemical Argo floats, and combining data collected from these platforms with satellite observations and model output, this project will aim to quantify the global distribution of this low-oxygen, low-light adapted Prochlorococcus ecotype. By integrating these results with ship-based measurements on the rates of production of carbon and oxygen, the project will aim to quantify the contribution of this ecotype to global and regional primary production. With anoxic regions predicted to expand in the future, results from this work will have implications for how Prochlorococcus are responding to climate change. This PhD project is aligned to a UKRI-funded project investigating the response of phytoplankton to climate change. It will primarily be a computational project using existing datasets, but with scope to do some field work, and would suit a student with a background in marine science (or related discipline) and with some experience in computer coding.