This project has furthered knowledge of phytoplankton nutrient limitation in several ways. Firstly, work conducted in the tropical North Atlantic has revealed how iron availability can control microbial access to the major nutrient phosphorus, through its requirement in alkaline phosphatase enzymes (enzymes that are used to hydrolyse organic phosphorus molecules). This new finding is significant, as it has elucidated a direct biochemical linkage between cycles of iron and phosphorus in the ocean, which could be important for predicting phytoplankton responses to the projected changing input of these nutrients in the future. Secondly, the work conducted in the South Atlantic has demonstrated how phytoplankton communities in broad regions of the surface ocean can be living under conditions of simultaneous nutrient co-limitation (where the availabilities of multiple nutrients are restricting phytoplankton growth at the same time). This finding is significant as it demonstrates that co-limitation is prevalent in the ocean, which has not been shown previously, and that this needs including in biogeochemical models that are used for projecting the impacts of climate change. Thirdly, the suite of experiments conducted has demonstrated how poorly studied nutrients such as cobalt and vitamin B12 can play a role limiting phytoplankton growth in the ocean. Forth, the experiments have demonstrated how reductions in phytoplankton fluorescence with increasing irradiance appear to be moderated significantly by iron availability. This last finding is important as it indicates significant additional complexity in developing a robust correction for phytoplankton NPQ (i.e. without firstly knowing the iron stress status of the phytoplankton community). This could represent a difficult obstacle to overcome in evaluating detailed patterns of iron stress from existing space-based passive sunlight induced fluorescence measurements (e.g. fluorescence line height products available from MODIS and MERIS). However, planned future sensors on satellites that will have the capacity for measuring phytoplankton fluorescence at multiple wavelengths, alongside enhanced sensitivity for fluorescence detection from low chlorophyll waters, could provide a route for overcoming this obstacle (e.g. the planned NASA PACE mission; see: https://pace.gsfc.nasa.gov
). When such data become available (estimated year 2023), the field-based observations collected in this study will provide crucial ground-level phytoplankton physiological information needed evaluate the signals carried in such data.