EXPAND · How is marine nitrogen availability being impacted by ocean desert expansion?

Often called ‘ocean deserts’, subtropical gyres have low biological productivity but, due to their immense size, contribute significantly to carbon sequestration regulating global climate. In gyres, biological productivity strongly depends on nitrogen supplied by microbes called diazotrophs, capable of fixing molecular nitrogen (N2) into bioavailable nitrogen forms. Earth system models predict increasing uncertainty in biological productivity towards the end of the 21st century. Such uncertainty responds to N2 fixation parametrisation in models and is largely driven by the scarce observations available in the Indian Ocean (IO), representing only 1% of the N2 fixation data available globally. Moreover, our current understanding of N2 fixation is mainly based on nutrient availability (phosphorus and iron) impacts on diazotrophs. However, my previous research shows that other controls including ocean circulation and diazotroph/non-diazotroph interactions are key in shaping N2 fixation inputs locally. None of these controls have been comprehensively examined over the vast extension of the IO, nor throughout seasons. We will conduct oceanographic expeditions covering the full extension of the IO gyre at its minimum and maximum expansion seasons. Gyre expansion will be tracked with satellite and in situ hydrographic and current speed measurements. The impact of chemical (nutrients), physical (ocean circulation), and biological (species interactions) controls on N2 fixation will be comprehensively measured, using at-sea experiments and up-to-date isotopic and molecular analyses. Moreover, seasonal variability will be monitored over a full year with mooring lines anchored at the centre, northern and southern edges of the gyre, equipped with DNA samplers and a newly designed automatic device measuring N2 fixation rates. These datasets will link cellular to ecosystem processes, bridging the gap between ocean desert expansion and N2 fixation in the world’s least explored gyre.