Impact of diazotrophs on marine food webs and the biological carbon pump: progress and remaining challenges.

Marine diazotrophs are microscopic planktonic organisms ubiquitous in the ocean, that play a major ecological role: they supply nitrogen to the surface ocean biosphere, an essential but scarce nutrient in ~60% of the global ocean. Over the past decades, they have attracted considerable attention, with numerous studies providing key insights into their diversity, lifestyle, biogeographical distribution, and biogeochemical role in planktonic ecosystems. An increasing number of studies show that these microbes regulate marine productivity and shape the food web by alleviating nitrogen limitation, thereby contributing to carbon sequestration to the deep ocean. Yet, the diazotroph-derived organic carbon exported to the deep ocean is still poorly quantified, limiting robust estimates of the ocean's contribution to CO₂ sequestration and climate change mitigation under present and future conditions. This knowledge gap reflects the complexity of diazotroph export pathways to the deep ocean, whose quantification and variability drivers remain difficult to resolve with current methods. This review aims to synthesize current knowledge on the role of diazotrophs in their interactions with the food web and the biological carbon pump (BCP), reanalyze existing datasets, identify key knowledge gaps, and propose future research directions.