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Abstract. Trichodesmium is a globally important marine microbe that provides fixednitrogen (N) to otherwise N-limited ecosystems. In nature, nitrogen fixationis likely regulated by iron or phosphate availability, but the extent andinteraction of these controls are unclear. From metaproteomics analysesusing established protein biomarkers for nutrient stress, we foundthat iron–phosphate co-stress is the norm rather than the exception for Trichodesmium colonies in theNorth Atlantic Ocean. Counterintuitively, the nitrogenase enzyme was moreabundant under co-stress as opposed to single nutrient stress. This isconsistent with the idea that Trichodesmium has a specific physiological state duringnutrient co-stress. Organic nitrogen uptake was observed and occurredsimultaneously with nitrogen fixation. The quantification of the phosphate ABCtransporter PstA combined with a cellular model of nutrient uptake suggestedthat Trichodesmium is generally confronted by the biophysical limits of membrane spaceand diffusion rates for iron and phosphate acquisition in the field. Colonyformation may benefit nutrient acquisition from particulate and organicsources, alleviating these pressures. The results highlight that topredict the behavior of Trichodesmium, both Fe and P stress must be evaluatedsimultaneously.