Protein markers of cyanobacterial stress: Nutrients including iron, nitrogen, and phosphorus limit primary productivity in the oceans. Determining how abundant cyanobacteria such as Prochlorococcus adapt to nutrient stress across marine settings requires accurate molecular assays. Saito et al. developed a proteomic and metaproteomic approach capable of targeting specific metabolic biomarkers from mixed communities in seawater (see the Perspective by Moore). Prochlorococcus proteins are indicative of a major limiting nutrient across a wide transect in the Pacific Ocean; however, they also show that the limitation of multiple nutrients at overlapping biomes is an additional source of stress. Science, this issue p. 1173 ; see also p. 1120