The C : N : P of nitrogen‐fixing cyanobacteria regulates phosphate‐limited nitrogen fixation in the ocean yet the causes and range in variation C : N : P is not well constrained. Here we quantify the C : N : P and underlying macromolecular composition of Crocosphaera . Crocosphaera is a nitrogen‐fixing cyanobacterium that substantively contributes to elemental biogeochemistry in the (sub)tropical oceans. We observed variation...