LAUSR

Nitrogen fixing plankton provide nitrogen to fuel marine ecosystems and biogeochemical cycles but the factors that constrain their growth and habitat remain poorly understood. Here we investigate the importance of metabolic specialization in unicellular diazotroph populations, using laboratory experiments and model simulations. In clonal cultures of Crocosphaera watsonii and Cyanothece sp. spiked with 15 N 2 , cellular 15 N enrichment developed a bimodal distribution within colonies, indicating that N 2 fixation was confined to a subpopulation. In a model of population metabolism, heterogeneous nitrogen (N 2 ) fixation rates substantially reduce the respiration rate required to protect nitrogenase from O 2 . The energy savings from metabolic specialization is highest at slow growth rates, allowing populations to survive in deeper waters where light is low but nutrients are high. Our results suggest that heterogeneous N 2 fixation in colonies of unicellular diazotrophs confers an energetic advantage that expands the ecological niche and may have facilitated the evolution of multicellular diazotrophs. Takako Masuda et al. show that individual cells in clonal populations of Crocosphaera watsonii and Cyanothece sp exhibit varied nitrogen fixation rates. This heterogeneity within the population decreases the energetic cost of respiration and expands the viable habitats for these unicellular diazotrophs.