is a major dinitrogen (N )-fixing microorganism, providing bioavailable nitrogen (N) to marine ecosystems. The N -fixing enzyme nitrogenase is deactivated by oxygen (O ), which is abundant in marine environments. Using a cellular scale model of sp. and laboratory data, we quantify the role of three O management strategies by sp.: size adjustment, reduced O diffusivity, and respiratory protection. Our model predicts that cells increase their size under high O Using transmission electron microscopy, we show that starch granules and thylakoid membranes are located near the cytoplasmic membranes, forming a barrier for O The model indicates a critical role for respiration in protecting the rate of N fixation. Moreover, the rise in respiration rates and the decline in ambient O with temperature strengthen this mechanism in warmer water, providing a physiological rationale for the observed niche of at temperatures exceeding 20°C. Our new measurements of the sensitivity to light intensity show that the rate of N fixation reaches saturation at a lower light intensity (∼100 μmol m s ) than photosynthesis and that both are similarly inhibited by light intensities of >500 μmol m s This suggests an explanation for the maximum population of occurring slightly below the ocean surface. is one of the major N -fixing microorganisms in the open ocean. On a global scale, the process of N fixation is important in balancing the N budget, but the factors governing the rate of N fixation remain poorly resolved. Here, we combine a mechanistic model and both previous and present laboratory studies of to quantify how chemical factors such as C, N, Fe, and O and physical factors such as temperature and light affect N fixation. Our study shows that combines multiple mechanisms to reduce intracellular O to protect the O -sensitive N -fixing enzyme. Our model, however, indicates that these protections are insufficient at low temperature due to reduced respiration and the rate of N fixation becomes severely limited. This provides a physiological explanation for why the geographic distribution of is confined to the warm low-latitude ocean.