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Recently, very precise measurements have detected the seasonal variability in the atmospheric O2/N2 ratio at several sites in the northern and southern hemispheres. In this paper, we derive marine primary productivity (PP) from satellite ocean color data. To infer air-sea oxygen fluxes, a simple one-dimensional diagnostic model of ocean biology has been developed that depends on only two parameters: a time delay between organic production and oxidation (set to 2 weeks) and an export scale length (50 m). This model gives a global net community production of 4.3 mol C m-2 yr-1 in the euphotic zone and 3.2 mol C m-2 yr-1 in the mixed layer. This last value corresponds to a global f ratio (net community production (NCP)/PP) at the base of the mixed layer of 0.37. The air-sea fluxes derived from this model are then used at the base of a three-dimensional atmospheric model to compare the atmospheric seasonal cycle of O2/N2 at five sites: Cape Grim (40.6S, 144.6E), Baring Head (41.3S, 174.8E), Mauna Loa (19.5N,154.8W), La Jolla (32.9N, 117.3W), and Barrow (71.3N, 156.6W). The agreement between model and observations is very encouraging. We infer from the agreement that the seasonal variations in O2/N2 are largely controlled by the photosynthesis rate but also by the remineralization linked to the deepening and shoaling of the mixed layer. Lateral ventilation to high latitudes may also be an important factor controlling the amplitude of the seasonal cycle.