THE equatorial Pacific Ocean is a ’high-nitrate, low-chlorophyll’ region where nitrate and phosphate are abundant all year round, These nutrients cannot therefore be limiting to phytoplankton production. It has been suggested that the bioavailability of iron-a micronutrient-may be preventing full biological utilization of the major nutrients(1-3). The results of a previous in situ iron fertilization experiment in this region provided support for this hypothesis(4), but the observed biological response resulted in only a small decrease in surface-water CO2 fugacity(5). Here we report a much larger, biologically induced uptake of surface-water CO2 that occurred during a second study(6). The fugacity of CO2 in the centre of the (iron-fertilized) patch of surface ocean fell from a background value near 510 mu atm to approximately 420 mu atm, corresponding to a transient 60% decrease in the natural ocean-to-atmosphere CO2 flux. We conclude that iron supply to this ocean region can strongly modulate the local shortterm source of CO2 to the atmosphere, but has little long-term influence on atmospheric CO2 partial pressure. However, if such a modulation also occurs in the Southern Ocean, then iron bioavailability at high southern latitudes could have a significant effect on atmospheric CO2 partial pressure(7-11), for example over glacial-interglacial periods.