We describe interactive effects of total phosphorus (total P 5 0.1-4.0 mmol L 21 ; added as H 2 NaPO 4 ), irradiance (40 and 150 mmol quanta m 22 s 21 ), and the partial pressure of carbon dioxide (P CO2 ; 19 and 81 Pa, i.e., 190 and 800 ppm) on growth and CO 2 -and dinitrogen (N 2 )-fixation rates of the unicellular N 2 -fixing cyanobacterium Crocosphaera watsonii (WH0003) isolated from the Pacific Ocean near Hawaii. In semicontinuous cultures of C. watsonii, elevated P CO2 positively affected growth and CO 2 -and N 2 -fixation rates under high light. Under low light, elevated P CO2 positively affected growth rates at all concentrations of P, but CO 2 -and N 2 -fixation rates were affected by elevated P CO2 only when P was low. In both high-light and low-light cultures, the total P requirements for growth and CO 2 -and N 2 -fixation declined as P CO2 increased. The minimum concentration (C min ) of total P and half-saturation constant (K K ) for growth and CO 2 -and N 2 -fixation rates with respect to total P were reduced by 0.05 mmol L 21 as a function of elevated P CO2 . We speculate that low P requirements under high P CO2 resulted from a lower energy demand associated with carbon-concentrating mechanisms in comparison with low-P CO2 cultures. There was also a 0.10 mmol L 21 increase in C min and K K for growth and N 2 fixation with respect to total P as a function of increasing light regardless of P CO2 concentration. We speculate that cellular P concentrations are responsible for this shift through biodilution of cellular P and possibly cellular P uptake systems as a function of increasing light. Changing concentrations of P, CO 2 , and light have both positive and negative interactive effects on growth and CO 2 -, and N 2 -fixation rates of unicellular oxygenic diazotrophs like C. watsonii.Within the past 5 yr, new attention has focused on the effects of elevated partial pressures of carbon dioxide (P CO 2 ) on marine dinitrogen (N 2 ) fixation. In particular, three studies initiated this interest by documenting increased N 2 -fixation rates by Trichodesmium erythraeum in response to elevated P CO 2 (Barcelos e Ramos et al. 2007;Hutchins et al. 2007;Levitan et al. 2007). It is, however, particularly important to answer questions about how multiple environmental factors might change and interact with rising P CO 2 to affect ocean biogeochemical cycles. Hutchins et al. (2007) examined the combined effects of P CO 2 , temperature, and P concentration on N 2 -fixation rates by two strains of T. erythraeum. That study concluded that P CO 2 limitation of N 2 -fixation rates by T. erythraeum was independent of P concentration because the relative magnitude of elevated P CO 2 effects on growth and N 2 -fixation rates was nearly identical in P-limited and Preplete cultures. This finding suggests that decreases in P concentrations, which are likely to accompany future changes in surface ocean warming and stratification (Hutchins et al. 2009), would not influence the effect of elevated P CO 2 on oceanic N 2 ...