We describe interactive effects of total phosphorus (total P = 0.1–4.0 µmol L−1; added as H2NaPO4), irradiance (40 and 150 µmol quanta m−2 s−1), and the partial pressure of carbon dioxide (pCO2; 19 and 81 Pa, i.e., 190 and 800 ppm) on growth and CO2– and dinitrogen (N2)-fixation rates of the unicellular N2-fixing cyanobacterium Crocosphaera watsonii (WH0003) isolated from the Pacific Ocean near Hawaii. In semicontinuous cultures of C. watsonii, elevated pCO2 positively affected growth and CO2– and N2-fixation rates under high light. Under low light, elevated pCO2 positively affected growth rates at all concentrations of P, but CO2– and N2-fixation rates were affected by pCO2 elevated only when P was low. In both high-light and low-light cultures, the total P requirements for growth and CO2– and N2-fixation declined as pCO2 increased. The minimum concentration (Cmin) of total P and half-saturation constant (K½) for growth and CO2– and N2-fixation rates with respect to total P were reduced by 0.05 µmol L−1 as a function of elevated pCO2. We speculate that low P requirements under high pCO2 resulted from a lower energy demand associated with carbon-concentrating mechanisms in comparison with low-pCO2 cultures. There was also a 0.10 µmol L−1 increase in Cmin and K½ for growth and N2 fixation with respect to total P as a function of increasing light regardless of pCO2 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, CO2, and light have both positive and negative interactive effects on growth and CO2-, and N2-fixation rates of unicellular oxygenic diazotrophs like C. watsonii.
Garcia N. S., Fu F.-X. & Hutchins D. A., 2013. Colimitation of the unicellular photosynthetic diazotroph Crocosphaera watsonii by phosphorus, light, and carbon dioxide. Limnology & Oceanography 58(4): 1501-1512. Article.