Colimitation of the unicellular photosynthetic diazotroph Crocosphaera watsonii by phosphorus, light, and carbon dioxide

We describe interactive effects of total phosphorus (total P = 0.1–4.0 µmol L −1 ; added as H 2 NaPO 4 ), irradiance (40 and 150 µmol quanta m −2 s −1 ), and the partial pressure of carbon dioxide (; 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 positively affected growth and CO 2 ‐ and N 2 ‐fixation rates under high light. Under low light, elevated positively affected growth rates at all concentrations of P, but CO 2 ‐ and N 2 ‐fixation rates were affected by elevated 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 increased. The minimum concentration (C min ) of total P and half‐saturation constant (K ʽ ) for growth and CO 2 ‐ and N 2 ‐fixation rates with respect to total P were reduced by 0.05 µmol L −1 as a function of elevated . We speculate that low P requirements under high resulted from a lower energy demand associated with carbon‐concentrating mechanisms in comparison with low‐ cultures. There was also a 0.10 µmol L −1 increase in C min and K ʽ for growth and N 2 fixation with respect to total P as a function of increasing light regardless of 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 .