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

Abstract

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 (; 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 positively affected growth and CO2‐ and N2‐fixation rates under high light. Under low light, elevated positively affected growth rates at all concentrations of P, but CO2‐ and N2‐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 CO2‐ and N2‐fixation declined as increased. The minimum concentration (Cmin) of total P and half‐saturation constant (Kʽ) 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 . 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 Cmin and Kʽ for growth and N2 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, 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.