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

Nathan S. Garcia, Fei-Xue Fu and David A. Hutchins

Limnol. Oceanogr., 58(4), 2013, 1501-1512 | DOI: 10.4319/lo.2013.58.4.1501

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 (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 . 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 K½ 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.

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