Physiological Response of Crocosphaera watsonii to Enhanced and Fluctuating Carbon Dioxide Conditions

We investigated the effects of elevated pCO(2) on cultures of the unicellular N(2)-fixing cyanobacterium Crocosphaera watsonii WH8501. Using CO(2)-enriched air, cultures grown in batch mode under high light intensity were exposed to initial conditions approximating current atmospheric CO(2) concentrations (∼400 ppm) as well as CO(2) levels corresponding to low- and high-end predictions for the year 2100 (∼750 and 1000 ppm). Following acclimation to CO(2) levels, the concentrations of particulate carbon (PC), particulate nitrogen (PN), and cells were measured over the diurnal cycle for a six-day period spanning exponential and early stationary growth phases. High rates of photosynthesis and respiration resulted in biologically induced pCO(2) fluctuations in all treatments. Despite this observed pCO(2) variability, and consistent with previous experiments conducted under stable pCO(2) conditions, we observed that elevated mean pCO(2) enhanced rates of PC production, PN production, and growth. During exponential growth phase, rates of PC and PN production increased by ∼1.2- and ∼1.5-fold in the mid- and high-CO(2) treatments, respectively, when compared to the low-CO(2) treatment. Elevated pCO(2) also enhanced PC and PN production rates during early stationary growth phase. In all treatments, PC and PN cellular content displayed a strong diurnal rhythm, with particulate C:N molar ratios reaching a high of 22∶1 in the light and a low of 5.5∶1 in the dark. The pCO(2) enhancement of metabolic rates persisted despite pCO(2) variability, suggesting a consistent positive response of Crocosphaera to elevated and fluctuating pCO(2) conditions.