Figure 1.

(a) Prochlorococcus MED4 cell concentration and (b) mean chlorophyll fluorescence per cell in cultures sparged with nine mixtures of O₂ and CO₂. All cultures were inoculated at t=−1 day. Experimental cultures (black) were sparged with the indicated gas mix (balanced with N₂), beginning at t=0 days; control cultures (gray) were inoculated and grown in parallel but not sparged. Data points are the mean of at least three cultures. Error bars indicate 1 s.d.; where error bars are not visible, they are smaller than the plotting symbol. The drop in chlorophyll fluorescence per cell in control cultures from t=3 days to t=4 days is expected for cultures entering stationary phase. In experimental cultures, the addition of CO₂ is expected to delay this transition (Moore et al., 2007).