Fig. S1. Flow cytogram of Euk-A and Euk-B populations showing their characteristic red autofluorescence from chlorophyll a and green fluorescence following staining with SYBR-Green I (see Experimental procedures). Euk-A can clearly be discriminated from Euk-B based on its lower fluorescence and scattering signals characteristic of its smaller mean cell size and lower intracellular chlorophyll a and nucleic acid contents (see Zubkov et al., 2007a).

Fig. S2. The contribution of Prymnesiophyceae (Prym), Chrysophyceae (Chryso) and Pelagophyceae (Pelago) to total eukaryote-hybridized cells (%) in Euk-A (A and B) and Euk-B (C and D) in relation to cell- (pgC cell−1 day−1; A and C) and biomass-specific (gC gC−1 day−1; B and D) CO2 uptake rates. Contributions by each class were tested separately for any correlation with cell- and biomass-specific uptake rates using a Pearson's test, for both Euk-A and Euk-B. No statistically significant correlation (P > 0.1 in all cases; data not shown) was found for any of the groups or rates, further indicating that cell- and biomass-specific CO2 uptake is independent of taxonomic composition.

Fig. S3. Comparison of Euk-B and Euk-A abundance ratios between (A) unconcentrated and syringe-pump concentrated samples, (B) unconcentrated and CellTrap-concentrated samples, and (C) between syringe-pump concentrated and CellTrap-concentrated samples. No significant differences (paired t-test) were observed between these ratios in unconcentrated and syringe pump-concentrated samples (P = 0.75), between the former and CellTrapTM-concentrated samples (P = 0.29), and between the two different concentration methods used in this work (P = 0.40). Solid line indicates the 1:1 relationship.

Fig. S4. A typical example of curve fitting to determine cell-specific CO2 uptake for Euk-A and Euk-B (small or large photosynthetic picoeukaryotes (also known as plastidic protists) populations respectively). In this case the same number of cells were sorted for both groups (400, 800, 1200 and 1600 cells), with higher activities, and therefore a higher uptake per cell, registered for Euk-B.

Fig. S5. A typical example of the green fluorescence signal obtained using the DNA staining dye SYBR-Green I (see Experimental procedures), for unconcentrated, syringe-pump concentrated and CellTrap-concentrated samples, showing clearly differentiated DNA peaks for Euk-A and Euk-B populations that are similar in each case irrespective of the concentration method used (changes in cellular DNA content between samples have previously been shown to be indicative of variation in community structure, see Veldhuis and Kraay, 2004).

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