Fig. S1. Abundance of cells within each probe-identified group actively incorporating 33P-Pi, AT33P and 3H-leucine in the control (left panel) and P-amended mesocosms (right panel).

Fig. S2. Phosphate (Pi) concentration estimated by conventional methods (Strickland and Parsons, 1972) in the control (open symbols) and P-amended mesocosms (filled symbols). Pi concentration in the P-amended mesocosms immediately after Pi addition was not measured, so the theoretical concentration (sum of existing Pi + 100 nM added Pi) is plotted instead. The 33P-Pi added ranged between 25 and 108 pM.

Fig. S3. Epifluorescence microscopy images of (A) DAPI-stained cells showing the presence of polyphosphates (yellow granules) on day 3 of the experiment in the P-amended mesocosm; (B) same field of view showing autofluorescence of cyanobacterial cells. Note that heterotrophic bacterial cells also contain polyphosphate granules. No polyphosphate granules were observed in the control mesocosm, where microbial cells where limited by phosphorus availability.

Fig. S4. Morphology of NOR5/OM60 cells on day 4 of the experiment in (A) the control mesocosm, (B) P-amended mesocosm. NOR5/OM60 cells in the control mesocosm are elongated probably as a response to phosphorus stress. Four days after P addition NOR5/OM60 cells had decreased considerably in size. This phenomenon was not observed with other Gammaproteobacteria like Alteromonadales. Other elongated non-NOR5/OM60 cells can as well be seen in the control mesocosms, but we were not able to identify them with any of the probes used in this study. Abundance of the NOR5/OM60 clade was determined with the probes NOR5-730, as described elsewhere (Ferrera et al., 2011).

Fig. S5. Microautoradiogram showing uptake of 33P-phosphate by Gammaproteobacteria cells in the control mesocosms. Blue dots represent DAPI-stained bacteria. Green dots represent gammaproteobacterial cells hybridized with the CARD-FISH Gam42a probe. Black spots are silver grains deposited in the photographic emulsion. Cells were scored as active (MAR+) when they were in contact with at least one silver grain.

Table S1. Presence of homologues to the ppk gene in genomes of cultured marine bacteria that belong to the phylogenetic groups investigated in this study.

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