Fig. S1. Amino acid sequence alignment of Prochlorococcus MIT9301 PtxD with previously characterized NAD-dependent phosphite dehydrogenases. Conserved amino acid residues involved in substrate binding or catalysis in P. stutzeri WM88 phosphite dehydrogenase (Fodor, 1997; Woodyer et al., 2003; 2005; Relyea and van der Donk, 2005) are marked in yellow. The G-X-G-X2-GX17-D motif characteristic of the Rossman fold of HA hydroxyacid dehydrogenases (Rossmann et al., 1974; Wierenga et al., 1985; Woodyer et al., 2003) is marked in purple.

Fig. S2. Growth of Prochlorococcus strain MIT9301 with different concentrations of P source. Prochlorococcus strain MIT9301 (axenic) was grown to mid-logarithmic phase in phosphate-containing Pro99 medium, washed on filters, then resuspended in Pro99 medium containing phosphite (left panel) or phosphate (right panel) as the sole P source at the indicated concentrations. Error bars represent the standard deviation of the mean relative fluorescence of duplicate cultures.

Fig. S3. Vertical nutrient distribution for the depth profiles of the North Pacific Hawaii Ocean Time Series HOT and Sargasso Sea Bermuda Atlantic Series BATS stations at the time the of sampling (October 2006). Concentration of phosphate (blue), and nitrate plus nitrite (red) are expressed as µmol kg−1. Data were obtained from Hawaii Ocean Time Series ( and the Sargasso Sea Bermuda Atlantic Series websites ( respectively.

Table S1. Best BLASTP hits for Prochlorococcus MIT9301 phosphonate/phosphite-related proteins in NCBI non-redundant protein database.

Table S2. Abundance and expression of Prochlorococcus phosphite and phosphonate genes in Sargasso Sea and North Atlantic pyrosequencing libraries.

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