Fig 5.

Results of standard addition experiments to test for matrix effects in Vineyard Sound samples (a and d), North Pacific samples (b and e), and a Synechococcus culture (c and f). Top panels show fluorescence measured in DAPI-stained extracts with and without added polyP standard. Each Vineyard Sound and North Pacific sample was extracted once, and the extract was split into two equal aliquots, one of which was left unamended and one which was amended with various amounts of polyP standard. The expected values were calculated based on the fluorescence in the unamended split and the fluorescence expected from each quantity of polyP, as calculated using an external calibration. For the Synechococcus culture, one sample was homogenized and divided into 18 equal aliquots and polyP was extracted from each. Nine were amended with polyP standard, and nine were left unamended; expected values were calculated from the mean fluorescence of the unamended aliquots and the fluorescence expected from the amount of standard added, based on an external calibration curve.