FIG. 4.

Amount of Synechococcus DNA as a function of density as determined by CsCl centrifugation and quantitative PCR, using the Synechococcus rbcL gene as a proxy in field samples. The vertical lines correspond to the vertical lines in Fig. 3 and indicate the density of Synechococcus DNA containing only ¹⁴N or only ¹⁵N. (A) Data for the ambient population (T0) and for incubation without addition of a ¹⁵N-labeled nitrogen source (NTC). (B to F) Data obtained after incubation in the presence of 2 μmol N liter⁻¹ of (B) [¹⁵N]urea, (C) ¹⁵NH₄⁺, (D) ¹⁵NO₃⁻, (E) ¹⁵N-amino acids (AA), and (F) [¹⁵N]glutamic acid (GA). The data are ratios of quantities, which were calculated by dividing the measured amount of Synechococcus rbcL DNA in each fraction by the highest value measured for any of the fractions collected from a CsCl column.