FIG. 2.

Secondary CsCl gradients containing bis-benzimide were used to disentangle the effects of ¹⁵N incorporation and G+C content on the buoyant density of DNA from targeted primary gradient fractions. The total number of 16S rRNA genes in each gradient fraction was determined by quantitative PCR. The secondary gradients were loaded with DNA that had a buoyant density of 1.727 to 1.733 g ml⁻¹ in primary gradients. The symbols correspond to DNA from soil incubated in the presence either of artificial air (○) or of artificial air containing 99.8 atom% ¹⁵N₂ (•).