Relative abundances of proteobacterial membrane-bound and periplasmic nitrate reductases in selected environments

Abstract

Dissimilatory nitrate reduction is catalyzed by a membrane-bound and a periplasmic nitrate reductase. We set up a real-time PCR assay to quantify these two enzymes, using the narG and napA genes, encoding the catalytic subunits of the two types of nitrate reductases, as molecular markers. The narG and napA gene copy numbers in DNA extracted from 18 different environments showed high variations, with most numbers ranging from 2 x 10(2) to 6.8 x 10(4) copies per ng of DNA. This study provides evidence that, in soil samples, the number of proteobacteria carrying the napA gene is often as high as that of proteobacteria carrying the narG gene. The high correlation observed between narG and napA gene copy numbers in soils suggests that the ecological roles of the corresponding enzymes might be linked.

FIG. 1.

Abundance of 16S rRNA, narG, and napA genes expressed as gene copy numbers per ng of extracted DNA, analyzed from three independent replicates per site. Error bars indicate standard deviations. S indicates significant differences (P < 0.05) between the narG and napA gene copy numbers (Student's t test) for each site.

FIG. 2.

Correlation between narG and napA gene copy numbers in the soil and sediment samples (white triangles) and in water and biofilm samples (black circles). Each replicate of the 18 selected environments is plotted.