Adsorption of Extracellular Chromosomal DNA and Its Effects on Natural Transformation of Azotobacter vinelandii

To better understand the influence of environmental conditions on the adsorption of extracellular chromosomal DNA and its availability for natural transformation, the amount and conformation of adsorbed DNA were monitored under different conditions in parallel with transformation assays using the soil bacterium Azotobacter vinelandii. DNA adsorption was monitored using the technique of quartz crystal microbalance with dissipation (QCM-D). Both silica and natural organic matter (NOM) surfaces were evaluated in solutions containing either 100 mM NaCl or 1 mM CaCl2. The QCM-D data suggest that DNA adsorbed to silica surfaces has a more compact and rigid conformation in Ca2+ solution than in Na+ solution and that the reverse is true when DNA is adsorbed to NOM surfaces. While the amounts of DNA adsorbed on a silica surface were similar for Ca2+ and Na+ solutions, the amount of DNA adsorbed on an NOM-coated surface was higher in Ca2+ solution than in Na+ solution. Transformation frequencies for dissolved DNA and DNA adsorbed to silica and to NOM were 6 x 10(-5), 5 x 10(-5), and 2.5 x 10(-4), respectively. For NOM-coated surfaces, transformation frequencies from individual experiments were 2- to 50-fold higher in the presence of Ca2+ than in the presence of Na+. The results suggest that groundwater hardness (i.e., Ca2+ concentration) will affect the amount of extracellular DNA adsorbed to the soil surface but that neither adsorption nor changes in the conformation of the adsorbed DNA will have a strong effect on the frequency of natural transformation of A. vinelandii.