Effects Of Several Anionic Ligands And Bacteria On DNA Adsorption By Soil Colloids And Minerals

Brown soil and Red soil were collected from temperate and subtropical area,respectively. The effects of two bacteria B. thuringiensis (G⁺) and P.putida (G^-), anionicligands such as phosphate, citrate and tartrate on the adsorption of DNA by soil colloidsand minerals were investigated. These studies are important for the understanding of thestate, behavior, binding mechanism with soil components and ecological significance ofDNA in soil environments. The main results are outlined as follows:1. Similar maximum adsorption of DNA by B. thuringiensis and P. putida wereobserved at pH 5.5 and pH 7.0. The affinity of DNA on P. putida was higher than that on B.thuringiensis at both pH values. The amount of DNA adsorbed by the two bacteria wasdecreased with the increase of pH.2. The two bacteria exerted different effects on the adsorption of DNA by soil colloids.The maximum adsorption of DNA on Brown and Red soil colloids was enhancedsignificantly while the affinity of colloids for DNA was decreased after the introduction ofB. thuringiensis. The addition of P.putida increased DNA adsorption on Brown soilcolloids by 88.4～90.7%, decreased the affinity of organic colloid for DNA and increasedthe affinity in inorganic colloid system. For red soil colloids, P.putida lowered theadsorption capacity of DNA in organic colloid system, enhanced the affinity of organiccolloid for DNA and reduced the affinity in inorganic colloid system.3. DNA adsorption on montrnorillonite was decreased after the introduction of B.thuringiensis and increased after the addition of P. putida. The two bacteria decreased theaffinity of montrnorillonite for DNA. B. thuringiensis enhanced DNA adsorption onkaolinite and goethite, while depressed their affinity for DNA. The presence of P. putidaalso raised the maximum adsorption of DNA on kaolinite and goethite at both pH values,but the affinity for DNA was increased at pH 7.0 and reduced at pH 5.5.4. Sequential desorption of DNA by Tris-HCl/NaAc-HAc buffer, NaCl and phosphatebuffer from bacteria, soil colloids and minerals as well as their bacterial compositesrevealed that electrostatic interaction is the major mechanism for the adsorption of DNAby bacteria. As for soil colloids and minerals, after the introduction of bacteria, theproportion of DNA adsorption by electrostatic force was increased and the interaction ofligand exchange was decreased. In kaolinite system at pH 5.5, the proportion ofelectrostatic interaction and ligand exchange were both increased as compared to the nobacteria system.5. Phosphate, citrate and tartrate depressed DNA adsorption on brown soil colloidsand goethite in the range of 0～200 or 0～80 mmol L^-1 at pH 7.0, and the inhibitory efficiency was in the order of phosphate＞citrate＞tartrate. As for montmorillonite, thethree ligands showed similar efficiencies in suppressing DNA adsorption. In kaolinitesystem, phosphate, citrate and tartrate showed a suppressive effect on DNA adsorption atlower concentrations (0～10 mmol L^-1) and an enhancing impact at relative higherconcentrations (10～200/80 mmol L^-1), with a higher promotive efficiency by phosphatethan by citrate and tartrate. It is assumed that the inhibition exerted by anionic ligands onDNA adsorption by brown soil colloids is mainly attributed to the presence of largeamount of 2:1-minerals.6. Phosphate and citrate depressed DNA adsorption by red soil colloids at lowerconcentrations (0～10/20 mmol L^-1) and enhanced the adsorption at higher concentrations(10～200/20～80 mmol L^-1) at pH 5.5. The two ligands always inhibit the binding of DNAwith goethite and promote the adsorption of DNA on kaolinite and montrnorillonite.Tartrate exhibited a steady stimulative effect on DNA adsorption by red soil colloids andminerals studied in the range of 0～80 mmol L^-1. The promotive efficiency of three ligandson DNA adsorption by kaolinite and montmorillonite followed the order of tartrate＞phosphate＞citrate.These findings indicated that the inhibition of DNA adsorption on redsoil colloids by ligands at low concentrations may be governed by the presence of ironoxids and the increase of DNA adsorption at higher ligand concentrations may be ascribedto the high amount of kaolinite in Red soil.7. The effect of addition order between ligands and DNA on DNA adsorption wasdissimilar in different system. At pH 7.0, the amount of DNA adsorbed by brown soilcolloids and goethite was greater when DNA was introduced before the addition of ligands.As for montmorillonite and kaolinite, DNA adsorption was greater when ligands wereintroduced before the addition of DNA. At pH 5.5, the amount of DNA adsorbed byorganic red soil colloids and goethite was greater when DNA was introduced before theaddition of ligands especially for phosphate and citrate. On the contrary, greater amountsof DNA molecules were adsorbed by inorganic red soil colloids, montrnorillonite andkaolinite when DNA was added after the introduction of ligands.