Effect of heavy metal cations on the fate of extracellular DNA adsorbed and bound on clay minerals.

The presence of high-valent metal cations on clay mineral surfaces is hypothesised to induce conformational changes in the secondary and tertiary structure of the DNA molecule adsorbed and bound onto clays, defined as M-conformation, and its condensation. The hypothesis that these reversible phenomena could enhance the resistance of DNA to enzymatic degradation strongly encourages the studies on the effects of heavy metal contamination in clay rich soils on the fate of extracellular soil DNA (eDNA). This lack of knowledge is relevant concerning the ecological role of soil eDNA in terms of persistence and availability for bacterial horizontal gene transfer by natural transformation, and as substrate for biofilm formation (Pietramellara et al., 2009). We assessed the effect of ionic Fe polymers on the adsorption (loosely versus tightly bound) of eDNA on clay minerals (dirty clay), that represent the mineral fraction of soil colloids. DNA-clay complexes (dirty eDNA/pure clay versus dirty eDNA/dirty clay) were analysed in terms of strength of DNA-clay interaction (adsorption isotherms). Challenging to conduct DNA-clay interaction studies in conditions as natural as possible, dirty eDNA was extracted by simulating natural cell lysis without any purification, in order to avoid possible bias coming from DNA purification processes.