Estimation of the fate of dissolved DNA in thermally stratified lake water from the stability of exogenous plasmid DNA

The fate of the exogenous plasmid DNA, pEGFP, was traced in lake waters collected from the epilimnion and the hypolimnion during a period of marked thermal stratification. Both gel electrophoresis analysis and dot-blot hybridization analysis demonstrated that the seeded pEGFP was completely degraded in epilimnion water within 170 h of incubation, while no pEGFP degradation was observed in hypolimnion water, Removal of microorganisms from the lake waters of the epilimnion and hypolimnion attenuated the degradation of pEGFP but did not stop it. Addition of EDTA to the lake water samples resulted in complete termination of the pEGFP degradation and indicated the involvement of a dissolved enzyme for DNA degradation, A measurable release of phosphate during pEGFP degradation indicated the existence of DNA-mediated phosphate regeneration in epilimnion water. In contrast, no DNA degradation in hypolimnion water suggested the possibility that it is less nutritionally important in the hypolimnion, Transformability of the pEGFP was not altered in hypolimnion water during the incubation, which suggested stability of genetic information that gradually decreased in epilimnion water. Dissolved DNA (dDNA) is commonly observed in aquatic environments and is thought to be an important nutritional source for microorganisms. However, our results suggest that dDNA in the hypolimnion environment may not be so important as a nutritional source for microbes, but might be more important as a genetic pool for further natural transformation.