Stabilization of extracellular DNA by transient binding to various soil surfaces

Soils are chemically complex and spatially heterogeneous environments that offer a variety ofDNA-adsorbing surfaces such as sands, silts, clays, and complex macromolecules such as humic acids and remnants of plant tissues. Different soil types vary in compositionwith respect to these surfaces, influencing the specific adsorption ofDNA(Ladd et al. 1996). After adsorption to surfaces of quartz, feldspar, oxides or hydroxides containing heavy minerals, humic acids and clay, the DNA becomes partially resistant to degradation by extracellular nucleases (Aardema et al. 1983; Romanowski et al. 1991; Khanna and Stotzky 1992; Crecchio and Stotzky 1998). DNA fragments adsorbed on these surfaces have been shown to transform bacteria in vitro (Lorenz and Wackernagel 1990; Stewart et al. 1991; Khanna and Stotzky 1992; Paget et al. 1992; Romanowski et al. 1993; Pietramellara et al. 1997; Demanèche et al. 2001). The adsorption of DNA by soil particles is therefore important since it may extend the time period that extracellular DNA is available for natural transformation of bacterial communities (Gallori et al. 1994; Nielsen et al. 1997a,b). Table 7.1 summarises the results of studies that have examined the effects of soil particulate material and humic acids on the biological activity of DNA. These studies were carried out using purified DNA, DNA present in cell lysates from dead bacteria, or DNA naturally released from living donor bacteria (Khanna and Stotzky 1992; Gallori et al. 1994; Pietramellara et al. 1997; Nielsen et al. 2000a,b, 2004; Demanèche et al. 2001). Both prokaryotic and eukaryotic DNA have been used as sources of transforming DNA, as reviewed by Nielsen (2003) and Nielsen et al. (1998, 2001). Here, studies investigating the interaction of DNA (present in pure solution, cell lysates, and live donor cell suspensions) with soil components (present as pure substances, in mixtures, and as natural soil samples) are examined. The accessibility of adsorbed DNA molecules to soil microbes is discussed, with focus on their possible role as contributors to natural transformation processes in the open environment. In addition, the interaction of extracellular proteins (enzymes) with various soil components is briefly discussed.