Free extracellular DNA has been retrieved in many environments, both aquatic and terrestrial . Naked DNA is actively excreted by growing cells, depending on biotic and abiotic factors [2–4], or is passively released in the environment by decaying cells. In hypersaline systems the water availability is reduced with the consequence of inferring water stress to cells. The osmotic stress in particular affects cell turgor and membrane integrity, leading to death by osmotic lysis of cells not adapted to hypersaline conditions , and in turn to the release of the cellular content, including nucleic acids.
Free dissolved DNA is used by microorganisms as source of N, P and C [6, 7], and in the deep-sea environment has been hypothesised to constitute a key trophic resource, substantially contributing to P cycling . Other than a nutrient source, dissolved DNA may have a genetic function as a source of genes acquirable by natural transformation. In terrestrial and aquatic environments several bacterial strains have been discovered to be naturally competent, i.e. to have the capacity to acquire exogenous naked DNA . The acquisition of new genetic traits by horizontal gene transfer can constitute an evolutionary strategy for the selection of natural microbial communities . In harsh and stressful conditions, in particular, gene exchange and rearrangements are estimated to increase in order to promote genome plasticity, increasing DNA repairing rates and evolutionary adaptation mechanisms [9, 10].
In natural ecosystems the majority of extracellular DNA is converted in deoxyribose, inorganic orthophosphate, purines and pyrimidines by the enzymatic hydrolytic action of nucleases, present in most of the microbial habitats [11, 12]. Besides the biological degradation, DNA is a chemically unstable molecule that decays spontaneously mainly through hydrolysis and oxidation . Several physical and chemical factors can moreover compromise the integrity of naked DNA molecules once they are released by cells. Once free in the environment, DNA fragments are no longer preserved by cellular DNA repair mechanisms and, even if not severely degraded, they accumulate environmentally inflicted damages. Despite all these factors, extracellular DNA has been demonstrated to be preserved in soil, sediments, freshwater and seawater for different time periods , and even geologically ancient DNA has been retrieved from fossil materials [14, 15]. Dell'Anno and Danovaro  estimated that the deep-sea sediments constitute the largest reservoir of extracellular DNA in the Earth's oceans, with 0.50 ± 0,22 Gt of extracellular DNA contained in the first 10 cm of sediments, and its residence time, resulted by the balance of release and degradation, is 9.5 years. In particular, in the sediments underlying the deep anoxic hypersaline lake l'Atalante Danovaro et al.  retrieved the highest concentration of extracellular DNA reported in a natural environment.
In hypersaline environments salt has been shown to have a stabilising effect on nucleic acids, protecting biological macromolecules against heat degradation [17, 18]. On the other side, the reduced water activity of a salty environment has consequences on DNA conformation. The reduction of the hydration of the DNA molecules decreases the stabilisation of the structure that in high water activity environments is conferred by weakly bound water molecules .
The aim of this work is to study the persistence of extracellular DNA and its potential for gene exchange in extreme environments characterised by hypersaline and anoxic conditions. The deep hypersaline anoxic lakes of the Eastern Mediterranean Sea are unique deep-sea habitats originated from the dissolution of buried salt deposits emerging at the topography due to the strong faulting activity of the area. They are characterised by a salinity above 30%, absence of light, elevated pressure, variable pH values and ionic compositions. The sharp density difference between brines and normal sea water acts as a barrier, avoiding oxygen exchange, therefore the brines become oxygen-free and rich in hydrogen sulphide. Despite these harsh conditions, the brines that fill the lakes contain highly adapted active microbial communities . The brines are separated from the upper seawater by a steep interface layer with salinity values ranging from seawater to the brine physio-chemical parameters. This layer is an enrichment phase for complex microbial networks rich in taxonomical and functional biodiversity that are stratified along the depth and salinity profile .