Bacteriophages participate in soil life by influencing bacterial community structure and function, biogeochemical cycling and horizontal gene transfer. Despite their great abundance, diversity, and importance in microbial processes, they remain little explored in environmental studies. The influence of abiotic factors on the persistence of bacteriophages is now recognized; however, it has been mainly studied under experimental conditions. This study aimed to determine whether the abiotic factors well-known to influence bacteriophage persistence also control the natural distribution of the known DNA bacteriophage populations. To this end, soil from eight study sites including forests and grasslands located in the Attert River basin (Grand Duchy of Luxembourg) were sampled, covering different soil and land cover characteristics. Shotgun metagenomics, reference-based bioinformatics and statistical analyses allowed characterising the diversity of known DNA bacteriophage and bacterial communities. After combining soil properties with the identified DNA bacteriophage populations, our in-situ study highlighted the influence of pH and calcium cations on the diversity of the known fraction of the soil DNA bacteriophages. More interestingly, significant relationships were established between bacteriophage and bacterial populations. This study provides new insights into the importance of abiotic and biotic factors in the distribution of DNA bacteriophages and the natural ecology of terrestrial bacteriophages.

<p>Bacteriophages are numerous, tremendously diverse and ubiquitous in the environment. Since the 1960s, bacteriophages have been proposed as new tracers to investigate the hydrological processes in addition to conventional tracers (i.e. isotopes, salts, dyes). Their dynamic into water (i.e. surface water, groundwater) have been well studied. However, the soil compartment known for its important microbial activity, have been few characterized in terms of bacteriophage diversity. Hence, in the present study, the objective is to investigate the transport of soil viral population from the soil matrix to the soil water compartment. This mobilization from the soil matrix is mainly driven by the adsorption/desorption mechanisms to which bacteriophages are subjected. Therefore, in order to understand the dynamics of the bacteriophage population, both soil and soil water were sampled from the Weierbach forest, located in the Attert River basin (Grand-Duchy of Luxembourg) at the topsoil level (i.e. 0-20 cm) over a period of one month. Due to a lower abundance of the microbial population in soil water, an enrichment method was carried out to increase the concentration. Subsequently, a shotgun metagenomics analysis was performed on the soil and soil water samples to obtain the DNA sequences, which were then sorted using bioinformatics and statistical analyses, allowing ultimately the identification of the viral populations. The moving of the bacteriophage populations from the soil to the soil water provides information on their transport capacity, in particular by taking into account environmental conditions such as air and soil temperatures, precipitation, soil humidity, soil pH, etc.  </p><p> </p><p><strong>Key words</strong>: bacteriophages, soil, water, transport, environmental conditions</p>

Hydrological tracers, commonly used for characterizing water flow paths and sources, do not often meet all the requirements of an ideal tracer. Trans-disciplinary approaches are advocated as the way forward to enlarge the number of tracers available for investigating hydrological processes. Since the 19th century, hydrological tracers have been increasingly used, particularly in underground areas. The tracer toolbox at hand includes a large variety of options, including fluorescent dyes, isotopes, salts or bacteriophages, with each tracer offering specific qualities and complementarities. While their potential for hydrological studies has been studied in karstic environments since the 1960s, bacteriophages remain insufficiently understood. According to the selection methodology used in this review, more than thirty experiments have been listed, involving in total around seventeen different bacteriophages. These have facilitated the investigation of groundwater, surface water (i.e., river, lake and marine water), wetland and wastewater hydrological processes. The tracing experiments have also highlighted the possible interaction between bacteriophages and the surrounding environments. Bacteriophages have successfully helped researchers to understand the water flow within watersheds. Certain advantages, such as the sensitivity of detection, the ease of producing high concentrations of bacteriophages to be injected, their specificity for a host and their non-pathogenicity for human and animal cells, make bacteriophages appreciable tracer candidates for tracing experiments. However, the adsorption process or environmental factors such as temperature, pH and UV light considerably impact the fate of bacteriophages, thereby leading to an attenuation of the phage signal. Considering both the flaws and the qualities of bacteriophages, their use as hydrological tracers requires new insight and further discussions regarding experimental tracing conditions.