Phages as vectors and indicators for biological information: Phage transport and phage-mycosphere interactions

Bacterial viruses, also known as phages, are intrinsic components of the Earth’s Critical Zone (CZ). Together with diverse communities of bacteria and fungi, they occupy habitats of the CZ extending from the vegetation canopy, through the soils and into the aquifers. In this thesis, I aimed to study the transport of phages in the upper CZ and their interactions with non-host soil bacteria and fungi to reveal their role in regulating the CZ microbial life. To reflect transport processes of CZ-inhabiting phages in soil tracers are highly useful. Thus, in the first study, I evaluated the transport efficiency and particle intactness of marine tracer phages passing through soil. Marine phages were selected as tracer, because they are non-pathogenic, non-toxic, naturally absent and thus non-multiplying in the terrestrial subsurface. I found that the marine phages PSA-HM1 and PSA-HS2 retained high phage particle intactness in contrast to commonly used Escherichia virus T4. This suggests their potential as particle tracers to mimic the transport of (bio-) colloids of similar traits in soil. Soil in the CZ is often unsaturated and restricts mobility of microorganisms. Fungi bridge unsaturated zones in soil and hence provide network for microbial transport. In the second study, I developed a hyphosphere model system mimicking unsaturated soil environment, and reported on the ability of hyphal-riding bacteria to co-transport lytic phages and to utilize phages as “weapons” for improved colonization of water-unsaturated habitats. As the findings emphasize the importance of hyphal transport of bacteria and associated phages, in the third study, I developed a hyphae-assisted approach and isolated five soil bacteria able to co-transport phages. In analogy to invasion frameworks in macroecology, the hyphosphere model system with bacteria and co-transported phages can be useful models to simulate processes of biological invasion at micro-scale. In the fourth study, I investigated dormant phages (i.e. prophages) that are widespread in the CZ and can be induced under environmental stress. I found that volatile fungal metabolites can act as triggers for prophage induction and may exert long-distance manipulation of prophage activity thereby affecting microbial community and nutrient cycling in soil. Altogether, the findings may help to elucidate transport processes of phages in the CZ and to reveal the role of phages in the CZ microbial ecosystem. Approaches (e.g. phage as tracers) and findings (e.g. phage-bacterial co-transport) may also serve as useful tools for testing hypothesis in other disciplines, such as hydrogeology, invasion ecology and chemical ecology