The ecological role of volatile mediated interactions belowground

Abstract

In the porous network of soil, microbes are unevenly distributed. Interactions between soil (micro-) organisms that are physically separated could be mediated by volatile organic compounds (VOCs). VOCs are small, partially very smelly, molecules that can diffuse through air- and water-filled soil pores. Microbes, similar to plants, produce a diverse set of VOCs. The importance of these compounds in communication and competitiveness between microbes and plants in soil is increasingly recognized. However, our understanding on the relevance of VOCs-mediated interactions belowground is still limited. The aim of this thesis was to reveal novel insights into the ecological role of VOCs in microbial interactions and community dynamics in soil. A soil model system which more closely reflects conditions of soil environment in and around the rhizosphere was designed to study VOCs-mediated interactions between bacteria-bacteria, bacteria-fungi and plant-bacteria. Furthermore, novel aspects on the ecological role of VOCs in interactions of soil microorganisms such as protists were examined. Results of this thesis revealed that microbial interactions and shifts in the community composition strongly affect the volatile emission in soil. In this context, bacteria associated to the fungus can significantly influence the VOCs production and fitness of the fungal host. Moreover, within this thesis it was demonstrated that VOCs produced by microbes in the rhizosphere or plant roots can have a significant long distance effect on microorganisms in the surrounding nutrient-depleted bulk soil. For instance, VOCs released by bacterial interactions in the rhizosphere could stimulate the activity of distant starved bacteria. Furthermore, it was shown that plants can attract (beneficial) bacteria by root-VOCs. These results suggest that the rhizosphere effect might not be restricted to narrow zone – the few millimeters around the roots – but is further expanded by VOCs-mediated interactions. Interestingly, VOCs can also play a role as long-distance messenger in interactions between bacteria and protists. It was shown that bacterial VOCs such as terpenes affect protist activity and motility. This was mostly correlated to responses in direct feeding-interactions. Accordingly, bacterial VOCs could serve as signals for protists to find suitable prey. Overall, findings of this thesis provide novel information on the complexity of VOCs-mediated interactions in soil and contribute to our knowledge on the importance of VOCs-mediated (chemical) communication in ecosystem functioning belowground.