Microbial communities respond to stressor-driven events, by changing their structure and activity, as shown by multiple rRNA gene-based studies. In this proposal we will focus on how the bacterial and viral microdiversity is affected by degradation under the influence of different stressors, and on how it recovers. A22 is built on the hypothesis that stressors modulate bacterial microdiversity in river sediments, by direct or indirect selection of genomovars with traits favourable for survival under the respective stressor conditions. Viral microdiversity will be driven by the necessity to successfully infect and counteract the anti-viral abilities of the host. While microdiversity can decrease during the degradation phase, at recovery will return to initial levels, but different genomovars will dominate (asymmetry of paths). To test these hypothesises, we will turn to metagenomics, supplemented in some cases with culture-dependent work. We will evaluate the microdiversity based both on single nucleotide variants (SNVs) and on structural variants (SVs). Various stressors will be investigated, either alone or in combination, by relying on data from several highly parallelized mesocosm experiments (the ExStream setup) from Phase I: low or high flow velocity, temperature and salt. In addition, the effects of drought will be investigated in ExStream/SIGMA experiments from Phase II, as well as in field streams, via sampling through the drying period in three consecutive years. All in all, we will explore changes in microbial microdiversity across different stressor types and intensities, both at degradation and recovery.