The roles of bacteria and fungi for CPOM degradation during stressor increase and release: A metatranscriptomic approach
Bacterial and fungal heterotrophs dominate microbial decomposer communities degrading coarse particulate organic matter (CPOM) in streams. Details on actively involved taxa and their interactions, enzymatic reactions and metabolic pathways in stream ecosystems are, however, largely unresolved yet and might vary depending on environmental factors. This project aims at elucidating the role of different fungal and bacterial groups and their particular role in enzymatic decomposition of CPOM, with and without stressors, using metatranscriptome sequencing.
The project addresses MH3 of the CRC, i.e. that functions recover faster than community composition to a pre-degradation stage, due to partial functional redundancy among taxa. More specifically, we hypothesise that the process of CPOM degradation is more resistant to stressors than bacterial and fungal community composition along gradients of single and multiple stressors, due to functional redundancy between these taxa. We expect that functions are shared between bacteria and fungi, and among different taxa within these microbial groups. Thus, if species or other taxonomic groups disappear following stressor exposure, their metabolic activity will be compensated for by other taxa. However, such compensatory mechanisms might be limited if a succession of taxa is necessary for the decomposition of CPOM, for example when bacteria depend on the initial fungal enzymatic activities to degrade particular CPOM constituents.
This could hinder a rapid recovery of functions. To test these hypotheses, we first need to determine the bacterial and fungal community composition contributing to the decomposition of leaf litter and the taxon-specific contribution by stable isotope probing (in collaboration with A02, A05) and amplicon and metatranscriptome sequencing in the absence of stressors. In the main AquaFlow and ExStream experiments, we will evaluate the effect of stressor increase and release for both fungi and bacteria with regard to the previously identified organisms and processes. Sampling will be performed in cooperation with Z02 according to a coordinated time-course in both the AquaFlow and ExStream experiments. In the ExStream experiments a fully factorial design will be used, including unstressed conditions, conditions of single and multiple stressors and conditions after stressor release. The limited number of AquaFlow flumes will make it necessary to achieve adequate replication by repeating runs over time.
Further, we will focus on the biotic interactions between fungi and bacteria concerning functional redundancy and recovery after stressor reduction. Accordingly, we will determine whether functions are more resistant and/or resilient than the community composition, and whether a succession of species with specific functions is necessary to restore the normal trajectory of CPOM degradation after stressor release. In a later phase of RESIST, the findings from these controlled experiments will be validated on samples taken from field studies using degraded and restored sections of the Emscher/Boye and Kinzig catchments.