In Phase I project A06 demonstrated the strength of microbial communities and in particular of protist communities for water quality analyses. We further found additive effects of salinity and temperature stress for some taxa while no stressor interactions occurred in other taxa. On the community level salinity acted as a significant stressor altering food selection and temperature as a significant stressor for growth and cryptic species turn-over in laboratory settings. Our results from Phase I suggest a decreasing diversity in communities when multiple stressors act at high levels and, most interestingly, we demonstrated a differential effect of salinity on the food preference of protists with respect to Proteobacteria and Actinobacteria. In Phase II project A06 will focus (i) on the underlying mechanisms and relationships that lead to the protist community responses that were characterized in Phase I – particularly the ecophysiological effects of varying salt concentrations on food selection and the associated prevalence of either Proteobacteria or Actinobacteria and (ii) on the effects of drought as well as recovery from drought. Experiments in Phase II will focus on community responses and specifically on recovery from stressors. In this context, we will investigate functional redundancy between protist taxa and its capacity to buffer and thus stabilize food web interactions. In particular, we will analyse stressor-induced shifts in protist food selection for single protist strains as well as for communities in order to identify differential effects of stressors on distinct taxa and link these to the community grazing effect. Further, drought and recovery from drought will be a focus of the project. We expect a decreasing diversity during drought and sequential recovery of diversity and relative abundance of different functional protist groups. In particular, we expect a dominance of small and fast growing (r-strategist) flagellate taxa during early stages of recovery. In workpackage I (WP I), we will address the ecophysiological basis of salinity stress responses in protist food selection. In particular, we will address whether food selection shifts gradually with increasing stressor or alternatively responds when a threshold value is exceeded. These insights are basic for the implementation of salinity stress in microbial food web models and for a mechanistic understanding of food web responses to stressors. For the strain-specific responses, we will select taxa from the Central Collection of Algal Cultures at UDE (CCAC) according to the results of Phase I that will be studied in more detail. In workpackage II (WP II), we will investigate differences between stressor onset and recovery with special attention on community composition and the development of food selection in response to the natural variation of stressors along ecological gradients in streams in collaboration with A02. The analyses will use the taxon-specific FISH-probes and protocols for the analysis of food vacuole content in protists developed in Phase I. Aside from food selectivity we will also analyse shifts of protist community composition along ecological gradients. Workpackage III (WP III) will then transfer these findings to the analysis of effects of drought and recovery from drought on natural communities in collaboration with A02, A03, A21 and A23. These experiments will focus on the field flumes as well as on field sites along the Boye. The analysis will include surface biofilm communities, sediment communities at different sediment depths as well as the stream water communities. Community development as well as chemical and physical parameters will be analysed during the drought and after rewetting. The field flumes will be used to simulate a drought on streams in a replicated design and allow for a controlled field experiment. In collaboration with A02 and A03 we will analyse the protist community development during and after drought in the sediment and the biofilm as well as downstream effects on stream and biofilm communities.

Lisa Boden (University of Duisburg-Essen)

Stressor modulated community responses and functional redundancy of microbial predator-prey interactions

Microbial predator-prey interactions play a crucial role in aquatic food webs. Therefore, this project investigates the impact of multiple stressors on food selectivity of predators. The project will focuse on protists feeding on bacteria but also investigate invertebrates grazing on protists.

Using Fluorescence in situ hybridization (FISH) uptake of target bacteria by protists can be visualized and quantified and the effect of abiotic stressors on food selectivity of predators can be investigated via direct microscopy. Further, amplicon data will be used to analyze gut content of invertebrates and to determine the composition of both prokaryotic and eukaryotic microbial communities.

Contact: lisa.boden@uni-due.de

First Supervisor: Prof. Dr. Jens Boenigk (University of Duisburg-Essen, Biodiversity)
Second Supervisor: Prof. Dr. Bánk Beszteri (University of Duisburg-Essen, Phycology)
Mentor: Dr. Karin Pfandl (Tyrolpath Obrist Brunhuber GmbH)