Project A02

Effect of multiple stressors on the microbial ecosystem functions DOC degradation and biomass recycling in water and sediment

Hypothesis 1 Hypothesis 1 Hypothesis 1 ARC 2 AquaFlow ExStream Field studies Bacteria DOC degradation Nutrient cycling

Project Summary

Our research goals are to elucidate the effects of multiple stressors on microbial DOC degradation and biomass recycling, which play crucial roles for river ecosystem functioning. As a first step, we want to monitor the natural dynamics of these two processes in two lotic systems, the rivers Boye (Emscher catchment) and Bieber (Kinzig catchment), for a period of 2 years. We will survey the composition of the bacterial communities over time and determine the relative contribution of protists and of different bacteria to biomass recycling.

Alongside we will incubate samples with DOC, nitrogen and phosphorus to investigate factors that naturally limit DOC degradation in the two rivers. As a second step, we will study acute and long-term effects of multiple stressors on microbial DOC degradation and biomass recycling, whereby we focus on the effects of salt and temperature stress, both individually and in combination. Effects of salt and temperature stress will be studied at microcosm and mesocosm scales using the AquaFlow and ExStream installations. DOC degradation will be quantified with our recently developed Reverse Stable Isotope Labelling (RIL). This novel method allows for detecting minute amounts of DOC degradation as CO2 production down to 10 μg/l.

Biomass recycling will be investigated in microcosms by adding 13C-labelled living E. coli biomass or necromass to environmental samples and measuring degradation activity as 13CO2 evolution. Specific inhibitors will also allow us to distinguish the relative importance of bacteria and protists and to identify specific necromass-degrading bacteria via stable isotope probing (SIP). We will apply flow cytometry for absolute quantification of different microorganisms and 16S rRNA gene amplicon sequencing for determining the composition of bacterial communities. The project will provide novel insights into the effects of multiple stressors on the microbial turnover of DOC and nutrients in lotic ecosystems.

PhD topic(s)

Daria Baikova (University of Duisburg-Essen)

Effect of multiple stressors on microbial DOC degradation

Dissolved organic carbon plays an important role in biogeochemical cycling. It serves as the main source of organic nutrients and energy in freshwater systems. Microorganisms are the key players in DOC degradation, which is of major importance for the functioning of river ecosystems. Microbial degradation of DOC in rivers is often nutrient-limited and consumes dissolved oxygen, which is the dominant parameter influencing macroorganisms in aquatic ecosystems. Since sensitive macroorganisms respond greatly to changes in oxygen concentration, already small changes in DOC degradation will lead to significant ecosystem effects.

Microbial DOC turnover in rivers is studied to a limited extent and it is so far unclear how it is affected by elevated temperature and salt levels. This project contributes to the investigation of the effects of these stressors on bacterial communities and their function to degrade DOC. Firstly, the natural variation in DOC degradation in rivers Boye and Bieber will be determined in a 2-year monitoring project. The relevance of different stressors will be assessed by relating stressor-induced changes to natural environmental fluctuations due to seasonal changes. As bacterial mineralization of organic carbon leads to carbon dioxide release, the latter will be monitored to assess the efficiency of DOC degradation with the help of reverse stable isotope labelling (RIL). Additionally, the dynamics of absolute prokaryotic abundance and species composition using flow cytometry and 16S rRNA gene amplicon sequencing will be monitored. The limitations of degradation such as nutrient availability will be also investigated. Secondly, the acute and long-term response of DOC-degradation to the stressors temperature and salinity will be investigated in microcosms in isolation as well as in combination. Lastly, the effects of multiple stressors on DOC degradation and community composition will be investigated in the mesocosm experiments AquaFlow.

Contact: daria.baikova@uni-due.de

First Supervisor: Prof. Dr. Rainer Meckenstock (University of Duisburg-Essen, Aquatic Microbiology)
Second Supervisor: Dr. Verena Brauer (University of Duisburg-Essen, Aquatic Microbiology)
Mentor: Dr. Thomas Held (Arcadis Germany GmbH)


Una Hadziomerovic (University of Duisburg-Essen)

Effect of multiple stressors on microbial biomass recycling

Aquatic ecosystems regulate vital environmental services like water purification. The functioning of an aquatic ecosystem is to a considerable extent driven by microorganisms, which recycle nutrients and carbon from dead biomass (necromass) within the so-called microbial loop.

The aim of this project is to investigate the effects of the stressors salt and temperature on a specific process within the microbial loop, namely the recycling of bacterial necromass, in river ecosystems. We will study the natural extent of bacterial necromass degradation in two rivers during a 2-years long monitoring project. To assess the acute and long-term stress response of bacterial necromass recycling we will investigate the effect of different intensities of the stressors salt and temperature in microcosm experiments as well as in the mesocosm experiments AquaFlow and ExStream.

Bacterial necromass recycling will be measured by adding 13C-labelled E. coli necromass to environmental samples in microcosms. The degradation activity will be measured over time as 13CO2 evolution with a Picarro G2131-i Isotope and Gas Concentration Analyzer with Auto Mate FX. To determine whether bacteria or protists are responsible for bacterial necromass degradation, specific growth inhibitors will be used. Stable isotope probing (SIP) will be applied to identify necromass degraders based on the incorporation of the 13C into the DNA of the necromass degraders during growth.

Contact: una.hadziomerovic@uni-due.de

First Supervisor: Prof. Dr. Rainer Meckenstock (University of Duisburg-Essen, Aquatic Microbiology)
Second Supervisor: Dr. Verena Brauer (University of Duisburg-Essen, Aquatic Microbiology)
Mentor: Dr. Thomas Held (Arcadis Germany GmbH)

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