Universität Tübingen
Investigation of the importance of trophic interaction and microbial food webs in aquifers for the natural attenuation potential of groundwater
Abstract
Heterotrophic protists represent a very important trophic link within food chains in different ecosystems, as they shuttle energy and organic matter from prokaryotes to higher trophic levels. In groundwater, they may even themselves represent the most relevant higher trophic level due to the scarcity of higher organisms. But their functional relevance in groundwater ecosystems and their influence on groundwater natural attenuation is only poorly understood. There is a severe lack of knowledge on the biodiversity and activity of natural protozoan assemblages in aquifers, most of all in anaerobic and contaminated zones. Furthermore, also the importance of other microeukaryotes such as fungi as potential contaminant degraders has not been systematically addressed yet. Partly, this can be attributed to a lack of adequate PCR and fingerprinting approaches for protists in aquifers. Within this thesis, essential tools for the molecular characterisation of microeukaryotic communities in aquifers were successfully established. The suitability of distinct eukaryote-targeted primer pairs were validated and found to differ widely in their ability to detect over-all protistan diversity. By in silico predictions, fingerprinting and sequencing of microeukaryote amplicons from hydrocarbon-contaminated aquifer sediment DNA, I could show that the Euk20f/Euk516r primer set in combination with Bsh1236I digestion is best suited for the recovery of diverse protistan 18S rDNA lineages with T-RFLP analysis. These newly developed molecular assays were used to assess protistan communities in distinct redox-zones of a tar-oil contaminated aquifer. A diverse microeukaryote community was detected with profound depth-related changes in diversity and composition hinting at a coupling to local biogeochemical and microbial parameters. Ciliates and cercozoa were found predominantly in the upper, highly BTEX contaminated part of the aquifer, where prokaryotic prey abundances were highest. Kinetoplastid nanoflagellates dominated protozoan communities in the sulphidogenic gradient zone, the ‘hot-spot’ of bacterial BTEX degradation, and the PAH contaminated strata below. Therefore, detected protozoan groups hint at an active microbial loop, which may influence biodegradation in situ to differing extents (quantitatively and qualitatively) in respective zones. Additionally, the detection of a diverse fungal community dominated by yeasts may also indicate a direct involvement of microeukaryotes in contaminant turnover, most of all, as the majority of fungi identified was related to isolates known for aerobic degradation of aromatic hydrocarbons. Their activity and utilisation of substrates under anaerobic conditions, however, remains to be elucidated. The applicability of DNA-SIP to groundwater protists was evaluated by feeding of 13C labelled prey to a protistan predator, revealing critical detection limits. My established assays were tested on DNA extracts of a SIP experiment addressing toluene degradation under sulphate reduction. These first results show that SIP can be applied to anaerobic contaminant degrading food chains and make further approaches in SIP very promising. The cultivation and isolation of groundwater protists, a prerequisite to unravel the potential role and relevance of distinct groups in groundwater ecosystem functioning, proved to be problematic for contaminated sediment samples due to excessive growth of fungi. Aerobic and anaerobic enrichment cultivations further emphasised the need of refined anaerobic cultivation techniques to be established. To summarise, molecular tools for the characterisation of protistan communities in contaminated aquifers were successfully established. At an exemplary site, I show that a protistan diversity far greater than previously recognised resides even in contaminated groundwater sediments, and that local protistan populations display similarly significant spatial distinctions in correlation to redox and contaminant scenarios as previously observed for Bacteria. The prospective further application of such molecular tools to study food webs and carbon flow in groundwater ecosystems may further add to an understanding of factors enhancing or limiting biodegradation in situ.
Author and committee
dc:creator, dc:contributor.*- Author
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- Euringer, Kathrin
Identifiers
dc:identifier.*- Identifier
- hdl:10900/49237