Contribution of natural microbial communities to the elimination of pathogenic viruses from aquatic environments
Final Report Abstract
Pathogenic viruses once released into the aquatic environment are subject to retardation and decay by a number of abiotic processes, some of which were studied by other partners in the consortium. They, however, also get involved as extrinsic participators in the microbial food web, and evidence for the significance of microbial antagonism in viral decay is increasing. A major result of our search for mechanism is that protozoan grazing on viruses takes place, and can substantially contribute to the elimination of ‘strangers’, i.e. viruses that do not find hosts for reproduction in situ. Furthermore, we showed that the impact of heterotrophic flagellates on viral density is related to the flagellates’ feeding behaviour: the model phage MS2 was actively removed by the suspension feeders Thaumatomonas coloniensis and Salpingoeca sp.; in contrast the actively raptorial grazer Goniomonas truncate had little impact on phage density. The decline of viral titre was demonstrated to be caused by ingestion rather than random absorption by both qPCR and locating protein fluorescently labelled MS2 inside the flagellates. These data demonstrate that protists in microbial food webs can have an important function in eliminating viruses. Another outcome of these experiments was that the presence of an active bacterial community also contributed significantly to virus decay. However, here the mechanisms behind still awaits to be unravelled. Investigations conducted in collaboration with the Berlin project on virus mobility and attenuation in the subsurface used sediment enclosures at the Marienfelde facility for simulating aquifers to elucidate the role of viral sorption to the sediment matrix for temporal retardation and survival. Results confirmed the strong sorption tendency of virus particles: spiking the model bacteriophage MS2 at a concentration similar to the total number of natural bacteriophages in surface water (i.e. 108 virus particles mL-1) resulted in a reduction of virus counts by two log units following the breakthrough of a pulse passing 1 m of natural sediments. Most interestingly, however, a simulated rain event, i.e. a pulse of deionized water, mobilized 103 to 105 virus active particles mL-1, which were initially sorbed to the sediment matrix. Sediments thus constitute temporal reservoirs for viruses. To distinguish between MS2 total number and active MS2 phages, a modified qPCR protocol was developed within the framework of the project, that will allow to quantify virus inactivation. Data evaluation with respect to long term survival and remobilization is still in progress in order to characterise microbially induced virus elimination in relation to individual microbial factors and physicochemical conditions. Finally, in cooperation with the projects on ‘Virus concentration’ and ‘Virus census and budgeting ’ we adopted the recently developed protein clusters (PCs) analysis for dissecting the first groundwater viral metagenomes from the specifically concentrated virus fraction. Comparative analysis with data from other aquatic environments revealed two major results. First, 94% of PCs in our metagenome represented double stranded (ds)DNA bacteriophage families of Myoviridae, Podoviridae and Siphoviridae which contrast to the previous description of the viral composition of two groundwater microbial metagenomes (generated from total biomass caught on a 0.22 µm filter) which were dominant by single stranded (ss)DNA viruses (72% and 21%). When compared with viral metagenomes from aquatic eukaryotes (e.g. fish), despite geographical proximity, the aquatic viral consortia were close to each other in terms of similarity However, within the viral metagenomes from different aquatic systems, differences in composition were huge, with the cluster richness and thus diversity of the groundwater viromes being highest.
Publications
- (2011) Pathogenic microorganisms and viruses in groundwater. Acatech (National Academy of Science and Engineering) Report – Nr. 6; pp. 300, ISSN: 2191-8481/ISBN: 978-3-942044-22-6
Krauss, S. and Griebler, C.
- (2013) Grazing of heterotrophic flagellates on viruses is driven by feeding behaviour. Environmental Microbiology Reports
Deng, L., Krauss, S., Feichtmayer, J., Hofmann, R., Arndt, H., and Griebler, C.
(See online at https://doi.org/10.1111/1758-2229.12119)
