VAAM-Jahrestagung 2011 Karlsruhe, 3.–6. April 2011

esistance exists as a continuum between highly sensitive and resistant.Acquisition of resistance and subsequent proliferation of resistant strainswas demonstrated on model particles as well as in solution. Loss ofsusceptibility to phage infection was associated with a reduction in thestrains' ability to metabolize various carbon sources. Our work with thismodel system indicates that phage-host dynamics are extremely complex. Ifthe observed patterns are valid for indigenous marine phage-host systems,they imply that i) continuous ranges of infectivity and susceptibility toinfection exist in phage-host system, ii) in turn, that the concept of virushostsystem has limited use, and iii) that functional diversification ofbacterial hosts occurs at the clonal level. Thus far, the ecologicalconsequences of this intriguing complexity are poorly understood.EMV029Aggregate-colonizing microbial communities - acomparison of marine vs. freshwater systemsM. Bizic-Ionescu* 1,2 , B. Fuchs 1 , R. Amann 1 , H.-P. Grossart 21 Department forMolecular Ecology, Max Planck Institute for MarineMicrobiology, Bremen, Germany2 Department of Stratified Lakes, Leibniz Institute of Freshwater Ecologyand Inland Fisheries, Stechlin, GermanyWe observed the formation of particulate organic matter (POM) aggregatesafter a dinoflagellate bloom in coastal waters at Helgoland Roads in autumn2009. These structures are known to function, both, as a sink for colloidalparticles that adhere to the aggregates during the process of sinking as wellas a source for dissolved substances due to rapid decomposition. The latter isgreatly due to the dense colonization by microorganisms. Such an enhancedmicrobial activity affects the biochemical cycles of carbon, nitrogen,phosphorus, iron and other potential nutrients in the respective aquaticenvironment. We analyzed samples applying CAtalyzed ReporterDeposition-Fluorescence In Situ Hybridization (CARD-FISH) andepifluorescence microscopy, using different oligonucleotide probes on sizefractionated plankton, >10 μm, 10-3 μm, and 3-0.2 μm. The total number ofcells in the 3-0.2 μm fraction decreased after the algal bloom beforestabilizing after 2 months. Despite this trend the total number remained highand the general ratio between Archaea and Bacteria was maintained. A morein-depth observation showed that different phyla followed different trends asa reaction to the environmental conditions. For example at the group level,in the 3-0.2 μm fraction, Alphaproteobacteria appeared to be the mostdominant ranging between 30-50% of the total community. The relativeabundances of the Bacteroidetes in this fraction decreased after the algalbloom (from 37% to 12%) whereas Gammaproteobacteria increased (from9% to 14%). In case of particle associated bacteria (>10 μm) the relativeabundance of Gammaproteobacteria (reaching 40%) was significantly higherthan that in the free-water phase (reaching 25%). Their number on theaggregates decreased after the bloom whereas the number of Roseobacterincreased. SAR11 as well as Crenarchaeota and Actinobacteria clearlypreferred the non-attached phase. These data are to be compared with similaranalysis on freshwater samples. Lake samples were chosen based on aDenaturing Gradient Gel Electrophoresis (DGGE) analysis which showedseasonal fluctuations in both the epilimnion and the hypolimnion of LakeStechlin similar to that observed in the marine system. Preliminary resultsshow that in the fraction smaller than 3 μm Gammaproteobacteria consist aminor part of the community (~1%) while the Betaproteobacteria are muchmore significant than in the marine system, reaching 11% in the epilimnionand 16% in the hypolimnion of Lake Stechlin.EMV015The bacterial community in the digestive tract of thesmall aquatic crustacean Daphnia magnaH. Freese*, B. SchinkDepartment of Biology, University Konstanz, Konstanz, Germanywhether they compete for food. The aim of this study was to characterise theintestinal microbial community and to estimate if Daphnia have aspecialised stable gut microbiota or if the community just reflectssurrounding bacteria. Therefore, the intestinal microbial community of D.magna clones was analysed via 16S rDNA clone libraries. To investigate thestability of their microbiota, Daphnia were incubated under differentconditions (food sources, exposure to defined bacteria) while changes in theintestinal community composition were followed by T-RFLP. The D. magnamicrobiota was dominated by clones affiliated to the β-proteobacteriaLimnohabitans sp., which were described to respond rapidly toenvironmental changes. Overall, the intestinal microbial community did notcontain known fermentative or obligately anaerobic gut bacteria.Limnohabitans spp. were also always prominent in the T-RFLP profilesdespite changing food sources and independent of applied bacteria, thusindicating that they are specialised stable community members. Otherintestinal microorganisms were stimulated by differing food sources butnever dominated the community. Just when Daphnia spp. were starved todeath their microbial community changed distinctly.EMV016Microbial engineers control sediment dynamics inaquatic habitatsS.U. Gerbersdorf* 1 , H.V. Lubarsky 1,2 , D.M. Paterson 2 , S. Wieprecht 1 ,W. Manz 31 Institut for Hydraulic Engineering, Department of Hydraulic Engineeringand Water Resources Management, University Stuttgart, Stuttgart, Germany2 Scottish Oceans Institute, Sediment Ecology Research Group, UniversitySt. Andrews, St. Andrews, United Kingdom3 Institute for Integrated Natural Sciences,University Koblenz, Koblenz,GermanyBackground. Sediments and their microbial communities (biofilms) featureto a great extend the essential functionality of marine and freshwater habitatsand provide important ecosystem services such as nutrient (re)-cycling orself-purification. This study addresses the ecosystem functionbiostabilisation where the microorganisms modify the response of theaquatic sediments to erosive forces (flow velocity, turbulence) by thesecretion of extracellular polymeric substances (EPS).Methods. The colonization of natural assemblages of estuarine bacteria anddiatoms, as well as freshwater biofilms, was studied over several weeksusing non-cohesive glass beads (< 63μm) as an artificial substratum. Theadhesion capacity and the substratum stability of the growing biofilms hasbeen determined by Magnetic Particle Induction (MagPI) and CohesiveStrength Meter (CSM), respectively. In parallel, bacterial cell numbers,microalgal biomass, the composition of the bacterial and microalgalassemblages as well as EPS quantity and quality (carbohydrates, proteins)have been monitored.Results. Microbial colonization resulted in significant enhancement ofadhesion and stability of the substratum as compared to the controls (up to afactor of 12) irrespective of the environment (freshwater, marine). Thestabilization potential of the bacteria exceeded that of the axenic diatomassemblages; however, the overall stabilization was highest in mixedassemblages. The assemblage composition, their physiology and thesecretion of EPS quantity and quality were important for sedimentstabilization, but strongly influenced by changing abiotic conditions.Conclusions. While biostabilisation has been mainly linked to microalgae(ecosystem engineers), our results point out the importance of bacterialassemblages for microbial sediment stabilization. Thereby, changes inabiotic conditions can significantly affect the ecosystem servicebiostabilisation by microbes. The data further suggest that the EPS matrixdetermines sediment adhesion and stability; however the binding strengthwas less related to quantity than to quality with possible synergistic effectsbetween proteins and carbohydrates. This information contributes to ourconceptual understanding of microbial sediment engineering that representsan important ecosystem service.In aquatic environments, bacteria play a key role in the carbon cycle buttheir importance in zooplankton guts remains mostly unknown, althoughtheir presence was regularly documented. Recently, denitrification byingested bacteria in anoxic guts of benthic aquatic invertebrates wasdemonstrated indicating their possible symbiotic participation in digestion.However, the guts of most important zooplankton organisms, e.g. smallDaphnia spp. which are a significant trophic link in freshwater systems, areprobably only partly anoxic if at all. This leads to the question how themicroorganisms interact with their host, i.e., whether they symbioticallyparticipate in digestion, whether they prevent success of pathogens orspektrum | Tagungsband 2011