Virus ecology of suspended in freshwater systems

Research project P 14721 Virus ecology of suspended matter in freshwater systems Peter PEDUZZI 09.10.2000 Due to their high abundance, viruses in aquatic sytems are considered as significant biological agents in microbial processes. They are thought to impact the activity, life strategy and diversity of their hosts and apparently influence organic matter fluxes. Suspended particulate material plays a significant role in biogeochemical cycles and for biological processes in natural aquatic environments. Riverine systems sometimes carry high loads of this material, thus underlining the particular relevance. Particle-bound microbial abundance and activity can contribute significantly to overall microbial processes depending on the quality of particles and the type of organisms involved. Research on the interaction between suspended matter and the natural assemblage of viruses infecting microplankton organisms is surprisingly scarce. Particularly in freshwater systems, where suspended matter is often a prominent factor determining a variety of processes, there is an intolerable lack of information. The proposed project will add substantial information to our current knowledge. We will attempt to answer several scientific key questions: * Does particle quality and quantity determine virus abundance and virus - bacteria interactions? * Is particulate material a means of removal versus survival and production of viruses? * Does the viral community structure differ between particle and ambient water environments? * Does viral activity influence the community structure of free-living and attached bacteria? * What is the potential role of viruses in particle formation/dissolution? The Danube river-floodplain system provides a broad spectrum of almost every type of particles (from mineral particulates to lake and river snow) and is an ideal study site for this type of investigation. Three subsystems with different hydrological characteristics and Particle regimes have been selected as study locations. Our approach is expected to provide further insights into the structure and function of aquatic microbial food webs and carbon cycling in a sequence of systems with varying particle loads.

River systems usually carry high loads of suspended particulate material, which is thought to play a significant role in biogeochemical cycles and for biological processes. The project intended to provide new insight into the interaction between suspended matter and the natural assemblage of bacteria and viruses under varying hydrological conditions in order to contribute to our understanding of the structure and function of aquatic microbial food webs. Particle size distribution and particle quality parameters were monitored and related to microbial parameters. Hydrology and "water age" influenced both particle abundance and quality. The presence of bacteria and viruses on particles was highly dependent on the size of the material, a small to medium size class with a high proportion of organic material, was most densely colonized by bacteria and virus-like particles. Several interdependencies between bacteria/viruses attached to particles and environmental parameters were detected. With the development of improved technology we are now able to visualize and reconstruct the architecture and microbial colonization of aquatic aggregates. For the first time viral, bacterial and other material distribution on such particles could be demonstrated on fully hydrated material. In laboratory experiments it was found that different particle qualities influenced bacterial and viral abundance and bacterial activity on particles and in the ambient water. Data from a tropical aquatic network (Sri Lanka) showed that most virus parameters, like abundance or frequency of visibly infected cells, were positively linked to bacterial abundance and production but also to some nutrient species. An adapted fingerprinting method for viral communities revealed considerable variability of virioplankton in different subsystems of the Danube River floodplain. In general diversity was lowest in the river channel. Viral richness was higher in the investigated floodplain pools. Simultaneously, the particle-associated and free-living bacterial assemblages were investigated. We developed a new software tool for the analysis of electrophoresis profiles. The particle-associated bacterial community exhibited a higher richness and was more heterogeneous in time and space than the free-living community. The temporal dynamic was generally higher in isolated floodplain pools. The bacterial community structures of the river and the various floodplain pools, as well as the particle- associated and free-living bacteria, differed significantly. This variability was related to changes in the algal biomass, temperature, and concentrations of organic and inorganic nutrients. Suspended particles, bacteria and viruses in the studied freshwater systems appear to be linked to various environmental conditions and may affect processes at the ecosystem scale.