Mechanisms controlling the diversity of freshwater bacterioplankton

The major goals of the project are to broaden the knowledge about species composition of the bacterial community in the oligo-mesotrophic Lake Mondsee, Austria, and other lakes, as well as to reveal the major mechanisms which influence the species composition of bacterial communities. Amongst the organisms inhabiting the water body of lakes and oceans (algae, zooplankton, fish, etc. ) the bacteria are the numerically dominant group. A water sample of one millilitre taken from a lake usually contains more then one million bacterial cells. The bacteria play a pivotal role in the metabolism of all aquatic, as well as of all other ecosystems. They degrade toxic compounds and recycle nutrients essential for other organisms. Furthermore, they use dissolved organic matter for production of new bacterial cells, which serve other organisms as food. Despite the numerical dominance and the knowledge of the ecological importance of bacteria in almost all ecosystems the knowledge on the species composition of the bacterial communities is very little. Furthermore, only a few is known about the mechanisms which might control this bacterial species composition. The major goals of the project are to broaden the knowledge about species composition of the bacterial community in the oligo-mesotrophic Lake Mondsee, Austria, as well as to reveal the major mechanisms which influence this species composition. The starting point of the project is the isolation of bacterial strains from water samples of Lake Mondsee. We grow the isolated bacteria in the laboratory, determine to which species they belong, and develop molecular probes which specifically label the selected bacterial species. These probes enables us, by use of microscopical techniques, to distinguish the selected bacterial species from other species, to detect those bacterial species in their natural environment, and to follow the seasonal dynamics of these bacterial species population over longer periods. In parallel we investigate the dynamics of parameters which are suspected to influence the development of bacterial populations. These parameters include the water temperature, the abundance of predators (i.e. protozoa, etc.), abundance of viruses which are able to infect the bacterial species, etc. Additionally, laboratory experiments, which deal with the influence of single environmental parameters on the growth of the investigated bacterial species are conducted.

The major goal of the project was to further reveal the mechanisms controlling the diversity of freshwater bacterioplankton. Planktonic bacteria are the most numerous organisms in freshwater lakes (as well as in all other non-extreme habitats on earth), and they are of great importance for the lake`s metabolism. Despite of the ecological importance of bacteria, only little is known about the species composition of bacterioplankton, as well as about the mechanisms controlling the composition and dynamics of bacterial communities. The investigation of these aspects is hampered by the low culturability of environmental bacteria (only <1% of cells can be cultivated by standard microbiological methods), as well as by the lack of morphological traits suitable for species identification. We applied an autecological research strategy, i.e. focusing on the interaction of selected bacterial species with their abiotic and biotic environment. We developed a method (acclimatization method) for cultivation of bacteria resistant to cultivation by standard microbiological methods, and used this method for the isolation and cultivation of a large number of bacterial strains from diverse freshwater habitats located all over the world. Furthermore, we developed molecular tools (probes for fluorescent in situ hybridization, FISH), which allowed specific detection of the cultivated bacteria in their natural environment. Application of these probes demonstrated that many of the cultivated bacterial strains represent abundant populations contributing up to 60% of the total bacterial numbers in the pelagic of lakes. The detailed investigation of two populations revealed recurrent seasonal population dynamics in consecutive years. This new finding is of importance for a holistic understanding of entire lake ecosystems. Combined field and laboratory investigations revealed several factors controlling the population dynamics of single bacterial populations, as well as controlling the composition of the entire bacterial communities. Water chemistry (pH, hardness, salinity), water temperature, and predation by protistan and mesozooplankton predators were revealed as important drivers of population dynamics. The gained knowledge on the ecology of the investigated bacterial taxa will allow predictions about their presence in particular habitats, as well as predictions about their population dynamics. This new knowledge opens new opportunities for the monitoring of lake ecosystems. Overall contributes the obtained knowledge about the ecology of the investigated freshwater bacteria, as well as the new knowledge about the mechanisms and parameters controlling the dynamics of single bacterial species, as well as the composition and dynamics of the entire bacterioplankton, significantly to a more holistic understanding of lake ecosystems.