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This thesis was prepared in the laboratories of the Max Planck Institute for Limnology in Plön (Germany). It is focused on bacteria-protozoan interactions and the underlying mechanisms of grazing-resistance in aquatic bacteria: phenotypic properties of aquatic bacteria with respect to their potential effect on bacterial vulnerability and survival in the presence of bacterivorous protists (such as heterotrophic nanoflagellates, HNF).
Interactions were studied mainly as impact of the presence of predators on both single bacterial strains and bacterial communities, considering variations of phenotype, genotype and fitness in the first case, and variations in diversity, productivity and distributions in the second part of the work. Chemostat systems with bacteria and bacterivorous protists were used to enrich and isolate bacterial species which had a potentially reduced vulnerability towards grazers. These strains were analysed with respect to their anti-grazer phenotypic properties, phenotypic plasticity and regulating mechanisms in the expression of these properties. Bacterial communities composed by strain isolated with the same methodology were tested in chemostat systems to analyze the impact of predation by HNF and of inter-specific competition for resources on their diversity and on their genetic and phenotypic composition.
One principal aim was to elucidate the diversity and effectiveness of the underlying mechanisms of grazing resistance in aquatic bacteria: cell-size and shape are important and well-documented features which determine bacterial vulnerability to grazers. However, other characters which might also impact upon the predator-prey interactions with protozoans were still poorly understood.
Of particular interest was the question as to whether some of the resistance mechanisms found (e.g., morphology, toxins, exopolymers) can be induced by predator chemicals, or whether their development is mainly related to substrate supply and elimination of competitors by grazing. A cost-benefit analysis for selected polymorphic bacterial strains (with resistant and vulnerable phenotypes) should reveal further insight into the evolution and the ecological consequences of grazing-resistant bacteria.
In the first part of the thesis it was studied the impact of grazing and substrate supply on the population size structure of a freshwater bacterial strain (Flectobacillus sp.) which showed a high morphological plasticity, with cell lengths in the range 2 - 40 µm, encompassing rods, curved cells and long filaments. Without grazers and with sufficient
substrate supply bacteria grew mainly in form of free-living medium-sized rods (4-7 µm) with a smaller proportion of filaments.
Batch grazing experiments with the bacterivorous flagellate Ochromonas sp. showed that cells < 7 µm were highly vulnerable to grazers and became eliminated whereas resistant, filamentous forms become enriched. Comparing long-term growth in carbon-limited chemostats with and without grazers revealed that bacterial biomass was on a similar level but the morphological composition strikingly different, with >80 % filaments in the grazer treatments. These morphological differences did not seem to have a strong impact on the physiological capabilities as revealed from substrate uptake and utilisation measurements. Carbon starvation resulted in a low degree of morphological plasticity and a fast increase of small rods which were highly vulnerable to grazing.
Dialysis bag experiments were used to test for chemical induction of bacterial morphological changes. They revealed that the excretion of organic substrates by flagellates stimulates bacterial growth. When these experiments were combined with continuous cultivation they gave strong evidence for chemical induction of resistant bacterial morphologies: The development of filamentous forms occurred even without direct contact with predators when bacteria were exposed for several generations to predator-released substances.
The second part of the thesis was focused on HNF predation and competition for resources can effect on bacterial populations. The effects of limiting factors on ecological systems and their possibility to influence community’s structure and composition represent one of the most debated topics in modern ecology.
The understanding of processes related to biological communities is extremely difficult, because of their extraordinary complexity. For these reasons, the most fruitful approaches have been the reduction of the complexity of the systems, and the development of related mathematical models. Because of their qualities, microbial communities are the most commonly used for these studies.
Using carbon-limited chemostats with differing substrate input, the influence of system productivity on predator-prey interactions in classical artificial communities of planktonic bacteria and bacterivores, was examined for two different experiments.
Analysing population dynamics, diversity and functional attributes of the mixed bacterial assemblages and of bacterivorous nanoflagellates it was assessed the relative importance of the classes of edible and inedible bacteria. In accordance with existing models on microbial predator-prey interactions, the relative importance of grazing-resistant prey increased with increasing productivity.
With a detailed analysis on the morphology and functionality of bacterial cells it was possible to recognize different mechanisms of resistance along the productivity gradient.
Performing a T-RFLP analysis of bacterial populations was noticed that predator-mediated increases in phenotypic diversity were not necessarily reflected in changes of the genetic diversity of our bacterial populations.
Starting from the huge background from former studies and models on simple communities, this approach tries to clarify the validity of the most debated ecological theories about relations between prey-predator interactions, productivity of the system and biodiversity on larger scale.
