Interactions between protists and suspended sediments

This project aims to assess the impact of suspended sediment on bacterivorous protists. Suspended sediments are introduced in rivers and lakes by surface runoff due to precipitation, snow melt or wind-induced resuspension. Depending on the weather conditions these sediment particles may become of quantitative importance in all freshwater systems and they are a dominating factor in systems of high sediment load such as Yellow River and Ganges. Microbial organisms (bacteria, protists, microalgae) are mainly responsible for ecosystem respiration and element cycling. Bacterivores are a key component for element flow from bacteria and the "microbial food web" to zooplankton and finally fish. In addition, microbial organisms play a pivotal role in the dynamic and degradation of pollutants. Selective feeding within the "microbial food web" alters the bacterial community structure and may thus indirectly affect biodegradation and bioaccumulation of harmful substances. Suspended sediments interfere with the feeding process of bacterivorous protists and play a pivotal role for the introduction of toxic substances into the food web. The effect of suspended sediments on protists has, however, only fragmentarily been investigated. The goal of this study is to gain insight into the selection behaviour of bacterivores to suspended sediments and subsequently to link population response (growth rate, fitness) to the individual (behavioral) response. The project is subdivided in three parts starting with laboratory investigations of effects of clay concentration and mineralogy. Later the impact of surface-attached substances (especially the heavy metals lead and cadmium) shall be investigated. The third phase shall focus on the effect of suspended clay in the field (examplarily for Lake Mondsee, Salzkammergut, Austria). These topics shall be investigated including the behavioural response, ingestion and digestion, selectivity, functional and numerical response of the protists. Different complementary methods shall be applied: Batch and continuous flow experiments and subsequent analysis using fluorescence microscopy and flow cytometry shall be combined with live observations by means of high resolution video- microscopy. This combination of different methods allow a more detailed analysis compared to the use of each method alone. We expect that this comprehensive project will reveal deeper insights into major abiotic factors controlling protist population dynamics.

Microbial organisms (bacteria, protists, microalgae) are mainly responsible for ecosystem respiration and element cycling. Studying the regulation of microbial processes and identyfiing the role of so far understudied abiotic factors is the key to understanding ecosystem functioning. Suspended sediments are introduced in rivers and lakes by surface runoff due to precipitation, snow melt or wind-induced resuspension. Depending on the weather conditions these sediment particles may become of quantitative importance in all freshwater systems and they are a dominating factor in systems of high sediment load. The main goal of this project was to investigate the significance of suspended sediments for aquatic protists. Further, as the underlying morphospecies concept proved to be problematic with respect to the ecophysiological resolution, the project was extended to studies on protist survival strategies and on intraspecific variation. Ecophysiology proved to be species-specific and vary considerably below the morphospecies level. For instance, Spumella is an r strategist and relies on mainly large bacteria for growth although small bacteria can contribute significantly to its diet. In contrast, Poterioochromonas is much more a K strategist. In general, simplified concepts of bacterial grazing vulnerability were not suitable. Size alone is not sufficient to define a refuge neither for filamentous bacteria nor for ultramicrobacteria from nanoflagellate predation. Despite these general trends, molecular and ecophysiological variation below the morphospecies level was high and cryptic species may differ with respect to habitat specifity and to perennation strategies. Flagellates, in general, discriminated strongly against sediment particles prior to food uptake, whereas the ciliates did not. However, freshwater flagellates proved to be susceptable to suspended sediment concentrations as realised in the field after precipitation. The presence of suspended sediments generally decreased the growth at any food concentration tested. This decrease was reflected in a positive correlation of the half saturation coefficient and the threshold food concentration and a negative correlation of the maximal growth rate with suspended sediment concentration. Dissolved components further modified the suspended sediment - protist interactions: Our investigations provided evidence that suspended sediments function as a sink to dissolved components making them less available to protists. Further, the above findings could, in general, be confirmed by field experiments. On the community level suspended clays did not affect flagellate abundance. This was different for specific taxa, i.e., the abundances of Spumella-like flagellates decreased significantly. Similarly, in a mountain stream the abundance of aquatic strains of Spumella decreased but these organisms were partly replaced by soil-borne flagellates. This effect was even more pronounced after precipitation.