Trophic state influences microbial predator-prey interaction

Research project P 14637 Trophic state influences microbial predator-prey interaction Roland RSENNER 09.10.2000 Both field studies and laboratory experiments showed that hetero- and mixotrophic flagellates and ciliates are the major consumers of bacteria in most aquatic systems. These protists are the essential link between bacteria and higher trophic levels in aquatic food webs. In addition, they play an important role in nutrient recycling by their feeding. Some bacterial strains can adapt to grazing pressure, -achieving decreased vulnerability or entire grazing resistance. Much less, however, is known on adaptations of protists to bacterial grazing-defense mechanisms. In this project, we want to investigate, how different trophic conditions, mimicked in a continuous cultivation system, influence a) the relationship between algae and bacteria and b) the development of bacterial grazing- defense mechanisms under different grazing pressure. In addition, (c) reactions of protists on bacterial defense strategies will be observed. A two stage continuous cultivation system will be fuelled by organic substrates released by the phytoflagellate Cryptomonas sp. that supports an associated mixed BC. At the beginning we will investigate the reaction of the algal-bacterial association, on four different P concentrations. In addition to dissolved P, particulate P shall be. measured in different ~size fractions. Via measurement of primary production in different size fractions we want to follow, the carbon flux from algae to bacteria. In the next experiments, the cultivation systems will be inoculated with protists, and we want to clarify if protistan grazing pressure can induce the same changes in bacterial size structure, activity and community composition at different productivity levels. For this purpose three experiments at different P loading shall be installed. Later on, we plan to maintain the systems but change the P loading. The lowest and highest P concentrations shall be continuously increased and decreased. Uptake rates of differently sized fluorescent latex beads and fluorescently labeled bacteria shall be detenrnined to investigate a possible change in the feeding behavior of protists at different trophic state of the experimental system. To measure the reaction of protists on bacterial defense strategies, protistan growth rates, biomasses and abundance will be determined.

Originally, interactions between aquatic organisms have been described by linear cascade models. Nowadays, these relationships are characterized by the term "food web" and in the case of microbial communities as "microbial loop". In the presented project, the principles and functions of an experimental microbial food web have been investigated by continuous cultivation techniques (chemostats) in the laboratory. The algae Cryptomonas phaseolus was at the basis of the experimental microbial community, releasing extracellular organic carbon (EOC) during the process of photosynthesis. This EOC served as natural organic substrate for a diverse bacterial community growing together with the algae. By inoculation of different protistan predators to the algae-bacteria community, we investigated how bacterial characteristics were changing due to grazing pressure by protists. This microbial interaction is the eldest (and the smallest) predator-prey system we know at present and we can expect a wide spectrum of adaptations within bacterial as well as protistan communities. The project was dealing with the question, how the microbial food web can be affected by changing environmental parameters. Eutrophication of freshwater systems, i.e., the increased loading with nutrients (mainly phosphorus), is one of the major anthropogenic changes affecting aquatic ecosystems. Phosphate is regarded as an essential nutrient not only for algae but also for bacteria. Therefore, we deal with an exciting interaction, as bacteria depend on algae as their natural carbon source but at the same time are in competition for phosphorus with the algae. Since bacteria are generally regarded as better competitors for phosphorus, a paradoxical situation exists. In several experiments, we tested the effects of different phosphorus concentrations on this algae-bacteria interaction. By this set-up, we could gain more details on the algae-bacteria paradox, which describes the situation that bacteria can reach an unexpected high biomass in relation to algae at very low phosphorous concentrations. Although bacteria compete with algae for phosphorus, algae seem to support their competitors to a much higher degree as expected. In addition to these studies, we investigated the adaptable response of the bacterial community to protistan grazing at different substrate concentrations. Such changes concerned e.g. the cell size of bacteria, which was regulated by certain groups of bacteria to escape strong grazing pressure and led to inedible morphologies. In general the grazing pressure of protists seemed to be such a strong structuring element for bacterial communities, that even the bacterial taxonomic composition (i.e. bacterial diversity) was strongly influenced. All experiments conducted in the course of the project showed clearly that the microbial food web is the essential biotic component of aquatic systems and the first to be affected by anthropogenic nutrient loadings.