| Popis: |
Biofilms are essential for the function of many natural ecosystems such as streams and rivers, but what governs their community composition (and thus potential ecosystem services) remains to a large extent elusive. Theories are mainly based on laboratory systems with few cultivable taxa, or on results from studies of planktonic communities. This is in part due to the methodological challenge to study substrate-associated microbial communities in their natural, complex and three-dimensional environment. In this study, the development and expansion of a set-up to test different factors potentially controlling such communities is described. It basically consisted of a river bypass, flow cells, and filter cartridges to allow for the fractionation of the community establishing the biofilm into different size classes and corresponding functional guilds (ciliates, heterotrophic flagellates (HF) and bacteria). First quantitative experiments using this set-up, described in chapter 1, tested the effects of protozoan grazing on the biofilm structure. It was shown that HF initially promote bacterial biofilm formation, but are able to graze on single cells in later successional stages. Ciliates in turn massively influenced the HF community, leading to a switch from benthivorous to planktivorous life forms and a subsequent release of single bacterial cells from grazing pressure (trophic cascade). After extending the set-up and incorporating molecular techniques to also analyse the diversity of the bacterial biofilms (chapter 2), it became obvious that the above described effects can vary depending on background factors like seasonal variations in temperature. Furthermore, it could be shown that both HF and ciliates influence bacterial diversity, and that this effect can be independent of morphological changes in the biofilm. In addition, HF generally increased the number of bacterial phylotypes, while ciliates tended to reduce them. In chapter 3, the set-up was again modified and this time applied to test the influence of different immigration potentials on HF and ciliate communities. In already established HF and ciliate communities, strongly reduced immigration possibilities from the plankton left all but one ciliate taxa uninfluenced. In just establishing HF communities, however, reduced immigration potential resulted in slower colonization of the substrate, but was followed by higher growth rates compensating for the slower colonization. After reaching the equilibrium abundance, the HF community did not longer depend on immigration but on local resource availability. Together, this study shows that natural biofilm communities are controlled by local factors like grazing, nutrients and competition, but also that regional factors like dispersal and abiotic and seasonal factors might alter the resulting effects depending on the successional stage or community composition. Generally, it was shown that conclusions from laboratory or planktonic studies are not always transferable to natural biofilm communities, thus testing ecological theories under natural background conditions remains crucial to understand the factors shaping them. The presented method is an important and further adaptable tool to achieve this goal. |
