Bacterial Influence on Diatoms from Photoautotrophic Freshwater Biofilms

XIII Bacterial influence on diatoms from photoautotrophic freshwater biofilms Photoautotrophic biofilms are characteristic for the littoral zones of seas, lakes and rivers. These green-brownish, mucous layers on submerged surfaces like stones or macroalgae are a niche of high primary production and play an important role in the aquatic food web. Biofilm formation also causes biofouling and hence immense costs each year for example in shipping and water resource management. Intensive investigations are therefore necessary to understand the complex mechanisms which lead to the development of this particular mode of living. Diatoms and bacteria are early colonizers of wet and illuminated surfaces and dominant members of mature photoautotrophic biofilms. In this thesis, interactions between diatom and bacterial isolates from photoautotrophic, epilithic biofilms from Lake Constance were investigated, that may considerably influence the formation of the whole biofilm. A protocol was established to purify xenic diatom strains from associated satellite bacteria, resulting in axenic diatom cultures. Long-term observations of diatom cultures revealed that bacteria may impact cell size and frustule morphology, features which commonly change during the life cycle of many benthic diatoms. Cell sizes of two of three investigated diatom species were strongly reduced and the frustules exhibited morphological abberations when cultivated under axenic conditions, whereas the xenic diatoms were well preserved. Two model systems were designed that enable the investigation of cellular mechanisms of diatom/bacteria interactions, which influence biofilm formation and diatom growth. It was shown that the diatom Achnanthidium minutissimum was strongly attached to surfaces and formed a biofilm only in the presence of co-isolated bacteria whereas the axenic diatom culture remained planktonic. The xenic culture produced high amounts of bound extracellular polymeric substances (EPS) and formed cellular capsules. In contrast, the axenic diatom culture produced predominantly soluble extracellular carbohydrates and capsules were absent. Capsulation and biofilm formation could be re-induced in the axenic diatom culture by a Bacteroidetes bacterium. Further investigations demonstrated that the interaction of A. minutissimum with the bacterium was mediated by soluble hydrophobic molecules produced by the bacterium. The same Bacteroidetes bacterium exhibited a growth-promoting impact in another model system using the diatom Fragilaria brevistriata. The diatom formed long cell chains up to macroscopic visible colonies when co-cultivated with the bacterium. Transposon mutagenesis and identification of bacterial mutants that lost growth-promotion revealed that two genes whose products are homologous to transcriptional regulators of the MarR family and to a calcium ATPase, respectively, are involved in the growth-stimulating potential of the Bacteroidetes bacterium. In this study it was shown that bacteria may have strong impacts on different physiological features of diatoms like morphological changes during the algal life cycle, culture growth and formation of EPS, features which may considerably influence the whole biofilm. With the designed bioassays convenient tools are now available which proved to be useful to study the cellular mechanisms of interactions between benthic diatoms and bacteria. This thesis contributes to a deeper understanding of the complex chemical and genetic basics of diatom/bacteria interactions.