Interactions of marine bacteria in the pelagic food web
A close link between bacteria and phytoplankton in the pelagic environment was observed in several studies. Generally, highly specific interactions of bacteria and phytoplankton (e.g. symbioses or parasitic relationships) can occur in addition to more unspecific interactions such as competition, commensalism or mutualism. In this thesis, bacteria-phytoplankton interactions were investigated with a focus on the association with microalgae considered to be key species at Helgoland Roads. Initially, bacterial communities of microalgal cultures were analysed concerning specificity of the association and related to algal growth phases. Seven freshly isolated key diatom and dinoflagellate species from Helgoland Roads, North Sea, were investigated. Members of Alpha - and Gammaproteobacteria and the Flavobacteria-Sphingobacteria group within the Bacteroidetes phylum predominated in the cultures. Shifts in the bacterial communities could not be correlated to changes of nutrient levels or algal growth phases. Regarding these results, it should not be generalised, that the compositions of the bacterial communities are strictly species-specific for microalgae. The hypothesis regarding species-specific interactions between bacteria and different diatom species was tested by resource competition experiments. It was investigated, whether microalgae or their exudates have a shaping influence on the structure of diatom-associated bacterial communities. Interestingly, competition of associated bacterial populations could not be observed in any treatment. Furthermore, the influence of exudates on the composition of the associated bacterial communities was estimated to be low. Shifts in the associated bacterial community structure resulting from isolation and cultivation of microalgae were also investigated. Generally, members of Alpha - and Gammaproteobacteria as well as members of the Bacteroidetes predominated the cultures. Strong shifts in the associated communities were observed during cultivation. Especially the number of phylotypes belonging to the Gammaproteobacteria increased. Additionally, the influence of phytoplankton on the bacterial community was studied in situ during winter-spring transition 2004. Generally, an influence of temperature and phytoplankton on the bacterial community during the sampling period was observed. Additionally, multivariate analysis revealed an influence on specific bacterial phylotypes by these factors.