Search

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.

Sea ice is an important structural component of polar marine ecosystems but also at lower latitude seas like e.g. the northern Baltic Sea. This study summarises observations on biological, chemical and physical characteristics of sea ice and under-ice water obtained during three expeditions to the Baltic Sea, the Fram Strait area (Arctic) and the Bellingshausen Sea (Antarctica). The study aimed at a better understanding and quantification of different components of the sea ice related food web. The seasonal Baltic sea ice is least studied and therefore the work in this area focused on an inventory determination of the abundance and biomass composition of the sympagic (=ice-associated) community of the Bothnian Bay and Bothnian Sea as well as on the importance of abiotic and biotic factors in the control of ice algal accumulation. The work on the better explored polar sea ice focused on the abundance, distribution and characteristics of transparent exopolymer particles (TEP) in Arctic and Antarctic sea ice. TEP are a recently described class of exopolymeric particles, which are formed abiotically and biotically from polysaccharid-rich precursors. High amounts of TEP-precursors are released by bacteria and algae especially in response to environmental stress. In the pelagic realm TEP are important in the aggregation of diatom blooms, provide the matrix of macroaggregates and serve as substrate and habitat for attached bacteria. High concentrations of TEP have been recently described for Arctic sea ice and may have an important impact on carbon dynamics in sea-ice systems. The present study related TEP concentrations to biotic and abiotic sea ice parameters, potential modes of TEP formation were elucidated and the importance of TEP for the sea-ice habitat was discussed.