Diatom-bacteria interactions are a crucial part of the global ocean and span from mutualistic via competitive to parasitic relationships. A lot of these interactions take place in the so-called “phycosphere” a diffusive boundary layer surrounding algal cells providing bacteria with a unique environment rich in algal-derived carbon sources. However, only few bacterial lines (Alpha-, Beta- and Gammaproteobacteria, Bacteroidetes) mainly genera such as Roseobacter, Sulfitobacter and Flavobacterium have been observed to reliably form symbiosis with diatom species. Thalassiosira rotula is a ubiquitous, bloom-forming, centric diatom found in a diverse number of marine habitats and was in this case isolated from a spring bloom event around the island of Helgoland. The main objective of this thesis was to investigate the influence of single strain bacteria on the growth of Thalassiosira rotula. To achieve this goal a co-cultivation method was designed that allowed for a high number of bacteria strains to be investigated at the same time. Moreover, it was investigated whether co-cultivation with two bacterial strains at the same time enhanced, diminished or did not affect the effect of single-strain bacteria on the growth of T. rotula. T. rotula was grown under vitamin-reduced conditions in Enriched Seawater, Artificial Water (ESAWred) in 24-well plates in co-cultivation with 33 bacterial strains isolated from the same spring bloom event or from the Arctic. Autofluorescence of algae was measured in a microplate reader as a proxy for algal growth over time. Moreover, two, dual-strain co-cultivations were conducted where T. rotula was combined with two different bacterial strains at the same time. During three single-strain co-cultivations it was observed that the designed method had two major sources of errors: 1) Algal growth was stunted by an unknown effect of MB medium introduced into cultures with bacterial inoculates 2) Bacterial OD600 varied greatly with the same dilution factor, therefore a fourth, improved single-strain co-cultivation was designed to avoid potential errors. The fourth single-strain co-cultivation showed that over two thirds of co-cultures grew to significantly higher autofluorescence signal than the axenic control within four days (p < 0.001, Dunnett’s test, α = 0.05). Moreover, no co-cultivation had autofluorescence lower than axenic control. Lastly, it was possible to observe family-wide trends of autofluorescence with members of the same bacterial family having similar effects on the growth of T. rotula. Dual-strain co-cultivation could not be properly interpreted due to contamination with MB medium in the first trial and lack of vitamin-reduced cultures in the second run. Overall, the experiments conducted in this thesis successfully showed that single-strain co-cultivations between T. rotula and bacteria from different families produce significantly different 8 growth of the algal partner. The observations made during experiments one, two and three gave crucial insights into potential areas of errors and were successfully used to improve experimental methods for observations of the effect of 30 bacterial strains on the growth of T. rotula in experiment four. This thesis laid the basis for close investigation of the interaction between T. rotula and marine bacteria. The observations made in this thesis can be used to select ecologically important bacteria and thoroughly investigate the interaction between the two partners.