We investigated the occurrence of Planktotalea frisia strain SH6-1T, a member of the Roseobacter clade, in the North Sea and interactions with phytoplankton algae with a special emphasis on the carbohydrate metabolisms. This bacterium was present in May 2006 throughout the North Sea. P. frisia SH6-1 was further present in the German Bight between February and early July with distinct peaks during and after phytoplankton blooms. Highest abundances, as detected by quantitative PCR, were 0.5-0.9% of total bacterial abundance. Comparison by CARD-FISH with a set of highly specific probes confirmed the high values in one sample. Between mid-July and October, P. frisia SH6-1 was not detected throughout the North Sea. Experimental studies in which P. frisia SH6-1 was grown in the presence of axenic cultures of the algae Phaeocystis globosa, Leptocylindrus danicus and Thalassiosira rotula exhibited distinctly different responses with the best growth together with P. globosa and T. rotula and very low growth together with L. danicus. The algae greatly differed in the composition of their exuded carbohydrates and that P. frisia SH6-1 was rather selective in their consumption, suggesting that the distinct carbohydrate metabolisms is a key feature to explain its seasonal occurrence in the North Sea.