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. Planktotalea frisia SH6-1 was further present in the German Bight between February and early July, with distinct peaks during and after phytoplankton blooms. The highest abundances, as detected by quantitative PCR, were 0.5-0.9% of total bacterial abundance. Comparison by catalyzed reporter deposition fluorescence in situ hybridization (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 in the fact that P. frisia SH6-1 was rather selective in consumption of algae, suggesting that the distinct carbohydrate metabolisms are a key feature to explain the seasonal occurrence of this bacterium in the North Sea. © 2013 Federation of European Microbiological Societies. Published by John Wiley & Sons Ltd. All rights reserved.