Haptophytes are important primary producers in the marine pelagic community. Haptophyte blooms may also have a major impact on the global carbon balance and on climate forcing, and toxins may cause fish-kills. We lack knowledge on their biodiversity and q uantitative distribution and dynamics at the species and population level. The main objective is to unravel the biodiversity, seasonal dynamics of haptophytes from North Atlantic waters, and how the species composition, diversity, life cycle and abundance are affected by environmental factors. The abundance and distribution of haptophyte specific viruses will be examined, and how they affect the haptophyte diversity. In the proposed project we will apply the latest technologies in molecular biology and mi crobiology to aquatic ecological problems. The biodiversity will be explored by a.o. environmental pyrosequencing, electron microscopy and culturing. Seasonal dynamics will be examined by combining the use of microscopy, molecular probes, flow cytometry a nd water chemistry upon the analyses of natural samples collected from coastal waters through the year. The project will improve the understanding of the dynamics in the marine pico- and nanophytoplankton community, where haptophytes play a significant ro le. This will increase our understanding on fundamental processes in the plankton.