AbstractVitamin B12(cobalamin) can control phytoplankton development and community composition, with around half of microalgal species requiring this vitamin for growth. B12dependency is determined by the absence of cobalamin-independent methionine synthase and is unrelated across lineages. Despite their important role in carbon and sulphur biogeochemistry, little is known about haptophytes utilization of vitamin B12and their ability to cope with its limitation. Here we report the first evaluation of B12auxotrophy among this lineage based on molecular data of 19 species from 9 families. We assume that all species encode only a B12-dependent methionine synthase, suggesting ubiquitous B12auxotrophy in this phylum. We further address the effect of different B12limitations on the molecular physiology of the model haptophyteTisochrysis lutea. By coupling growth assays in batch and chemostat to cobalamin quantification and expression analyses, we propose that haptophytes use three strategies to cope with B12limitation. Haptophytes may assimilate dissolved methionine, finely regulate genes involved in methionine cycle and B12transport and/or limit B12transport to the mitochondrion. Taken together, these results provide better understanding of B12metabolism in haptophytes and represent valuable data for deciphering how B12-producing bacteria shape the structure and dynamics of this important phytoplankton community.