This program aims to increase our knowledge about the biology and biodiversity of ‘blue diatoms’, unique among microalgae by their ability to produce specific water-soluble bluish pigments. Up to now the terms ‘blue diatoms’ have referred exclusively to the pennate marine diatom Haslea ostrearia, and to its blue pigment, the so-called marennine, a polyphenolic molecule with biological activities, which is produced during algal growth and ageing. In oyster ponds of western France, H. ostrearia may year on year become dominant, and marennine released in the seawater is responsible for the ’greening’ of oysters, a phenomenon which has a noticeable local economical impact. The diatom H. ostrearia has long been considered the only microalga ever known, worldwide in distribution, to produce marennine. Indeed in the literature, diatoms with blue tips described as H. ostrearia were reported in almost all seas and oceans in northern and southern hemispheres. Their presence was deduced either directly from the observation of living cells, or indirectly from a greening effect of bivalves. In most cases, a careful identification of the ‘blue diatoms’ at the species level did not rely on detailed studies of morphology and morphometrics. This paradigm has been recently questioned following the discovery, in phytoplanktonic samples taken on rocky shores of Crimea (Ukraine), of diatoms presenting similarities with H. ostrearia, with bluish apices, but in which the pigment seemed different from marennine. Preliminary results indicate that these diatoms could be two different species, which raises a concern about occurrences of ‘blue diatoms’ and their identification as H. ostrearia in all other marine environments. Hence the present project is aiming to deepen our knowledge of the biology, physiology and pigments of ‘blue diatoms’, to study their biodiversity, and to unravel their taxonomy, especially regarding the genus Haslea.