Haslea ostrearia-like diatoms: biodiversity out of the blue

Diatoms are usually referred to as golden-brown microalgae, due to the colour of their plastids and to their pigment composition, mainly carotenoids (fucoxanthin, diadinoxanthin, diatoxanthin), which mask chlorophylls a and c. The species Haslea ostrearia Gaillon/Bory (Simonsen) appears unique because of its extraplastidial bluish colour, a consequence of the presence of a water-soluble blue pigment at cell apices, marennine. When released in seawater, marennine can be fixed on gills of oysters and other bivalves, which turn green. This greening phenomenon is economically exploited in Southwestern France, as it gives an added value to oysters. For decades, this singularity ascribed a worldwide distribution to H. ostrearia, first as Vibrio ostrearius, then Navicula ostrearia, last as H. ostrearia, when the genus Haslea was proposed by R. Simonsen (1974). Indeed, this ‘birthmark’ (presence of blue apices) made H. ostrearia easily recognisable without further scrutiny and identification of the microalga as well as its presence easily deduced from the greening of bivalves. Consequently, the widely admitted cosmopolitan character of H. ostrearia has only been questioned recently, following the discovery in 2008, of a new species of blue diatom in the Black Sea, Haslea karadagensis. The biodiversity of blue diatoms suddenly increased with the finding of other blue species in the Mediterranean Sea, the Canary Islands, etc., the taxonomic characterization of which is in progress. This review thus focuses on the unsuspected biodiversity of blue diatoms within the genus Haslea. Methods for species determination (morphometrics, chemotaxonomy, genomics), as well as a new species, are presented and discussed.