## 2 Biodiversity, Chemodiversity, Phylogeny and Phylogeography of the _Haslea_ Genus  
The species \(H\). _ostrearia_ has long been known as the only blue diatom ever described. For decades, any record worldwide of a blue diatom, i.e., a pennate diatom with blue tips, was assigned to \(H\). _ostrearia_. However, recent works on blue _Haslea_ strains using scanning electron microscopy (SEM) observation and molecular markers have uncovered unsuspected biodiversity of this taxon, with four new species of blue _Haslea_ recently described: _Haslea karadagensis_ [PERSON]. [PERSON], [PERSON] and [PERSON] [36], collected in the Black Sea; _Haslea provincialis_ [PERSON], [PERSON] and [PERSON] [39], collected in the Mediterranean Sea; _Haslea unsantara_ [PERSON], [PERSON] and [PERSON] [40], collected in the Java Sea; and _Haslea silbo_ [PERSON], [PERSON] and [PERSON] [41], from La Gomera (Canary Islands, Spain). In the last decade, our knowledge about the diversity of non-blue _Haslea_ has increased as well [42; 43; 44]. So far, the genus _Haslea_ encompasses 44 taxa, of which 36 are taxonomically accepted, as listed in _AlgaeBase_[45]. Most of these species are benthic organisms encountered in the Northern hemisphere, possibly due to a more intense research effort in this part of the globe, or to a higher proportion of land to water, hence the benthic environments. In a time-calibrated molecular phylogeny [46], _Haslea_ belongs to a Naviculoid clade along with _Navicula_, _Hippodenta_, _Seminavis_ and _Pseudogomphonema_. Interestingly, _Haslea_ diversified as the first of the above clade at the boundary between Cretaceous and Tertiary (ca. 66 my BP), followed by _Hippodenta_ with _Navicula_ as the last genus [46]. The early divergence of _Haslea_ has recently been confirmed [47]. _Haslea antiqua_ Fenner has been described based on well-studied specimens (LM and SEM). This new species, found in a deep-sea sediment in the SW Pacific, has apparently been drifted by hazardous events to the deep-sea sediment. Sediment containing \(H\). _antiqua_ are well dated and represent the Late Paleocene. The specimens illustrated in [47], in terms of morphology, represent characteristics typical of the genus, including the striation, valve layering and particular hasleoid helictoglossae. It is missing the accessory rib on the raphe sternum interior, but evidently possesses a fascia in the valve center, present in some extant species (e.g., _Haslea crucigera_ ([PERSON]) [PERSON]) [48]. Among the 36 recognized taxa, four species (\(H\). _karadagensis_, \(H\). _provincialis_, \(H\). _usantara_ and \(H\). _silbo_) produce blueish compounds, the so-called \"marenmine-like pigments\", in reference to marenmine produced by \(H\). _ostrearia_. Furthermore, at least three other species of blue _Haslea_ have been discovered, but they have not been formally described yet. Thus, it can be hypothesized that the number of species recognized to produce marenmine-like pigments will increase with time, especially if the research effort is sustained. Nevertheless, blue _Haslea_ are rarely observed in natural environments, with a few exceptions where they can bloom (see below). Given the scarcity of observations of diatoms, and beyond microalgae with blue cell-compartments, it seems that the ability to synthesize marenmine-like pigments is a taxon-specific characteristic that is present in very few taxa, especially with regard to the enormous biodiversity of microalgae. Several molecular markers (mainly from mitochondrial and chloroplast DNA) have been characterized in _Haslea_ species, namely 16S, 18S, COI and _rbcl_. Nevertheless, these markers are not identified or not available for all recently described _Haslea_ and for many _Navicula_ species. Only the _rbcl_ dataset allowed the establishment of a consistent molecular phylogeny comparing different _Haslea_ and _Navicula_ species.