Species delimitation within the colonial flagellates <i>Uroglena, Uroglenopsis</i> and <i>Urostipulosphaera</i> (Chrysophyceae)

Until recently, there was no agreement on species delimitation within the morphologically similar chrysophycean genera <i>Uroglena, Uroglenopsis</i> and <i>Urostipulosphaera</i>. In this study, we aimed at a modern taxonomic revision based on the combination of morphological characters (ultrastructure of cysts, cell and colony features) and a multigene phylogeny (SSU, ITS rDNA and <i>rbc</i>L sequences), with ecology taken into account. Of more than 650 explored localities, only approximately one in 10 hosted a viable and detectable population of these colonial chrysophytes at the time of sampling. We established and examined 189 short-term cultures along with single colony isolates, derived mostly from blooming or encysting populations. We obtained the cyst morphology for four species and two lineages of <i>Uroglena</i>, two species of <i>Uroglenopsis</i>, and four species of <i>Urostipulosphaera</i>. A total of 12 resolved lineages could be attributed to previously described species or new species (<i>Uroglena imitata</i> sp. nov., <i>Urostipulosphaera granulata</i> sp. nov.). Based on our molecular analyses and morphological observations, we assign all the previously described <i>Uroglena</i>-like taxa to newly recognized genera and propose a key to identification. Consequently, <i>Uroglena</i> now includes 16 species and two varieties, <i>Uroglenopsis</i> contains four species and <i>Urostipulosphaera</i> encompasses nine species. Within <i>Uroglena</i> and <i>Urostipulosphaera</i>, species are defined by the ultrastructure of their cysts. On the contrary, as <i>Uroglenopsis</i> has simple cysts, species are defined by cell and colony characteristics.