Studies on Peridinium aciculiferum and Peridinium malmogiense (=Scrippsiella hangoei): comparison with Chimonodinium lomnickii and description of Apocalathium gen. nov. (Dinophyceae)

The fine structure of the freshwater Peridinium aciculiferum and the closely related Baltic Sea species currently known as Scrippsiella hangoei were examined in serial sections. The species name Peridinium malmogiense is shown to be an earlier synonym of Scrippsiella hangoei and is restored as the name of the species. Although both species have been included in the genus Peridinium, their phylogenetic positions are within the Thoracosphaeraceae, close to the specialized predators known as the pfiesteriaceans and the photosynthetic freshwater Chimonodinium lomnickii. The fine-structural features of the two species proved to be very similar, including the details of flagellar bases and roots, and the type of pyrenoid, which consisted of dilated areas of the chloroplast crossed by two-thylakoid lamellae and not associated with starch sheaths. Comparison with Chimonodinium revealed significant differences, in particular the absence of an eyespot and any trace of microtubules associated with a peduncle, which contrast with the multilayered eyespot and the distinct microtubular basket (MB) of C. lomnickii. The absence of a MB in P. aciculiferum and P. malmogiense is regarded as a character loss within a group of species hypothesized to be derived from a MB-containing ancestor. A phylogenetic analysis based on concatenation of nuclear-encoded small subunit rDNA, internal transcribed spacers 1 and 2 including 5.8S sequences agreed with published phylogenies based on genes of the ribosomal operon in closely grouping P. aciculiferum, P. malmogiense and two other species of peridinioids with a similar amphiesmal plate arrangement: P. euryceps and P. baicalense. The four species are regarded as members of the same genus. While one of the closest known relatives of these four species is C. lomnickii, the variable association of this species to several other groups of species in published phylogenies and the differences in fine-structure revealed in the present work advise against transferring the studied species to Chimonodinium. The new genus Apocalathium is described with P. aciculiferum as type species.