Environmental sampling in Poland and the United States and phylogenetic analyses based on 567 sequences of four genes (155 sequences of nuclear SSU rDNA, 139 of nuclear LSU rDNA, 135 of plastid‐encoded SSU rDNA, and 138 of plastid‐encoded LSU rDNA) resulted in description of the new genus Flexiglena, which has been erected by accommodating Euglena variabilis, and enriching the Discoplastis and Euglenaformis genera with five new species. Four of them have joined the Discoplastis genus, currently consisting of six representatives: D. adunca, D. angusta (=Euglena angusta), D. constricta (=Lepocinclis constricta), D. excavata (=E. excavata), D. gasterosteus (=E. gasterosteus), and D. spathirhyncha. One of them has enriched the Euglenaformis genus, currently represented by two species: Euf. chlorophoenicea (= E. chlorophoenicea) and Euf. proxima. For most studied species, the diagnostic descriptions have been emended and epitypes were designated. Furthermore, the emending of Discoplastis and Euglenaformis diagnoses was performed.

Although Lepocinclis ovum is recognized as a cosmopolitan and common species, and Lepocinclis globulus is the type species of the genus Lepocinclis, their correct identification is nearly impossible. The reason is that over 30 morphologically similar taxa appear in the literature, but no good diagnostic features exist to distinguish amongst them. Using environmental sampling and nuclear SSU rDNA sequencing, we delimited species within the group of Lepocinclis ovum‐like taxa. Morphological and molecular features were analyzed for taxa isolated from Poland and six cultured strains from algal collections. In the case of environmental sampling, DNA was obtained from a small number of cells (20–400) isolated with a micropipette without setting up laboratory cultures (52 isolates), and phylogenetic analyses were based on the variation in nSSU rDNA. Apart from L. ovum and L. globulus, 13 other species were distinguished and four taxa (Lepocinclis conica comb. nov., L. fominii comb. nov., L. gracilicauda comb. nov., and L. pseudofominii nom. nov.) had their taxonomic ranks changed. For all verified species, diagnostic descriptions were emended and epitypes designated. The only exception was L. ovum, for which the epitype was questioned and thus, a new candidate for the epitype was suggested for future adoption.

Morphological and molecular features were analyzed for a species of Phacus to better understand the phylogenetic relationships among them and establish the taxonomy. Phylogenetic analyses were based on nSSU rDNA and the research resulted in 55 new sequences. The study included species available in algal collections and those isolated directly from the environment in Poland and the Czech Republic. As a result, the obtained phylogenetic tree of Phacus includes 50 species, out of which 7 are represented on a tree for the first time (Phacus anacoelus, P. anomalus, P. curvicauda, P. elegans, P. lismorensis, P. minutus and P. stokesii) and many have been taxonomically verified. For all verified species, diagnostic descriptions were amended, the naming was reordered and epitypes were designated.

DNA sequences were analyzed for three groups of species from the Lepocinclis genus (L. acus‐like, L. oxyuris‐like, and L. tripteris‐like) along with cellular morphology. Phylogenetic analyses were based on nuclear SSU rDNA, LSU rDNA, and plastid‐encoded LSU rDNA. DNA sequences were obtained from species available in culture collections (L. acus SAG 1224‐1a and UTEX 1316) and those isolated directly from the environment in Poland (48 isolates), resulting in 79 new sequences. The obtained phylogenetic tree of Lepocinclis included 27 taxa, five of which are presented for the first time (L. convoluta, L. gracillimoides, L. longissima, L. pseudospiroides, and L. torta) and nine taxonomically verified and described. Based on morphology, literature data, and phylogenetic analyses, the following species were distinguished: in the L. acus‐like group, L. longissima and L. acus; in the L. tripteris‐like group, L. pseudospiroides, L. torta, and L. tripteris; in the L. oxyuris‐like group, L. gracillimoides, L. oxyuris var. oxyuris, and L. oxyuris var. helicoidea. For all verified species, diagnostic descriptions were emended, nomenclatural adjustments were made, and epitypes were designated.

Plastids, similarly to mitochondria, are organelles of endosymbiotic origin, which retained their vestigial genomes (ptDNA). Their unique architecture, commonly referred to as the quadripartite (four-part) structure, is considered to be strictly conserved; however, the bulk of our knowledge on their variability and evolutionary transformations comes from studies of the primary plastids of green algae and land plants. To broaden our perspective, we obtained seven new ptDNA sequences from freshwater species of photosynthetic euglenids – a group which obtained secondary plastids, known to have dynamically evolving genome structure, via endosymbiosis with a green alga. Our analyses have demonstrated that the evolutionary history of euglenid plastid genome structure is exceptionally convoluted, with patchy distribution of inverted ribosomal operon (rDNA) repeats, as well as several independent acquisitions of tandemly repeated rDNA copies. Moreover, we have shown that inverted repeats in euglenid ptDNA do not share their genome stabilizing property documented in chlorophytes. We hypothesize that the degeneration of the quadripartite structure of euglenid plastid genomes is connected to the group II intron expansion. These findings challenge the current global paradigms of plastid genome architecture evolution, and underscore the often-underestimated divergence between the functionality of shared traits in primary and complex plastid organelles.