Phylogenetic relationships between geographically separate Phormidium cyanobacteria: is there a link between north and south polar regions?

We present a phytogeographical comparison between polar (Arctic and Antarctic) and non-polar strains of the cyanobacterial genus Phormidium, which plays a key role in Arctic and Antarctic ecosystems as primary producer. A total of 26 Phormidium strains were studied using a polyphasic approach, 18 from Arctic (Svalbard, Ellesmere Island and Scandinavian Arctic—Abisko) and Antarctic (Antarctic Peninsula—King George and James Ross Island) regions, and 8 from temperate sites (mostly situated in Central Europe). A phylogenetic tree was constructed and compared with similar 16S rRNA sequences retrieved from Genbank. Within the Phormidium autumnale cluster, genetic similarity of 16S rDNA was more related to geographical proximity of strain origin than to morphological similarity. No genetic identity of Phormidium strains from north and south polar regions was found. The cluster Phormidium autumnale apparently belongs to generic entities in which geographical limitation plays a prominent role. However, the cyanobacterial strains found in Europe suggest that the distribution areas of some Phormidium cyanobacteria overlap. The Phormidium autumnale cluster is evidently a very characteristic type and represents an isolated clade within the traditional genus Phormidium. According to morphological features and the structure of trichomes, it is most similar and thus probably belongs to the genus Microcoleus.     

Comprehensive comparisons of three pennate diatoms, Diatoma tenuae, Fragilaria vaucheriae, and Navicula pelliculosa, isolated from summer Arctic reservoirs (Svalbard 79°N), by fine-scale morphology and nuclear 18S ribosomal DNA

Here we report morphological and molecular characteristics of dominant freshwater diatoms in summer Arctic reservoirs of Svalbard (Norway), using four culture isolates, when we collected the samples in the field on 15 August 2005. Analyses of morphology and BLAST searches with 18S rDNA sequences identified them to Diatoma tenue (HYNP006, HYNP013), Navicula pelliculosa (HYNP021), and Fragilaria vaucheriae (HYNP022), respectively. Comparative studies of morphology revealed that the body shapes of the three polar diatoms were nearly identical to the known morphology of each species; however, they were considerably shorter in body length than previously described identical species from other locations. The 18S rDNA sequences of the diatoms were nearly identical to the same species from temperate and other regions. Phylogenetic analysis showed that the polar diatoms each formed a clade with their identical species and genera according to their taxonomic positions. This suggests that the polar diatoms may possess little or no genetic or morphological variation compared to more temperate strains.     

Identifying monophyletic groups within Bugula sensu lato (Bryozoa,Buguloidea)

Species in the genus Bugula are globally distributed. They are most abundant in tropical and temperate shallow waters, but representatives are found in polar regions. Seven species occur in the Arctic and one in the Antarctic and species are represented in continental shelf or greater depths as well. The main characters used to define the genus include bird's head pedunculate avicularia, erect colonies, embryos brooded in globular ooecia and branches comprising two or more series of zooids. Skeletal morphology has been the primary source of taxonomic information for many calcified bryozoan groups, including the Buguloidea. Several morphological characters, however, have been suggested to be homoplastic at distinct taxonomic levels, in the light of molecular phylogenies. Our purpose was to investigate the phylogenetic interrelationships of the genus Bugula, based on molecular phylogenetics and morphology. A Bayesian molecular phylogeny was constructed using original and previously published sequences of the mitochondrial genes cytochrome c oxidase subunit 1 (COI) and the large ribosomal RNA subunit (16S). Morphological characteristics from scanning electron and light microscopy were used to confirm the clades detected by the molecular phylogeny. Our results suggest that the genus is composed of four clades, for which we provide diagnoses: Bugula sensu stricto (30 species), Bugulina (24 species), Crisularia (23 species) and the monotypic Virididentula gen. n. Ten species could not be assigned to any of those genera, so they remain as genus incertae sedis. Our findings highlight the importance of using molecular phylogenies in association with morphological characters in systematic revisions of bryozoan taxa.     

Global phylogeography of pelagic Polynucleobacter bacteria: Restricted geographic distribution of subgroups,isolation by distance and influence of climate

The free‐living planktonic freshwater bacterium Polynucleobacter necessarius subspecies asymbioticus (> 99% 16S rRNA similarity) represents a taxon with a cosmopolitan distribution and apparently ubiquitous occurrence in lentic freshwater habitats. We tested for intra‐taxon biogeographic patterns by combining cultivation‐independent and cultivation methods. A culture collection of 204 strains isolated from globally distributed freshwater habitats (Arctic to Antarctica) was investigated for phylogeographic patterns based on sequences of two markers, the 16S–23S internal transcribed spacers and the glutamine synthetase gene (glnA). Genetic distance between isolates showed significant geographic distance‐decay patterns for both markers, suggesting that an isolation‐by‐distance mechanism influences the global phylogeography. Furthermore, a couple of subgroups showed restricted geographic distributions. Strains of one subgroup were exclusively obtained from tropical sites on four continents (pantropical subgroup). Cultivation‐independent methods were used to confirm the restricted geographic distributions of two subgroups. The pantropical taxon could be detected in 63% of investigated tropical habitats but not in any of 121 European freshwater samples. Physiological tests indicated that almost all strains of the pantropical subgroup failed to grow at temperatures of 4°C, while strains affiliated with other subgroups showed good growth at this temperature. This suggests that thermal adaptation is involved in phylogeographic structuring of the global Polynucleobacter population.     

MOLECULAR PHYLOGENY OF PHYCOCYANIN‐CONTAINING CRYPTOPHYTES: EVOLUTION OF BILIPROTEINS AND GEOGRAPHICAL DISTRIBUTION1

Of 34 strains assigned to the cryptophyte genera Chroomonas Hansg., Hemiselmis Parke, and Komma D. R. A. Hill, distribution patterns of biliproteins, habitats, and sampling sites across a phylogenetic tree have been examined. The combined data set assembled from nuclear SSU rDNA, partial nuclear LSU rDNA, and nucleomorph SSU rDNA sequences comprised 4,083 positions and yielded an almost completely resolved tree. Spectrophotometry of the biliproteins and mapping of the different types of biliproteins onto the phylogenetic tree unveiled a complex evolutionary history. Different from other cryptophyte clades, the types of biliproteins were not generally congruent with clades or subclades of the genera Chroomonas (paraphyletic, phycocyanins [PCs] 645 or 630), Hemiselmis (PCs 612, 630 or phycoerythrin [PE] 555), and Komma (PC 645). At least one putative character reversal took place in the genus Chroomonas. Several changes in biliproteins have been found in the genus Hemiselmis, including two new biliprotein variants that probably originated by slight modifications from PC 612 and PE 555, respectively (PC 577 and PE 545/555). Freshwater and marine/brackish taxa were intermingled across the tree without displaying a specific pattern. In four terminal clades, genetically identical strains have been found to occur both in Europe and in the USA. The Chroomonas/Hemiselmis/Komma clade proved to be the most diverse of all cryptophyte clades concerning types of biliproteins and distribution of clades across marine or freshwater habitats.     

Marine trophic niche use and life history diversity among Arctic charr Salvelinus alpinus in southwestern Greenland

Life history strategies and potential marine niche use of Arctic charr Salvelinus alpinus (n = 237, 84–652 mm, total body length, L_T) were determined during the ice-free season (2012) at three different watercourses in south-western Greenland. All Arctic charr were collected from freshwater habitats. Based on stable isotopes of δ³⁴S, the Arctic charr were categorized as either marine- or freshwater-dependent feeders. The use of time-integrated trophic tracers (stable isotopes of δ¹³C, δ¹⁵N, δ³⁴S) suggested that several trophic groups of Arctic charr operate alongside within each fjord system. The groups suggested were one group that specialized in the marine habitat, in addition to two freshwater resident morphs (small-sized resident and/or large-growing cannibalistic individuals). Stomach contents consisted entirely of freshwater and terrestrial prey (i.e., insects), indicating that marine-dependent feeders also fed in freshwater habitats after return from their marine migration. Growth and maturity patterns further supported variable life history strategies within each watercourse. The life history strategy patterns and marine trophic niche use were consistent across the watercourses along several hundred kilometres of coastline. This study represents the first ecological baseline for partially anadromous populations of Greenland Arctic charr.     

DISTRIBUTION,MORPHOLOGY, AND PHYLOGENY OF KLEBSORMIDIUM (KLEBSORMIDIALES,CHAROPHYCEAE) IN URBAN ENVIRONMENTS IN EUROPE1

Klebsormidium is a cosmopolitan genus of green algae, widespread in terrestrial and freshwater habitats. The classification of Klebsormidium is entirely based on morphological characters, and very little is understood about its phylogeny at the species level. We investigated the diversity and phylogenetic relationships of Klebsormidium in urban habitats in Europe by a combination of approaches including examination of field‐collected material, culture experiments conducted in many different combinations of factors, and phylogenetic analyses of the rbcL gene. Klebsormidium in European cities mainly occurs at the base of old walls, where it may produce green belts up to several meters in extent. Specimens from different cities showed a great morphological uniformity, consisting of long filaments 6–9 μm in width, with thin‐walled cylindrical cells and smooth wall, devoid of false branches, H‐shaped pieces, and biseriate parts. Conversely, the rbcL phylogeny showed a higher genetic diversity than expected from morphology. The strains were separated in four different clades supported by high bootstrap values and posterior probabilities. In culture, these clades differed in several characters, such as production of a superficial hydro‐repellent layer, tendency to break into short fragments, and inducibility of zoosporulation. On the basis of the taxonomic information available in the literature, most strains could not be identified unambiguously at the species level. The rbcL phylogeny showed no correspondence with classification based on morphology and suggested that the identity of many species, in particular the type species K. flaccidum (kütz.) P.C. Silva, Mattox et W. H. Blackw., needs critical reassessment.     

Mantoniella beaufortii and Mantoniella baffinensis sp. nov. (Mamiellales,Mamiellophyceae), two new green algal species from the high arctic1

Members of the class Mamiellophyceae comprise species that can dominate picophytoplankton diversity in polar waters. Yet, polar species are often morphologically indistinguishable from temperate species, although clearly separated by molecular features. Here we examine four Mamiellophyceae strains from the Canadian Arctic. The 18S rRNA and Internal Transcribed Spacer 2 (ITS2) gene phylogeny place these strains within the family Mamiellaceae (Mamiellales, Mamiellophyceae) in two separate clades of the genus Mantoniella. ITS2 synapomorphies support their placement as two new species, Mantoniella beaufortii and Mantoniella baffinensis. Both species have round green cells with diameter between 3 and 5 μm, one long flagellum and a short flagellum (~1 μm) and are covered by spiderweb-like scales, making both species similar to other Mantoniella species. Morphologically, M. beaufortii and M. baffinensis are most similar to the cosmopolitan M. squamata with only minor differences in scale structure distinguishing them. Screening of global marine metabarcoding data sets indicates M. beaufortii has only been recorded in seawater and sea ice samples from the Arctic, while no environmental barcode matches M. baffinensis. Like other Mamiellophyceae genera that have distinct polar and temperate species, the polar distribution of these new species suggests they are cold or ice-adapted Mantoniella species.     

Teleomorph-anamorph relationships and reclassification of <Emphasis Type="Italic">Cordyceps cuboidea</Emphasis> and its allied species

Based on morphological characteristics and molecular phylogeny, we reclassified Cordyceps cuboidea and allied species C. alboperitheciata, C. prolifica, and Ophiocordyceps ryogamiensis. We investigated their teleomorph-anamorph relationships and revealed that these four species have Hirsutella-like anamorphs with morphological differences between them. By analyzing their molecular phylogeny, inferred from DNA sequences of internal transcribed spacer (ITS) and large subunit (LSU) D1/D2 region of rDNA, they were separated into four close-knit clades. Although C. prolifica and O. ryogamiensis formed their own clades, isolates of C. cuboidea separated into two clades, i.e., a true C. cuboidea clade and one resembling a new species, the O. paracuboidea clade. The latter two species are distinguished by the fruiting region of the stroma. In addition, C. alboperitheciata is regarded as a synonym of C. cuboidea. From the morphology, teleomorph-anamorph relationships, and molecular phylogeny, we concluded these species should be assigned to the genus Ophiocordyceps.     

Molecular and morphological characterization of the Achnanthidium minutissimum complex (Bacillariophyta) in Petuniabukta (Spitsbergen,High Arctic) including the description of A. digitatum sp. nov.

Recent morphology-based investigations of freshwater Arctic diatoms suggest that many species remain to be discovered, reflecting a unique polar flora. During a survey of the freshwater diatom flora of northern Billefjorden, including the Petuniabukta fjord region (Spitsbergen), several morphodemes belonging to the Achnanthidium minutissimum species complex were recorded. Molecular phylogenies based on rbcL, 28S and 18S sequences, including single cells from Canada and strains from Marion Island (sub-Antarctica) and GenBank revealed the presence of 12 distinct A. minutissimum complex lineages, of which three contained strains from Spitsbergen. One Arctic lineage is described as a new species. Achnanthidium digitatum sp. nov. is morphologically characterized by narrow, linear to only slightly lanceolate valves and usually two areolae per stria. The two remaining Arctic lineages are in need of a more complete morphological and molecular comparison with other representatives of the A. minutissimum complex to clarify their taxonomic identity. It is argued that implementation of molecular data in the taxonomy of Achnanthidium will be essential to solve the taxonomic problems associated with this group, eventually resulting in a better understanding of the biogeography and niche differentiation of different species belonging to the A. minutissimum complex. Ideally, this should be based on more variable genes than the currently widely used 18S, which does not have a species level resolution in the A. minutissimum complex.     

Genotypic Diversity of Oscillatoriacean Strains Belonging to the Genera <Emphasis Type="Italic">Geitlerinema</Emphasis> and <Emphasis Type="Italic">Spirulina</Emphasis> Determined by 16S rDNA Restriction Analysis

Genotypic diversity of several cyanobacterial strains mostly isolated from marine or brackish waters, belonging to the genera Geitlerinema and Spirulina, was investigated by amplified 16S ribosomal DNA restriction analysis and compared with morphological features and response to salinity. Cluster analysis was performed on amplified 16S rDNA restriction profiles of these strains along with profiles obtained from sequence data of five Spirulina-like strains, including three representatives of the new genus Halospirulina. Our strains with tightly coiled trichomes from hypersaline waters could be assigned to the Halospirulina genus. Among the uncoiled strains, the two strains of hypersaline origin clustered together and were found to be distant from their counterparts of marine and freshwater habitat. Moreover, another cluster, formed by alkali-tolerant strains with tightly coiled trichomes, was well delineated. Received: 14 May 2002 / Accepted: 22 July 2002     

Isolation and molecular identification of free-living amoebae of the genus Naegleria from Arctic and sub-Antarctic regions

Twenty-three freshwater samples with sediment taken from two regions in the Arctic, Spitzbergen and Greenland, and one region in sub-Antarctica, Ile de la Possession, were cultured for amoebae at 37 degrees C and room temperature (RT). Only two samples yielded amoebae at 37 degrees C and the two isolates were identified from their morphological features to belong to the genus Acanthamoeba. Vahlkampfiid amoebae were isolated from 11 samples at RT. Morphological analysis of the cysts identified all 11 isolates as belonging to the genus Naegleria, although only about half of them (45%) transformed into flagellates. Ribosomal DNA sequence analysis demonstrated that these isolates represent novel species and that N. antarctica, N. dobsoni and N. chilensis are their closest relatives. Not surprisingly, these three species also grow at lower temperatures (<37 degrees C) than the majority of described Naegleria spp. Two of the eight new species were found in both Arctic and sub-Antarctic regions, and other new species from the Arctic are closely related to new species from the sub-Antarctic. Therefore, it seems the Naegleria gene pool present in the polar regions is different from that found in temperate and tropical regions.     

Relationships between Arctic and Antarctic Shewanella strains evaluated by a polyphasic taxonomic approach

As a result of the recent application of DNA-based technologies to investigate bacterial diversity in the polar marine environment, bacterial strains sharing significant 16S rRNA gene sequence similarities were isolated from both the Arctic and the Antarctic sea waters. Three selected marine bacterial isolates (BSw20248, BSw20661 and BSw10166) from two Arctic regions (the Greenland Sea and Canada Basin) and from Antarctica (Prydz Bay) were studied to determine their evolutionary relationships and phylogenetic position using a polyphasic taxonomic approach, including phenotypic characterization and genotypic analyses. These bacterial isolates proved to belong to the same species and were identified as Shewanella frigidimarina. Differences in phenotypic properties observed among them revealed a diversity of ecotypes for adaptation to the changing ecological or geographical conditions. It provided the evidence that Shewanella frigidimarina has a bipolar, or even global, distribution in the marine environment.     

Salt tolerance and polyphyly in the cyanobacterium Chroococcidiopsis (Pleurocapsales)

Chroococcidiopsis Geitler (Geitler 1933) is a genus of cyanobacteria containing desiccation and radiation resistant strains. Members of the genus live in habitats ranging from hot and cold deserts to fresh and saltwater environments. Morphology and cell division pattern have historically been used to define the genus. To better understand the evolution and ability of the Chroococcidiopsis genus to survive in diverse environments we investigated how salt tolerance varies among 15 strains previously isolated from different locations, and if salt tolerant strains are monophyletic to those isolated from freshwater and land environments. Four markers were sequenced from these 15 strains, the 16S rRNA, rbcL, desC1, and gltX genes. Phylogenetic trees were generated which identified a distinct clade of salt‐tolerant strains. This study demonstrates that the genus is polyphyletic based on saltwater and freshwater phenotypes. To understand the resistance to salt in more details, the strains were grown on a range of sea salt concentrations which demonstrated that the freshwater strains were salt‐intolerant whilst the saltwater strains required salt for growth. This study shows an increased resolution of the phylogeny of Chroococcidiopsis and provides further evidence that the genus is polyphyletic and should be reclassified to improve clarity in the literature.     

Polar freshwater cyanophage S-EIV1 represents a new widespread evolutionary lineage of phages

Cyanobacteria are often the dominant phototrophs in polar freshwater communities; yet, the phages that infect them remain unknown. Here, we present a genomic and morphological characterization of cyanophage S-EIV1 that was isolated from freshwaters on Ellesmere Island (Nunavut, High Arctic Canada), and which infects the polar Synechococcus sp., strain PCCC-A2c. S-EIV1 represents a newly discovered evolutionary lineage of bacteriophages whose representatives are widespread in aquatic systems. Among the 130 predicted open reading frames (ORFs) there is no recognizable similarity to genes that encode structural proteins other than the large terminase subunit and a distant viral morphogenesis protein, indicating that the genes encoding the structural proteins of S-EIV1 are distinct from other viruses. As well, only 19 predicted coding sequences on the 79 178 bp circularly permuted genome have homology with genes encoding proteins of known function. Although S-EIV1 is divergent from other sequenced phage isolates, it shares synteny with phage genes captured on a fosmid from the deep-chlorophyll maximum in the Mediterranean Sea, as well as with an incision element in the genome of Anabaena variabilis (ATCC 29413). Sequence recruitment of metagenomic data indicates that S-EIV1-like viruses are cosmopolitan and abundant in a wide range of aquatic systems, suggesting they have an important ecological role.     

Phylogenetic relationships in Interfilum and Klebsormidium (Klebsormidiophyceae, Streptophyta)

Members of the genus Klebsormidium have cosmopolitan distribution and occur in a very wide range of freshwater and terrestrial habitats. Due to its simple filamentous morphology, this genus represents a taxonomically and systematically complex taxon in which phylogenetic relationships are still poorly understood. The phylogeny of Klebsormidium and closely related taxa was investigated using new ITS rRNA and rbcL sequences generated from 75 strains (isolated from field samples or obtained from culture collections). These sequences were analyzed both as single-marker datasets and in a concatenated dataset. Seven main superclades were observed in the analyses, which included sixteen well-supported clades. Some species of Klebsormidium, including the type species Klebsormidium flaccidum, were polyphyletic. Interfilum was recovered with high statistical support as sister taxon to a clade of Klebsormidium formed mainly by strains identified as K. flaccidum. Whereas some clades could be easily associated with described species, this was not possible for other clades. A new lineage of Klebsormidium, isolated from arid soils in southern Africa and comprising undescribed species, was discovered. Several morphological characters traditionally used for taxonomic purposes were found to have no phylogenetic significance and in some cases showed intra-clade variation. The capacity to form packet-like aggregates (typical of Interfilum), features of the morphology of the chloroplast and the type of habitat were the main phylogenetically relevant characters. Overall, Klebsormidium and Interfilum formed a more diverse algal group than was previously appreciated, with some lineages apparently undergoing active evolutionary radiation; in these lineages the genetic variation observed did not match the morphological and ecological diversity.     

Morphological Identities of Two Different Marine Stramenopile Environmental Sequence Clades: Bicosoeca kenaiensis (Hilliard, 1971) and Cantina marsupialis (Larsen and Patterson, 1990) gen. nov., comb. nov.

Although environmental DNA surveys improve our understanding of biodiversity, interpretation of unidentified lineages is limited by the absence of associated morphological traits and living cultures. Unidentified lineages of marine stramenopiles are called “MAST clades”. Twenty‐five MAST clades have been recognized: MAST‐1 through MAST‐25; seven of these have been subsequently discarded because the sequences representing those clades were found to either (1) be chimeric or (2) affiliate within previously described taxonomic groups. Eighteen MAST clades remain without a cellular identity. Moreover, the discarded “MAST‐13” has been used in different studies to refer to two different environmental sequence clades. After establishing four cultures representing two different species of heterotrophic stramenopiles and then characterizing their morphology and molecular phylogenetic positions, we determined that the two different species represented the two different MAST‐13 clades: (1) a lorica‐bearing Bicosoeca kenaiensis and (2) a microaerophilic flagellate previously named “Cafeteria marsupialis”. Both species were previously described with only light microscopy; no cultures, ultrastructural data or DNA sequences were available from these species prior to this study. The molecular phylogenetic position of three different “C. marsupialis” isolates was not closely related to the type species of Cafeteria; therefore, we established a new genus for these isolates, Cantina gen. nov.     

Tracking the history of an invasion: the freshwater croakers (Teleostei: Sciaenidae) in South America

In this study, the competing hypotheses of single vs. double colonisation events for freshwater Pachyurinae (Sciaenidae) in South America is tested and the historical biogeography of the expansion of this clade within the continent is reconstructed based on phylogenetic analysis. Parsimony and Bayesian inference (BI) for 19 marine and freshwater species assigned to Sciaenidae, Haemulidae and Polypteridae were determined based on partial sequences of the mitochondrial 16S and cytochrome b genes and fragments of the nuclear Tmo‐4C4 and rhodopsin genes. A parsimonious ancestral character reconstruction of euryhalinity was performed on a clade of families of closely related fishes to evaluate the role of ecological fitting in the colonisation of freshwater by a marine sciaenid. The parsimony and BI phylogenetic hypotheses for the concatenated sequences supported the monophyly of the freshwater Sciaenidae. Divergence of the two freshwater clades of Sciaenidae, Pachyurinae and Plagioscion, occurred within the Amazon Basin. Within Pachyurinae, two clades were recovered: one composed of species from the Amazon and the Paraná Basin and a second with representatives from the São Francisco and south‐eastern Atlantic basins. The results were compatible with the hypothesis of a single colonisation event of South American freshwater habitats by a marine lineage. The hypothesis of gradual adaptation to freshwater was rejected in favour of the hypothesis of ecological fitting. Sciaenidae, or a subordinate lineage within the family, is ancestrally capable of withstanding exposure to low‐salinity habitats, which putatively facilitated the colonisation of freshwater habitats. The subsequent diversification and expansion of Pachyurinae across South America followed this colonisation and replicated the general pattern of the area relationships of South American river basins for several other fish groups.     

Desulfuromonas acetexigens sp. nov., a dissimilatory sulfur-reducing eubacterium from anoxic freshwater sediments

Five strains of obligate anaerobic sulfur-reducing eubacteria that exclusively use acetate as energy and carbon source have been enriched and isolated from anoxic sulfide-containing freshwater mud. The strains were unable to grow in the presence of 2% NaCl. Morphologically the strains were not uniform, cells were either rod-shaped or elongated ovoid. All strains were flagellated with a single polar to subpolar flagellum. They stained gram-negative. Two of the strains were studied in detail. Malate or fumarate was used alternatively to elemental sulfur as electron acceptor. The capacity to grow on acetate as sole organic substrate and to reduce elemental sulfur or polysulfide to sulfide are traits in common with the genus Desulfuromonas. The strains differ from Desulfuromonas acetoxidans by their freshwater origin, morphology, metabolic specialization and their DNA base ratio. Therefore we consider the new isolates as a new species for which the name Desulfuromonas acetexigens is proposed.Friedhelm Bak deceased December 1992     

New species Micractinium kostikovii (Chlorellaceae,Trebouxiophyceae) from Russia

Correctly identifying species of Chlorella-like microalgae is difficult because of their morphological simplicity and high phenotypic plasticity. The use of molecular tools has revolutionized research on algal diversity, enabling such advances as the discovery of numerous new taxa. This article presents the results of a study of strains that are newly isolated from a freshwater Lake Prudovikov (Samara region, Russian Federation). These strains had the typical Chlorella morphology, exhibiting spherical cells and a cup-shaped parietal chloroplast. The chloroplast contained a single pyrenoid enveloped by starch grains. However, 18S–ITS1–5.8S–ITS2 sequence analyses indicated that the studied strains are strongly allied with the well-supported genus Micractinium. Based on the results of a comparative analysis of the morphological, molecular and environmental characteristics of the studied strains (employing a polyphasic approach), we propose that they are new species of Micractinium: Micractinium kostikovii sp. nov.