The family Metopidae Kahl, 1927 comprises diverse anaerobic ciliates, prompting several taxonomic re-evaluations. Among its constituent genera, Planometopus Rotterová et al., 2018 was established as a monotypic genus based on the type species P. contractus (Penard, 1922) Rotterová et al., 2018, and its diagnosis was derived solely from that species. This study presents an integrative revision of this genus, including a new species, Planometopus striatulus n. sp. This new species is characterized by a body size of 100-130 × 50-70 μm in vivo, conspicuous cortical ribs, obliquely arranged somatic kineties comprising 15-22 rows of densely packed dikinetids, an adoral zone with 29-37 membranelles, and an almost entire portion of the perizonal stripe segmented into 71-86 false kineties. The Korean population of P. contractus exhibits a diplostichomonad-type paroral membrane, a feature that has not been described previously. Based on the new species and a comparative reassessment of P. contractus, our results suggest an improved diagnosis of the genus. Body size and the arrangement of somatic dikinetids demonstrate interspecific variability. Furthermore, molecular phylogeny based on 18S rRNA gene sequences robustly confirms the monophyly of Planometopus and supports the distinctiveness of P. striatulus n. sp.
A new amoeba species, Vannella russkaya sp. nov., is described based on morphological (light and transmission electron microscopy) and molecular (SSU rRNA and Cox1) analyses. The strain was isolated from soil near a brackish stream, Solonik (Novgorod Region, Russia), and was cultivated under marine conditions. Phylogenetic analysis placed the new species within a clade comprising predominantly marine Vannella, with the closest affinity to V. salarenaria, which was also found in soil and cultivated under marine conditions. The studied strain exhibits a broad tolerance to salinity from 5‰ to 50‰, with trophic forms observed. Notably, it does not form cysts under the tested salinities. These features suggest adaptation to saline-influenced terrestrial environments rather than strictly aquatic habitats. The phylogenetic relationship with marine taxa, combined with isolation from soil, suggests an ecological shift across habitat boundaries. This highlights the significance of transitional environments, such as brackish soil, as reservoirs of previously unrecognized protist diversity, demonstrating that saline-influenced terrestrial habitats can harbor lineages typically regarded as marine, thereby extending the known ecological range of the genus Vannella.
The genus Leptomyxa unifies two morphologically distinct groups of species: reticulate and non-reticulate. Previously, it was believed that only large reticulate species were capable of adopting a fan-shaped form. In recent years, fan-shaped cells have been discovered in four small non-reticulate species. In this study, we describe Leptomyxa echinata n. sp., isolated from a bottom sediment sample from Sobachiy Pond, Izmailovsky Park, Moscow, Russia. Amoebae of this species are non-reticulate, flattened, and branched, and can adopt a monopodial form. Interestingly, the morphology of monopodial cells differs on the surface of glass slides and in culture dishes. Leptomyxa echinata n. sp. is capable of adopting a remarkable fan-shaped form with numerous pointed outgrowths along the anterior margin. In addition, the karyoplasm of these amoebae, besides rounded nucleoli, contains tiny rounded granules. These features have not previously been described for other species of the genus Leptomyxa. The 18S rRNA gene sequence differentiates L. echinata n. sp. from all other species of this genus.
Recent works have demonstrated the importance of convergences in the evolutionary history of Arcellinida. Notably, test traits, traditionally used as diagnostic criteria in taxonomy, often present morphological convergences when species live in similar habitats. Understanding how strongly these characteristics are conserved within lineages or acquired independently is fundamental for interpreting Arcellinida evolution and for functional analyses in community ecology. This work focused on two well-characterised functional traits related to nutrient acquisition: aperture morphology (associated with large-prey hunting) and symbiosis with microalgae, and placed their occurrence in a phylogenetic context with the aid of a new COI primer designed for Arcellinida. In both cases, traits appeared to be labile across the trees, emphasizing the importance of convergence in Arcellinida evolution. These results call for a better assessment of trait conservatism in the group in order to use phylogenetic signals to improve our interpretation of environmental data.
A new species from a yet undescribed group of small monotactic amoebae was isolated from a cage with farmed rainbow trout (Oncorhynchus mykiss). Artificial salinization of the original water sample to 18‰ was required to recognize and isolate this amoeba, whereas in cultures with a salinity level corresponding to freshwater, the amoebae were in an encysted state. The studied amoeba is closely related to the clade of strains isolated from the internal tissues of various fish: the liver of tench (Tinca tinca), the liver of carp (Cyprinus carpio), and the kidney of freshwater stinging catfish (Heteropneustes fossilis) in the Czech Republic. Members of this clade were never described taxonomically. Based on the integrative approach, this clade is described as a new genus of Tubulinea, Tuberomeba gen. Nov., while the studied isolate is described as a new species, T. capitovia sp. nov.
The microbial eukaryote diversity of the Amazonian rainforest remains mostly undescribed, particularly for higher-altitude sites located within indigenous lands that are characterized by very difficult access. We obtained a bryophyte sample during the Brazilian expedition to the Serra do Imeri in 2022, one of the highest elevations in the forest. We preserved part of the sample and examined another part to determine the identity of bryophyte species and to search for Arcellinida shelled amoebae. In this single sample, we found 142 shells distributed across 17 different nominal taxa. We found one shell of a potentially new taxon, similar to representatives of the genus Padaungiella. Here we provide a morphological characterization of the recorded taxa, using both optical and scanning electron microscopy, including morphometrics and morphological descriptions. We conclude with taxonomic remarks and a discussion of morphological variation in the specimens found at Serra do Imeri.
Hydrological regimes of river ecosystems are increasingly altered by climate change and other human-induced stressors, yet how these alterations affect aquatic metacommunity assembly remains unclear. Here, we investigated seasonal zooplankton dynamics in the subtropical Chishui River in southern China by sampling 80 sites across the dry and wet seasons. Using the morphology-based approach, we identified 146 zooplankton taxa, including protists, rotifers, cladocerans, and copepods, with protists accounting for nearly half of the total taxa and being dominated by ciliates. By integrating physicochemical, climatic, and land-use data with neutral community models and co-occurrence network analysis, we found a clear seasonal shift in assembly mechanisms: stochastic dispersal dominated during the hydrologically connected wet season, whereas deterministic environmental filtering prevailed during the dry season. Unexpectedly, higher α-diversity was associated with lower cohesion-based network stability, suggesting that seasonal community assembly processes may influence the diversity-cohesion relationship. During the dry season, deterministic assembly processes were more prominent, and community composition was associated with nutrient-related variables. These findings suggest that maintaining natural flow variability and reducing nutrient inputs may help sustain habitat heterogeneity and seasonal metacommunity dynamics in river ecosystems.
Acanthamoeba is a widespread free-living amoeba that can cause rare but serious opportunistic infections in humans and animals. Various species have been described based on morphological criteria that have proven unreliable, and strain identification is currently based on genotype assignment using nuclear 18S rDNA sequences. Most Acanthamoeba strains belong to genotype T4, which is the most abundant in the environment as well as the most frequently reported in clinical samples. Due to its richness, the T4 genotype has been further subdivided into subtypes, which correspond almost perfectly to the subtypes of the mitochondrial small subunit (mtSSU) rDNA. The 18S/mtSSU subtype clusters provide a clearer picture of species groups, and other subtypes remain to be identified. In this study, we provide evidence, through 18S/mtSSU co-clustering, for the existence of additional subtypes of the Acanthamoeba T4 genotype, two nuclear and four mitochondrial, and describe a new species, Acanthamoeba pussardi sp. nov. The data are supported by the analysis of two mitochondrial protein-coding genes, cytochrome c oxidase subunit 1 (cox1) and nicotinamide adenine dinucleotide (NADH) dehydrogenase subunit 5 (nad5), as well as the mitochondrial rDNA spacer.
Endosymbiotic ciliates of the subclass Trichostomatia (Litostomatea) live in the digestive tract of various vertebrates. Trichostomes show remarkable morphological diversity, which has yet to be studied thoroughly using molecular methods. In this paper, we investigated intraspecific variability of three species (Cochliatoxum periachtum, Tripalmaria dogieli, and Cycloposthium edentatum) from the intestine of equids for the first time. Two molecular markers were used, the 18S rDNA and the internal transcribed spacer (ITS) region, the latter of which is often considered a robust molecular marker for the recognition of closely related taxonomic groups. We analyzed 182 sequences of the 18S rRNA gene and 141 sequences of the ITS region from both rumen and intestinal ciliates, belonging to different families within the orders Vestibuliferida, Entodiniomorphida, and Macropodiniida. Both original and publicly available data were used. The differences in variability of both molecular markers for representatives of different genera and families of ciliates were compared.
There is a paucity of ultrastructural data on hypotrichous ciliates. In the present work, a comprehensive investigation of the ultrastructure, morphology, morphogenesis, and molecular phylogeny of a hypotrichous species, Parentocirrus hortualis, was performed. Historically, the taxonomy of Parentocirrus has been complicated by morphological variability. While P. brasiliensis is nearly identical to the type species P. hortualis in most respects, it differs in the presence of additional dorsal kinetids, a key distinction that now appears unstable. Consequently, we propose that P. brasiliensis should be recognized as a synonym of P. hortualis. In terms of the ultrastructure of P. hortualis, we found that: (1) the semi-rigid or rigid cortex of stylonychine species might be related to the presence of a single or double layer(s) of subpellicular microtubules, respectively; (2) the highly degenerated kinetosome of the dorsal bristle unit, i.e., the parakinetosomal body, has only been observed in two distantly related species of the subfamily Stylonychinae, suggesting this feature might have evolved independently; and (3) the ultrastructure of other organelles, such as the perilemma, buccal lip, buccal seal, and pharyngeal disks, was also documented. This is the first report on the ultrastructural features of P. hortualis and of the genus Parentocirrus, which should help improve the diagnosis and definition of this widely distributed species.
The myxomycete genus Collaria has long received limited research attention and is frequently confused morphologically with the genera Comatricha and Lamproderma. During a myxomycete diversity survey in Shanghai, China, two distinctive specimens were obtained via moist chamber cultures of leaf litter collected from Gongqing National Forest Park. These specimens are characterized by an early-deciduous peridium leaving a distinct and persistent basal collar, a columella that terminates abruptly near the middle of the sporotheca and branches near its apex to form the capillitium, and capillitial threads bearing conspicuous spinose, dentate, or semi-annulate ornamentation at their free ends. Based on detailed morphological examination and a comprehensive comparison with all known species of both Collaria and Comatricha, this taxon is considered to represent a new species. Although attempts to obtain DNA sequences (nuclear 18S rDNA, translation elongation factor 1-alpha, and mitochondrial ribosomal small subunit RNA genes) using universal primers for dark-spored myxomycetes were unsuccessful, thereby preventing the determination of its molecular phylogenetic position, its stable combination of morphological characteristics aligns it more closely with the current concept of Collaria. Consequently, it is formally described here as the new species Collaria spinifila. Detailed photographic plates and a taxonomic discussion are provided.
Iron (Fe) is an essential element for living organisms and plays a central role in numerous biological processes. Although Fe uptake mechanisms have been extensively characterized in plants, they remain poorly understood in microalgae, particularly in dinoflagellates. In this study, we identified two novel ferric reduction oxidase genes (FROs) in the marine dinoflagellate Alexandrium pacificum (designated ApFRO1 and ApFRO2) and examined their expression under varying Fe conditions. ApFRO1 and ApFRO2 encode open reading frames of 2205 bp and 2688 bp, respectively, each containing ferric reductase family domains. Both proteins possess multiple transmembrane regions, consistent with predicted localization at the plasma membrane. Significant differences in cell growth were observed across Fe treatments. Strong upregulation of both ApFROs (up to 34.9-fold) occurred under Fe-depleted conditions over a three-day period. Both Fe deficiency and Fe excess reduced chlorophyll autofluorescence (CAF), indicating impaired photosynthetic performance. Together, these findings demonstrate that fluctuations in Fe availability markedly influence the physiology of A. pacificum, and that cells activate an Fe3+-reductase-mediated Fe acquisition pathway involving FROs.
Microorganisms are vital to trophic dynamics and biogeochemical cycling in estuarine-offshore ecosystems, yet integrated analyses of prokaryotic and microeukaryotic communities across water and sediment habitats remain limited for the Yangtze River Estuary (YRE). We combined 16S and 18S rDNA amplicon sequencing to characterize prokaryotic and microeukaryotic communities in surface water, bottom water, and sediment samples collected along an estuary-offshore transect in the YRE. Microeukaryotes showed higher alpha diversity in the water column than in sediments, whereas prokaryotes were more diverse in sediments. These contrasting diversity patterns were accompanied by clear shifts in dominant taxa, from benthic and anaerobic lineages in sediments and hypoxic bottom waters to phototrophic and copiotrophic groups in nutrient-rich estuarine surface waters. Along the estuary-offshore continuum, salinity, nutrients, and dissolved oxygen were the main drivers of community turnover, with a transition from eutrophic, low-salinity assemblages to saline, oligotrophic, and low-oxygen offshore communities, including potentially harmful dinoflagellates. Co-occurrence networks revealed widespread associations between microeukaryotes and prokaryotes, with microeukaryotes exhibiting a relatively sparse and modular interaction structure and prokaryotes showing a dense, highly clustered, and less compartmentalized network. These findings underscore the combined influence of habitat heterogeneity and environmental gradients on microbial diversity and community structure in the YRE.
An eyespot-bearing armored dinoflagellate from the Philippines is described as a novel Pentapharsodinium species based on distinct cellular morphology, thecal plate tabulation, and phylogenetic position inferred from ITS and LSU rDNA sequences. Cells were ovoid to teardrop-shaped, with an episome slightly larger than the hyposome. An eyespot was present in the sulcal region, and the nucleus was located centrally to posteriorly. The plate tabulation Po, cp, X, 4', 3a, 7'', 5c, 5s, 5''', 2'''', with a spine on plate C1, was consistent with Pentapharsodinium, although variation occurred in the number of postcingular and antapical plates. Both a round apical pore plate with a shallow indentation forming a V-shaped concavity and a rectangular X-plate were flanked by ridges of plates 2' and 4'. Plate 7'' was isodiametric, and plate Sa was small. Pentapharsodinium ocelliferum sp. nov. can be distinguished by the presence of an eyespot and distinctive thecal plate features, including a reduced Sa plate, an isodiametric 7'' plate, and characteristic features of the apical pore. Temporary cysts were common, while resting cysts with smooth, thick walls were rare in culture. The new species was positioned within the Pentapharsodinium clade, with P. imariense as its closest relative in the ITS phylogeny.
The subclass Suctoria is considered one of the most distinctive groups among ciliates due to the absence of cilia in the trophont stage. Biological control using suctorians may promote sustainable aquaculture while mitigating risks associated with pesticide use and drug residues. In this study, four species from wetland habitats in China were investigated using morphological and molecular methods. These species include one new species, Metacineta perimystacina n. sp., and three poorly known species: Spelaeophrya polypoides, Trichophrya epistylidis, and Discophrya robusta. Metacineta perimystacina is distinguished by a stalk comprising about 70% of the total body length and by six slits on the lorica. Small subunit rDNA sequences were obtained for all four species to determine their systematic positions. Phylogenetic analyses indicate that the orders Exogenida and Endogenida are polyphyletic, and they further suggest that the semicircumvaginative reproductive mode may represent a transitional state between the exogenous and endogenous types. Spelaeophrya polypoides exhibits a distinctive mode of budding known as vermigemmy, characterized by the lateral production of a vermiform, unciliated swarmer. Its distinct phylogenetic position highlights the uniqueness of this lineage and suggests that it may represent a new order. Finally, some species may show substrate/host specificity, providing new insights into ecological management and pathogen control.
Ciliate conjugation represents a fundamental biological process that involves complex nuclear reorganization and genetic exchange. In this study, we employed an integrated set of cytological and molecular approaches to investigate Euplotes songi-a phylogenetically basal species of the genus Euplotes-with the aim of determining the entire cascade of nuclear events during conjugation and obtaining new information on the macronuclear pheromone-coding genes. As is common in Euplotes species, the micronucleus of mating cells enters meiosis following a mitotic duplication; two of the resulting eight haploid nuclei undergo a further mitotic division to generate four haploid products, from which the migratory and stationary gametic nuclei are ultimately selected. Unlike phylogenetically more recent Euplotes species, in which mating cells completely resorb the parental macronucleus during conjugation, E. songi only resorbs the anterior fragment of the old macronucleus, while the posterior portion is likely to be functionally integrated into the primordium of the new macronucleus. Furthermore, we identified novel pheromone gene variants via crosses between wild-type strains, revealing that most polymorphisms occur in the 3' trailer region, one occurs in the 5' leader region, and only three are located within the coding region without altering the amino acid sequences of the known E. songi pheromones. Phylogenetic comparison of these pheromone sequences with homologous sequences from other Euplotes species, based on the well-established Euplotes SSU rDNA phylogenetic analysis, supports the conclusion that E. songi forms the earliest-diverging Euplotes clade together with E. petzi, E. sinensis, and E. huizhouensis. These findings provide valuable insights into the ancestral biological and genetic traits of Euplotes.
Polystyrene microplastics, as emerging environmental contaminants, have exerted widespread impacts on multicellular organisms in aquatic ecosystems. However, little is known about how they affect ecologically crucial unicellular eukaryotes, especially filter-feeding ciliates that connect the microbial and classical trophic networks in aquatic environments. Here, we investigated the filter-feeding ciliate Stentor coeruleus as a model organism, revealing that it exhibited strong survival capabilities under high-concentration microplastic stress, with phenotypic changes limited to altered pigment production and reduced predatory capacity. Notably, these phenotypes returned to normal within 24 h after microplastic removal, indicating a reversible adaptation mechanism. Our transcriptomic profiling revealed that microplastics activate redox stress responses in S. coeruleus, including the upregulation of key oxidoreductase genes (e.g., GST, GPx, SOD, and aldo-keto reductases), modulation of lipid metabolism, membrane protein modification, proteostasis maintenance (mediated by HSP70, DnaJ, and ubiquitin-related genes), and xenobiotic clearance through upregulation of the ABC transporter family. Furthermore, our feeding recovery experiments provide the first evidence that short-term exposure to high microplastic concentrations, despite not affecting survival rates, significantly disrupts energy transfer efficiency within microbial food webs. Overall, these results uncover the underlying molecular mechanisms and offer a novel ecological perspective on the threat posed by microplastics to microbial food webs.
Permafrost ecosystems serve as unique reservoirs for ancient microbial life, yet the diversity and long-term viability of protists, particularly centrohelid heliozoans, remain poorly understood. In this study, we investigated an enrichment culture of a centrohelid heliozoan isolated from a Siberian permafrost core sample retrieved from a depth of 9 m and dated to the Late Pleistocene (approximately 39,000 years BP). Using an integrative taxonomic approach combining light and scanning electron microscopy with 18S rRNA gene phylogeny, we describe a new species, Acanthocystis yamallongha sp. nov. This novel centrohelid is characterised by a unique spiral-like ridge ornamentation on its plate-scales, a morphological feature that has not been detected in species from contemporary aquatic or soil habitats, as well as by spine-scales with bifurcated tips. It exhibits a high level of genetic divergence from known species and likely represents an independent evolutionary lineage sister to the clade comprising freshwater representatives of A. takahashii and soil-derived environmental sequences. Our findings provide evidence for the ability of Acanthocystidae cysts to retain viability over geological timescales. The identification of A. yamallongha highlights that a close investigation of permafrost ecosystems is essential for uncovering the hidden diversity of ecologically important protists and clarifying their phylogenetic relationships.
The paper describes Nolandella shirvanensis sp. nov., a newly identified species of the marine amoeba genus Nolandella Page, 1983 (Tubulinea, Nolandida). It represents the first record of this genus from an athalassohaline biotope: a cold mud volcano with methane seeps in Gobustan National Park (Azerbaijan). Amoebae were isolated and cultivated in seawater with a salinity of 30‰, but they had an experimentally determined salinity tolerance range of 10-120‰. Morphological and molecular phylogenetic analyses show that the genus Nolandella comprises three named species (i.e., N. hibernica (Page, 1980), N. abertawensis (Page, 1980), and N. shirvanensis sp. nov.) and a fourth species, distinct from other members of this genus, known from the strain Nolandella sp. ATCC 50913. Molecular analysis of the diversity of Nolandella spp. expands the number of known actin paralogs in this genus and demonstrates a unique insertion in the mitochondrial cytochrome c oxidase subunit 1 (Cox1) gene, with a 590 bp group I intron nested inside it. This is the second detection of a group I intron in the Cox1 gene in a member of the Tubulinea.
