Ciliated Protist Research Articles

Comparative study on the larvicidal effect of some ciliated protists on Culex gelidus, Culex tritaeniorhynchus, and Aedes aegypti in Sri Lanka

Rice fields in Sri Lanka create suitable breeding places for vector mosquitoes. Such sites provide habitats for diversified naturally occurring microbiota. Ciliated protists, Zoothamnium sp., Chilodonella sp., and Vorticella microstoma are among such microbiota found in vector mosquito habitats especially in rice field habitats in Sri Lanka. The present study was carried out to determine the comparative larvicidal effect of these ciliated protists collected from naturally infested mosquito larvae in some rice-field habitats in Kurunegala, Sri Lanka, against vector mosquito larvae. Vector mosquito larvae, Culex tritaeniorhynchus, and Culex gelidus were reared in the laboratory from field-collected water samples while Aedes aegypti mosquito larvae were reared using egg sheets, for the laboratory bioassays. V. microstoma showed the potential for infection and mortality of Cx. tritaeniorhynchus larvae (71.33% ± 5.23). Results revealed a minimum of 1000V. microstoma is required to kill a single third instar larva of Cx. tritaeniorhynchus at 69.60 ± 2.40h of exposure. Cx. gelidus larvae showed 41.33% ± 3.43 mortality due to V. microstoma infestation. However, none of the ciliates were effective against Ae. aegypti larvae. Chilodonella sp. was very occasionally reported during this study hence was not possible to the mass rear for experimentations. This study concludes that V. microstoma is an effective ciliated parasite of Cx. tritaeniorhynchus larvae. Due to their effectiveness and eco-friendly nature, this species can be developed as an effective bio-controlling agent against Cx. tritaeniorhynchus mosquito species.

Characterization of a Pseudokeronopsis Strain (Ciliophora, Urostylida) and Its Bacterial Endosymbiont “Candidatus Trichorickettsia” (Alphaproteobacteria, Rickettsiales)

Symbiotic associations between bacteria and ciliate protists are rather common. In particular, several cases were reported involving bacteria of the alphaproteobacterial lineage Rickettsiales, but the diversity, features, and interactions in these associations are still poorly understood. In this work, we characterized a novel ciliate protist strain originating from Brazil and its associated Rickettsiales endosymbiont by means of live and ultrastructural observations, as well as molecular phylogeny. Though with few morphological peculiarities, the ciliate was found to be phylogenetically affiliated with Pseudokeronopsis erythrina, a euryhaline species, which is consistent with its origin from a lagoon with significant spatial and seasonal salinity variations. The bacterial symbiont was assigned to “Candidatus Trichorickettsia mobilis subsp. hyperinfectiva”, being the first documented case of a Rickettsiales associated with urostylid ciliates. It resided in the host cytoplasm and bore flagella, similarly to many, but not all, conspecifics in other host species. These findings highlight the ability of “Candidatus Trichorickettsia” to infect multiple distinct host species and underline the importance of further studies on this system, in particular on flagella and their regulation, from a functional and also an evolutionary perspective, considering the phylogenetic proximity with the well-studied and non-flagellated Rickettsia.

Ciliate Communities Respond via Their Traits to a Wastewater Treatment Plant With a Combined UASB–Activated Sludge System

Assessing functional diversity of communities is an efficient method to link community composition to ecosystem quality. Still, studies using functional traits of microeukaryote ciliate communities in biological wastewater treatment plants are lacking. The present work explores the functional diversity of the ciliate protist community in a wastewater treatment plant (WWTP) operating with a combined UASB-activated sludge system, and specifically to: 1) investigate the taxonomic and functional composition of the ciliate communities over time; 2) compare taxonomic and functional diversity indices with regard to its applicability in WWPS; 3) assess the relationship between the ciliate community’s functional composition and the WWTPs temporal conditions; and 4) investigate the potential use of functional diversity as an indicator of WWTP efficiency. Totally, we recorded 21 ciliate species throughout 37 samplings. The number of species was low compared to other plants. Bacterivorous and flake-forming species were the main functional strategies found in the samples. The correlation between taxonomic and functional richness was significant, indicating a functionally redundant community. There was a correlation between the Simpson and Rao’s quadratic entropy indexes suggesting that loss of taxonomic diversity leads to a loss of functional diversity. The homogeneity of the measured physical and chemical data led to functional homogenization and redundancy (homogenous CWM) of the ciliate community. The functional diversity is positively correlated with parameters of removal efficiency, indicating a promising application in WWTPs. Future studies will broaden knowledge on functional diversity in biological wastewater treatment systems, this being a first step with the unprecedented application of this methodology in artificial ecosystems.

Estimation of biocidal effect of metal and bioelement nanoparticles in a unicellular eukaryotic test system

The results of the study of biocidal properties of silver, copper and silicon dioxide nanoparticles are presented. Questions about the safety of nanocomponents in connection with their unstudied impact on the environment are considered. To evaluate the biocidal effect of noble metal nanoparticles and bioelements, a unicellular eukaryotic test-system, represented by a ciliated protist microorganism Paramecium caudatum inhabiting pond water bodies, was used. It was found that solutions of noble metal nanoparticles and bioelements are not bioinert and biostimulating. Colloidal solutions of silver, copper and silicon dioxide nanoparticles have a biocidal effect and show a similar dosedependent effect if the concentration of nanoparticles in the initial colloidal solutions is the same (300 µg/ml). The colloidal silver solution is characterized by a more pronounced toxic activity in a unicellular protist biological model, since full biocidal activity against paramecium is provided by dilutions of the colloidal solution of nanoparticles to the value 1: 6 of the initial one. Compared to it, solutions of copper nanoparticles and silicon oxide have a biocidal index of 100% achieved only in values of two- or three-times dilution of the initial solution. Colloidal solutions of nanoparticles in concentrations that do not cause complete mortality of the infusoria (1: 5 of the original for copper and silicon oxide nanoparticles and 1: 7 of the original for silver nanoparticles) inhibit their reproduction intensity by approximately the same value of 55-61% (paramecium reproduction intensity index of 0.455 to 0.390 respectively).

Morphology and phylogeny of two anaerobic freshwater ciliates: Brachonella comma sp. nov. and the widely distributed but little‐known caenomorphid, Ludio parvulus Penard, 1922

Hypoxic, sulfidic freshwater sediments typically support a diffuse consortium of distinctive ciliated protists, including caenomorphids, metopids, and odontostomatids among others. A recent resurgence of interest in these important members of sapropelic food webs has resulted in the description of many new species and an effort, still in its infancy, to characterize them from a morphologic, molecular, and metabolic standpoint and to determine their phylogenetic relationships. Their seemingly invariable association with prokaryotic endosymbionts and, less commonly, ectosymbionts has become a focus for many researchers. In this report, based on morphologic and molecular data, we describe a Brachonella species (Ciliophora, Metopida) new to science and analyze its phylogeny. We also provide a morphologic and molecular characterization of the smallest representative of the Caenomorphidae Poche, 1913, Ludio parvulus Penard, 1922. The phylogenetic analysis confirms the inclusion of this species in the Caenomorphidae.

How Ciliated Protists Survive by Cysts: Some Key Points During Encystment and Excystment.

Forming cysts is a common and important bionomic strategy for microorganisms to persist in harsh environments. In ciliated protists, many species have been reported to form cysts when facing unfavorable conditions. Despite traditional studies on the morphological features of cysts and the chemical composition of cyst wall, recent research has focused more on the molecular mechanisms of encystment. The present work reviews studies on developmental features and molecular information of resting cysts in ciliates, and pays more attention to the following questions: what are the inducing factors of encystment and excystment? How does the cell change morphologically during these dynamic processes? And what molecular mechanisms underlie those changes? We also present and summarize the characteristics of cysts from diverse ciliate lineages in a phylogenetic framework, aiming to provide new perspectives for studies on adaptive evolution of unicellular eukaryotes.

The hidden genomic diversity of ciliated protists revealed by single-cell genome sequencing

BackgroundCiliated protists are a widely distributed, morphologically diverse, and genetically heterogeneous group of unicellular organisms, usually known for containing two types of nuclei: a transcribed polyploid macronucleus involved in gene expression and a silent diploid micronucleus responsible for transmission of genetic material during sexual reproduction and generation of the macronucleus. Although studies in a few species of culturable ciliated protists have revealed the highly dynamic nature of replicative and recombination events relating the micronucleus to the macronucleus, the broader understanding of the genomic diversity of ciliated protists, as well as their phylogenetic relationships and metabolic potential, has been hampered by the inability to culture numerous other species under laboratory conditions, as well as the presence of symbiotic bacteria and microalgae which provide a challenge for current sequencing technologies. Here, we optimized single-cell sequencing methods and associated data analyses, to effectively remove contamination by commensal bacteria, and generated high-quality genomes for a number of Euplotia species.ResultsWe obtained eight high-quality Euplotia genomes by using single-cell genome sequencing techniques. The genomes have high genomic completeness, with sizes between 68 and 125 M and gene numbers between 14K and 25K. Through comparative genomic analysis, we found that there are a large number of gene expansion events in Euplotia genomes, and these expansions are closely related to the phenotypic evolution and specific environmental adaptations of individual species. We further found four distinct subgroups in the genus Euplotes, which exhibited considerable genetic distance and relative lack of conserved genomic syntenies. Comparative genomic analyses of Uronychia and its relatives revealed significant gene expansion associated with the ciliary movement machinery, which may be related to the unique and strong swimming ability.ConclusionsWe employed single-cell genomics to obtain eight ciliate genomes, characterized the underestimated genomic diversity of Euplotia, and determined the divergence time of representative species in this subclass for the first time. We also further investigated the extensive duplication events associated with speciation and environmental adaptation. This study provides a unique and valuable resource for understanding the evolutionary history and genetic diversity of ciliates.

New Finds on the Systematics of Cycloposthiid Ciliates (Ciliophora: Entodiniomorphida: Cycloposthiidae) Based on New 18S-rDNA Sequences from a Brazilian Capybara.

The family Cycloposthiidae (Entodiniomorphida) comprises ciliated protists that are symbionts of the gastrointestinal tract of several herbivore mammals, such as rodents, elephants, equids, primates, hippopotamus, marsupials, rhinoceros and tapir, where they contribute to the digestion of their host's plant-based diet. Despite the significance of these ciliates to the evolution of their hosts, many characters used in the taxonomy of the group are homoplastic and most of the valid species do not have molecular data available. For these reasons the systematics of this family is poorly understood. Here, we sequenced the 18S-rDNA of ten cycloposthiids, including nine Cycloposthium spp. and Monoposthium cynodontum, all of them isolated from the cecum of a Brazilian capybara. Our phylogenetic analyses indicate that the family Cycloposthiidae might be polyphyletic, while M. cynodontum and Cycloposthium spp. constitute a single monophyletic group. Given the great morphological and molecular similarities between members of M. cynodontum and Cycloposthium ciliates, it is possible that this species, although it has been described in the genus Monoposthium, is actually a Cycloposthium ciliate.

New contribution to epigenetic studies: Isolation of micronuclei with high purity and DNA integrity in the model ciliated protist, Tetrahymena thermophila

The ciliated protist Tetrahymena thermophila is a well-known model organism with typical nuclear dimorphism containing a somatic macronucleus (MAC) and a germline micronucleus (MIC). The presence in the same cell compartment of two nuclei with distinctly different structural and functional properties provides an ideal model system to explore mechanisms of genome maintenance. Although methods for the isolation of MIC have been available for many years, cross-contamination and DNA degradation remain unresolved. Here, we describe a reliable and quick method to isolate MIC with high purity and DNA integrity in T. thermophila. Different factors are examined to optimize the MIC purification. The MAC contamination ratio in purified MIC is about 0.19% and DNA integrity of purified MIC is maintained. We also establish a more accurate method to detect the contamination rate of nuclei including microscopic observation and PCR detection. This study will facilitate further epigenetic research in Tetrahymena.

Nanowatt simple microcalorimetry for dynamically monitoring the defense mechanism of Paramecium caudatum

Microcalorimetry has been widely used in measuring cellular metabolic heat to study bioprocesses, such as metabolism. However, it is still limited by insufficient sensitivity and system complexity, especially for the direct measurements of individual cells. Here, we present a droplet-based simple differential microcalorimetric system for determining the real-time energy balance of the ciliate protist, Paramecium caudatum. We utilized the platform to dynamically monitor its defensive behavior and measured the temperature change. Then we used heat balance equation and calculated corresponding dissipated power and energy with an ultimate resolution of ≈14 nW. The results showed that the defensive behavior by the ejection of the trichocysts’ content consumed energy of ≈0.75 mJ per cell, and the dissipated power was ≈303.8 μW. This differential microcalorimetric platform can be used to study the metabolic heat of protist metabolism as well as other individual cells, helping us to understand this widespread, yet little-known, biological phenomenon from a new perspective.

Relative contributions of disparate animal vectors to the development of freshwater ciliate communities

Here, we evaluate the role of two disparate animal groups, amphibians and odonates, in the dispersal of ciliates. We performed a 33-day outdoor experiment from July to August 2018 with four treatments: (i) a control, with only wind action; (ii) a treatment with the addition from propagules of odonates; (iii) a treatment with propagules from amphibians; and (iv) a treatment with the addition of propagules from both animals. We recorded 54 species of ciliates from 11 groups, with Peritrichia the most representative. Species richness and abundance increased markedly after the 12th day. The species composition of the ciliate species showed differences between treatments within each time period, as well as between the different treatments throughout the experiment. As expected, our results not only evidenced that the dispersal of ciliate protists was improved when mediated by biological vectors, but also demonstrated that the impact depends on the animal vector, and that the effect is even more relevant when propagules are carried by both animal vectors. Our findings support the importance of animal vectors in the dispersal and structuring of ciliates, and highlight the potential differences in the effectiveness of amphibians and odonates for the dispersal of this group.

Comparative Evaluation on Combined Ecotoxic Effects of Nitrofurazone and NH4CI Using Two Ciliated Protists as Model Organisms: Population Dynamics and Growth Rates

Ecotoxic effects of antibiotics or ammonium have been confirmed independently in aquatic animals, but few studies have investigated their combined effects. In aquaculture ecosystems, these pollutants frequently coexist, and often in high concentrations. In this study, the combined effects of antibiotic nitrofurazone and NH4Cl on the population dynamics and growth rates of two species of ciliated protists, Euplotes vannus and Pseudokeronopsis rubra, were investigated. Profiles of the dose-responses were visualized, and interactions between the two pollutants were quantified by the response surface method (RSM). Results showed that 1) the dynamics of the population growth differed significantly between the testd ciliates and varied with the concentrations of the pollutants; 2) the relative growth rate (RGR) of both ciliates decreased significantly with increased pollutant concentrations, while the difference in RGR between the two ciliates was not significant; 3) RSM analysis demonstrated an additive effect of nitrofurazone and NH4Cl on the RGR in both ciliates. In brief, ecotoxic effects can be caused by nitrofurazone and ammonium independently on the two test ciliates, and such effects can be strengthened when they present at the same time. These findings offer a valuable reference for evaluating combined ecotoxic effects caused by multiple pollutants in aquaculture ecosystems.

Morphogenesis of the Ciliature During Sexual Process of Conjugation in the Ciliated Protist Euplotes raikovi

Morphogenesis is an important process that widely occurs in almost all the organisms, including the ciliated protists. Ciliates are a large group of single-celled eukaryotes that can reproduce asexually (e.g., binary fission) and perform sexual process (e.g., conjugation). Morphogenesis happens in both asexual reproduction and sexual process in ciliates and the reorganization during conjugation is more complex. However, studies of morphogenesis focusing on conjugation are very limited. Here we studied the morphogenetic process during conjugation in the marine species Euplotes raikovi Agamaliev, 1966. The results indicate that: (1) the ciliature in the ventral side reorganizes twice during sexual process, i.e., conjugational and postconjugational reorganization; (2) the adoral zone of membranelles (AZM) is generated de novo in a pouch beneath the cortex during both reorganizations, with the anterior part generated during the first reorganization, while the posterior part formed during the second reorganization; (3) the frontoventral-transverse (FVT) cirri anlagen are formed de novo in both processes with the fragmentation pattern of 2:2:3:3:2; (4) one left marginal cirrus is generated de novo during both reorganizations; and (5) the dorsal ciliature remains intact during the whole process, except that the two caudal cirri originate from the end of the right-most two dorsal kineties during both reorganizations. Comparisons of the morphogenetic process during conjugation demonstrate a considerably stable pattern within Euplotes while the patterns vary dramatically among different ciliate groups.

Functional Ecology of Two Contrasting Freshwater Ciliated Protists in Relation to Temperature.

We conducted microcosm experiments with two contrasting freshwater ciliates on functional traits (FTs) related to their growth rate (numerical response, NR) and ingestion rate (functional response, FR) over a range of ecologically relevant temperatures. Histiobalantium bodamicum and Vorticella natans are common planktonic ciliates but their abundance, swimming behavior, and temperature tolerance are different. In contrast to most sessile peritrich species, the motile V. natans is not strictly bacterivorous but also voraciously feeds upon small algae. We observed three main alterations in the shape of NR of both species with temperature, that is, change in the maximum growth rate, in the initial slope and in the threshold food level needed to sustain the population. Similarly, maximum ingestion rate, gross growth efficiency (GGE), and cell size varied with temperature and species. These findings caution against generalizing ciliate performance in relation to the ongoing global warming. Our results suggest that V. natans is the superior competitor to H. bodamicum in terms of temperature tolerance and bottom‐up control. However, the abundance of V. natans is usually low compared to H. bodamicum and other common freshwater ciliates, suggesting that V. natans is more strongly top‐down controlled via predation than H. bodamicum. The taxonomic position of V. natans has been debated. Therefore, to confirm species and genus affiliation of our study objects, we sequenced their small subunit ribosomal RNA (SSU rDNA) gene.

Morpholino-Mediated Knockdown of Ciliary Genes in Euplotes vannus, a Novel Marine Ciliated Model Organism.

Cilia are highly conserved organelles present in almost all types of eukaryotic cells, and defects in cilia structure and/or function are related to many human genetic disorders. Single-celled ciliated protists, which possess diverse types of cilia, are remarkable model organisms for studying cilia structures and functions. Euplotes vannus is a representative ciliate with many intriguing features; for example, it possesses extensively fragmented somatic genomes and a high frequency of + 1 programmed ribosomal frameshifting. However, the molecular mechanisms underlying these remarkable traits remain largely unknown, mainly due to the lack of efficient genetic manipulation tools. Here, we describe the first application of a morpholino-based strategy to knockdown gene expression in E. vannus. Through interfering with the function of two ciliary genes, ZMYND10 and C21ORF59, we showed that these two genes are essential for the ciliary motility and proliferation of E. vannus cells. Strikingly, both ZMYND10- and C21ORF59-knockdown cells developed shorter cilia in the ventral cirri, a special type of ciliary tuft, suggesting a novel role for these genes in the regulation of cilia length. Our data provide a new method to explore gene function in E. vannus, which may help us to understand the functions of evolutionarily conserved cilia-related genes as well as other biological processes in this intriguing model.

The insights into the systematic relationship of Gastrostyla-affinitive genera, with report on a new saline soil ciliate genus and new species (Protozoa, Ciliophora)

BackgroundHypotrichia are a group with the most complex morphology and morphogenesis within the ciliated protists. The classification of Gastrostyla-like species, a taxonomically difficult group of hypotrichs with a common ventral cirral pattern but various dorsal and ontogenetic patterns, is poorly understood. Hence, systematic relationships within this group and with other taxa in the subclass Hypotrichia remain unresolved.Results18S rRNA gene sequence of a new Gastrostyla-like taxon was obtained. Phylogenetic analyses based on the 18S rRNA gene sequences indicate that this ciliate represents a new genus that is closely related to Heterourosomoida and Kleinstyla within the oxytrichid clade of the Hypotrichia. However, the position of this cluster remains unresolved. All three genera deviate from the typical oxytrichids by their incomplete (or lack of) dorsal kinety fragmentation during morphogenesis. Morphology and morphogenesis of this newly discovered form, Heterogastrostyla salina nov. gen., nov. spec., are described. Heterogastrostyla nov. gen., is characterised as follows: more than 18 fronto-ventral-transverse cirri, cirral anlagen V and VI develop pretransverse cirri, and dorsal ciliature in Urosomoida-like pattern.ConclusionsSimilar to the CEUU-hypothesis about convergent evolution of urostylids and uroleptids, we speculate that the shared ventral cirral patterns of Gastrostyla-like taxa might have resulted from convergent evolution.

Rain-fed granite rock pools in a national park: extreme niches for protists

Rain-fed granite rock pools are geological landforms with a worldwide distribution. However, their role as habitats for microorganisms has been barely explored. We carried out a detailed morphological inventory of the ciliated protists in the sediments of three granite rain-fed rock pools from a Spanish National Park. The ciliate seed bank in the rock pools comprised 54 morphospecies. The species number inferred for each pool by rarefaction analysis was similar to that observed by microscopy. The most representative species were small bacterivorous ciliates, although the distribution of ciliate groups was significantly different in each rock pool. Testate amoebae were also found to be rich in species. This study demonstrates the existence of a diverse ciliate community adapted to persist in these ephemeral and extreme rock pool habitats through species resistance structures, many of which have not been described previously. The presence of competitive species in dormant stages prevents local extinctions through a sequential excystation over time. Our results provide a benchmark for studying protist and microbial biodiversity within rain-fed granite rock pools, a unique habitat that merits bio- and geo- conservation.

Gamma Radiation Tolerance and Protein Carbonylation Caused by Irradiation of Resting Cysts in the Free-living Ciliated Protist Colpoda cucullus

The ciliate Colpoda cucullus forms resting cysts to survive unfavorable environmental stresses. In this study, we have shown that Colpoda resting cysts survived exposure to a gamma radiation dose of 4000 Gy, although vegetative cells were killed by 500 Gy. After 4000 Gy irradiation, more than 90% of resting cysts and approximately 70% of dry cysts could excyst to form vegetative cells. In both cases, the excystment gradually increased after the induction of excystment. In addition, we also showed that protein carbonylation level was increased by gamma irradiation, but decreased by incubation in the cyst state. These results indicated that cell damage was repaired in resting cysts. Colpoda probably developed tolerance to gamma radiation by forming resting cysts as a strategy for growth in terrestrial environments, as part of contending with the stress due to reactive oxygen species caused by desiccation.

The mitochondrial genome of Tetrahymena rostrata

Tetrahymena rostrata is a ciliated protist which can parasitize the gray field slug, Deroceras reticulatum. Here, we report the sequence of the mitochondrial genome (mt genome) of a strain of T. rostrata that was isolated from the egg of D. reticulatum. Whole cell genomic DNA was sequenced using Illumina® MiSeq and the mitochondrial DNA sequence reads were extracted and assembled. The resulting 47,235 bp assembly contained rRNAs, tRNAs, and 45 protein coding DNA sequences of which 21 encoded proteins of unknown function. Phylogenetic analysis showed T. rostrata clustered with Tetrahymena thermophila, Tetrahymena pigmentosa, Tetrahymena pyriformis, Tetrahymena paravorax, and Tetrahymena malaccensis.

Tripartite Symbiosis of an Anaerobic Scuticociliate with Two Hydrogenosome-Associated Endosymbionts, a Holospora-Related Alphaproteobacterium and a Methanogenic Archaeon.

A number of anaerobic ciliates, unicellular eukaryotes, intracellularly possess methanogenic archaea and bacteria as symbiotic partners. Although this tripartite relationship is of interest in terms of the fact that each participant is from a different domain, the difficulty in culture and maintenance of those host species with symbiotic partners has disturbed both ecological and functional studies so far. In this study, we obtained a stable culture of a small anaerobic scuticociliate, strain GW7. By transmission electron microscopic observation and fluorescent in situ hybridization with domain-specific probes, we demonstrate that GW7 possesses both archaeal and bacterial endosymbionts in its cytoplasm. These endosymbionts are in dependently associated with hydrogenosomes, which are organelle producing hydrogen and ATP under anaerobic conditions. Clone library analyses targeting prokaryotic 16S rRNA genes, fluorescent in situ hybridization with endosymbiont-specific probes, and molecular phylogenetic analyses revealed the phylogenetic affiliations and intracellular localizations of these endosymbionts. The endosymbiotic archaeon is a methanogen belonging to the genus Methanoregula (order Methanomicrobiales); a member of this genus has previously been described as the endosymbiont of an anaerobic ciliate from the genus Metopus (class Armophorea), which is only distantly related to strain GW7 (class Oligohymenophorea). The endosymbiotic bacterium belongs to the family Holosporaceae of the class Alphaproteobacteria, which also comprises several endosymbionts of various aerobic ciliates. For this endosymbiotic bacterium, we propose a novel candidate genus and species, "Candidatus Hydrogenosomobacter endosymbioticus."IMPORTANCE Tripartite symbioses between anaerobic ciliated protists and their intracellular archaeal and bacterial symbionts are not uncommon, but most reports have been based mainly on microscopic observations. Deeper insights into the function, ecology, and evolution of these fascinating symbioses involving partners from all three domains of life have been hampered by the difficulties of culturing anaerobic ciliates in the laboratory and the frequent loss of their prokaryotic partners during long-term cultivation. In the present study, we report the isolation of an anaerobic scuticociliate, strain GW7, which has been stably maintained in our laboratory for more than 3 years without losing either of its endosymbionts. Unexpectedly, molecular characterization of the endosymbionts revealed that the bacterial partner of GW7 is phylogenetically related to intranuclear endosymbionts of aerobic ciliates. This strain will enable future genomic, transcriptomic, and proteomic analyses of the interactions in this tripartite symbiosis and a comparison with endosymbioses in aerobic ciliates.