Focusing on Genera to Improve Species Identification: Revised Systematics of the Ciliate Spirostomum

Effect of microplastic particles on the population growth rate and clearance rate of selected ciliates (Protista, Ciliophora)

Microplastics (MPs), due to their micro size, which overlaps with the typical food size of various aquatic organisms, can be ingested and move up the food chain, accumulating in the bodies of organisms at higher trophic levels. Few studies have focused on the uptake of MPs by ciliates, which are an important element of the microbial cycle. Three different ciliate species were used in this study: Blepharisma japonicum, Euplotes sp., and Spirostomum teres, as well as polystyrene beads with diameters of 1 and 2 µm at two concentrations (106 and 107 beads × mL-1). The results of the experiments showed that MPs have a variable, species-specific effect on the population growth rate of ciliates, which is directly dependent on their concentration in the environment (P < 0.01). It was also observed that the number of MPs ingested changed over time depending on their concentration and size. On average, the highest number of ingested MPs (883.11 ± 521.47) was recorded at 60 min of exposure to a low concentration of small beads in B. japonicum. The lowest number of beads was ingested after 5 min of exposure to a low concentration of large beads in the same species. The rate of MP uptake by the ciliate species was significantly dependent on their concentration, exposure time, and size (P < 0.001). The highest clearance rate was observed in the fifth minute of the experiment in the environment with the lowest MP concentration.

Giant proteins in a giant cell: Molecular basis of ultrafast Ca2+-dependent cell contraction

The giant single-celled eukaryote, Spirostomum, exhibits one of the fastest movements in the biological world. This ultrafast contraction is dependent on Ca²⁺ rather than ATP and therefore differs to the actin-myosin system in muscle. We obtained the high-quality genome of Spirostomum minus from which we identified the key molecular components of its contractile apparatus, including two major Ca²⁺ binding proteins (Spasmin 1 and 2) and two giant proteins (GSBP1 and GSBP2), which act as the backbone and allow for the binding of hundreds of spasmins. The evidence suggests that the GSBP-spasmin protein complex is the functional unit of the mesh-like contractile fibrillar system, which, coupled with various other subcellular structures, provides the mechanism for repetitive ultrafast cell contraction and extension. These findings improve our understanding of the Ca²⁺-dependent ultrafast movement and provide a blueprint for future biomimicry, design, and construction of this kind of micromachine.

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.

The widely reported but poorly studied ciliate family Folliculinidae (Protozoa, Ciliophora, Heterotrichea): a revision with notes on its taxonomy, morphology and phylogenetic relationships

Ciliates of the heterotrich family Folliculinidae are widespread in various habitats and are distinguished by their transparent loricae of various shapes, conspicuous peristomial lobes, and dimorphic life cycles. They usually attach firmly to the surface of substrates, feed on bacteria and microalgae, and play a significant role in energy flow and material cycling in the microbial food web. However, little is known regarding their biodiversity and systematics. In this work, we establish the terminology of the family Folliculinidae and select six crucial features for genus recognition. Based on previous studies, we revise the classification of Folliculinidae, supply improved diagnoses for each of the 33 folliculinid genera, and provide a key to their identification. Moreover, phylogenetic analyses based on small subunit ribosomal DNA (SSU rDNA) sequences revealed that the family is monophyletic and comprises two subclades (subclades I II) which can be identified by the flexibility of their peristomial lobes and the sculpturing of their necks. Furthermore, we investigate the evolutionary relationships of folliculinids using the six chosen generic features.

Supplementary Information

The online version contains supplementary material available at 10.1007/s42995-022-00152-z.

Evolutionary insights and brief review of Loxodes Ehrenberg, 1830 (Ciliophora, Karyorelictea, Loxodidae) with description of a new species from Mexico

Karyorelictids are a group of ciliates inhabiting marine and freshwater biotopes and possessing a non-dividing macronucleus. We describe a new freshwater species based on morphology and the 18S rRNA gene sequence data. Loxodes tziscaensis n. sp. can be easily distinguished from other Loxodes species by the arrangement of the nuclear apparatus and features of the buccal and somatic ciliature. The current proposed 18S rRNA phylogeny of Loxodes, including seven Loxodes species, shows two morphologically well-supported groups. Group A (L. rostrum, type species; L. vorax and L. tziscaensis n. sp.) includes species with a single nuclear group (two macronuclei and one micronucleus), in contrast to species of group B, which possess more than one nuclear group (L. striatus, L. magnus, L. kahli, L. penardi, and L. rex). We propose that the last common ancestor of Loxodes was a marine Remanella-like species possessing a single nuclear group. The division and differentiation of the micronucleus into a new macronucleus and the retention of the old macronuclei, independently of cell division, may have been two crucial processes during the evolution and diversification of Loxodes species with one nuclear group into species with multiple nuclear groups.

A microbial eukaryote with a unique combination of purple bacteria and green algae as endosymbionts

Oxygenic photosynthesizers (cyanobacteria and eukaryotic algae) have repeatedly become endosymbionts throughout evolution. In contrast, anoxygenic photosynthesizers (e.g., purple bacteria) are exceedingly rare as intracellular symbionts. Here, we report on the morphology, ultrastructure, lifestyle, and metagenome of the only "purple-green" eukaryote known. The ciliate Pseudoblepharisma tenue harbors green algae and hundreds of genetically reduced purple bacteria. The latter represent a new candidate species of the Chromatiaceae that lost known genes for sulfur dissimilation. The tripartite consortium is physiologically complex because of the versatile energy metabolism of each partner but appears to be ecologically specialized as it prefers hypoxic sediments. The emergent niche of this complex symbiosis is predicted to be a partial overlap of each partners' niches and may be largely defined by anoxygenic photosynthesis and possibly phagotrophy. This purple-green ciliate thus represents an extraordinary example of how symbiosis merges disparate physiologies and allows emergent consortia to create novel ecological niches.

Macronuclear Plasticity in Two South American Populations of Spirostomum (Ciliophora, Heterotrichea) Warns About its Use for Species Classification: Revision and New Insights

Spirostomum is a widely distributed heterotrichean genus composed of well-known species with described ecology and phylogenetic affinities. The morphological classification of Spirostomum species is mostly based on the body size/shape, number of cortical granule rows and macronuclear characteristics. These features along with molecular phylogenies based on ribosomal genes divide the genus into two phylogroups, one including species with a compact macronucleus, and another including species with a moniliform macronucleus. Here, we present our observations on atypical Spirostomum specimens with unusually two distinct macronuclei and shortened adoral zone of membranelles. These atypical forms appeared in the cultures of S. minus and S. yagiui, sampled at different sites in South America (Chile and Brazil) and associated with unrelated substrate types. Morphological observations of living and stained cells, 18S rRNA gene analyses, and a thorough investigation of the literature suggest that the atypical phenotype may be a result of uncommon pathways during the conjugative process. Thus, we demonstrate that studies of ciliate natural populations and their morphological variations, especially from undersampled biogeographical regions, can reveal the boundaries of widely used morphological characters for Spirostomum taxonomy and species identification.

Transcriptome Profiling Revealed Multiple rquA Genes in the Species of Spirostomum (Protozoa: Ciliophora: Heterotrichea)

Adaptation to life at different oxygen tensions plays a role in protozoan ecology and controls the distribution of different species in anoxic habitats. The ciliate genus Spirostomum inhabiting fresh or low salinity water globally where these species are considered as bioindicators. Under anaerobic or low oxygen conditions, the rhodoquinol-dependent pathway has been reported in the species from the class Heterotrichea. With the help of RNA sequencing (RNAseq) data, Spirostomum spp., are suitable for deep molecular investigations on rquA for rhodoquinone (RQ) biosynthesis. In this study, Spirostomum ambiguum, Spirostomum subtilis, and Spirostomum teres collected from densely vegetated freshwater habitat in Fuzhou, China, explored the evidence of rquA. Based on transcriptome analysis, two to three RquA proteins were identified in S. ambiguum, S. teres, and S. subtilis, respectively. The presence of a key Motif-I of RquA and mitochondrial targeting signals (MTS), also confirmed the identity of these as RquA. Furthermore, Spirostomum RquA proteins could be sorted into two groups based on their conserved amino acid (CAA) residues. Phylogenetic analysis also exhibited RquA division into two subclades contained RquA1 and RquA2/RquA3 and supports two to three paralogs of rquA genes in the genomes Spirostomum spp. Additional transcriptomes and genomes analysis of Blepharisma spp., and Stentor spp., respectively, also revealed at least two paralogs of rquA in members of the class Heterotrichea. The present study provides evidence for the presence of RquA and rhodoquinol dependent fumarate reduction pathway in Spirostomum species potentially use to respire in the oxygen-depleted habitats and two to three diverse rquA genes.

New contributions to the phylogeny of the ciliate class Heterotrichea (Protista, Ciliophora): analyses at family-genus level and new evolutionary hypotheses

Heterotrichous ciliates play an important role in aquatic ecosystem energy flow processes and many are model organisms for research in cytology, regenerative biology, and toxicology. In the present study, we combine both morphological and molecular data to infer phylogenetic relationships at family-genus level and propose new evolutionary hypotheses for the class Heterotrichea. The main results include: (1) 96 new ribosomal DNA sequences from 36 populations, representing eight families and 13 genera, including three poorly annotated genera, Folliculinopsis, Ampullofolliculina and Linostomella; (2) the earliest-branching families are Spirostomidae in single-gene trees and Peritromidae in the concatenated tree, but the family Peritromidae probably represents the basal lineage based on its possession of many "primitive" morphological characters; (3) some findings in molecular trees are not supported by morphological evidence, such as the family Blepharismidae is one of the most recent branches and the relationship between Fabreidae and Folliculinidae is very close; (4) the systematic positions of Condylostomatidae, Climacostomidae, and Gruberiidae remain uncertain based either on morphological or molecular data; and (5) the monophyly of each genus included in the present study is supported by the molecular phylogenetic trees, except for Blepharisma in the SSU rDNA tree and Folliculina in the ITS1-5.8S-ITS2 tree.

Spirostomum teres: A Long Term Study of an Anoxic-Hypolimnion Population Feeding upon Photosynthesizing Microorganisms

The pelagic / anoxic hypolimnion population of Spirostomum teres was investigated as a part of the long-term ciliates’ monitoring (2003–2016) in an oligo- to mesotrophic monomictic hyposaline crater lake Alchichica (Puebla / Veracruz, Mexico), including an analysis of picoplankton (both heterotrophic, HPP and autotrophic, APP) and inorganic compounds of nitrogen (ammonium, nitrite, nitrate), phosphorus (dissolved reactive phosphorus, DRP) and silicon. Additionally, detailed studies of the ciliate vertical distribution and feeding activity measured upon fluorescently labelled APP (picocyanobacteria) were carried out. The results were compared with those from a neighbour freshwater crater lake La Preciosa and with a meromictic karstic lake La Cruz (Cuenca, Spain). The ciliate vertical distribution within the water column was very well defined: During the first decade, the benthic population was frequently found throughout a developing stratification of the lake. The established stratification of the lake turned the conditions favourable for the formation of an oxycline / hypolimnion population, typically, several meters below the deep chlorophyll maximum (formed basically by diatoms); the population preferred the layers without detectable dissolved oxygen. However, an observed gradient of light (PAR) could support both oxygenic and anoxygenic photosynthesis. Late stratification after deepening of the thermocline reduced the layers with S. teres population to a minimum apparently due to the drastic change in physicochemical conditions within a metalimnion, coupled with an oxycline, and limited to 1 to 2 meters; microstratification was found. Last years, the very bottom population disappeared or it was reduced and the late stratification S. teres peaks were smaller or did not appeared. Generally, S. teres oxycline / anoxic hypolimnion population was observed from June through November. Optimum picoplankton numbers in conditions that supported the ciliate growth were found: The ciliate was peaking at APP of 0.6 to 1 × 105 cells mL–1; the optimum of HPP was observed round 1.4 × 106 cells mL–1. S. teres was efficiently feeding upon picocyanobacteria in numbers of 105 cells mL–1 reaching the clearance rate of 2000 nL cell–1h–1, which represented in average 130 to 210 cells cell–1h–1 ingested. Feeding upon purple sulphur bacteria was observed but only during the end of the lake stable stratification when the ciliate population was already dropping. On the other hand, the volume specific clearance of S. teres upon picocyanobacteria (103 h–1) did not support the hypothesis that they could serve as a sole prey. Feeding upon eukaryote phytoplankton (chlorophytes Monoraphidium minutum, diatoms Cyclotella choc tawhatcheeana) could be of higher importance that previously supposed. Additionally, a use of ingested and retained photosynthetic prokaryotes is hypothesized.

A new contribution to the taxonomy and molecular phylogeny of three, well-known freshwater species of the ciliate genus Spirostomum (Protozoa: Ciliophora: Heterotrichea)

Members of the heterotrich genus Spirostomum are commonly found in freshwater or low salinity biotopes. In the present study, three species (S. minus, S. subtilis and S. teres) collected from freshwater habitats in Qingdao, China, are investigated using morphological and molecular methods. Detailed morphometric data are documented and improved diagnosis are supplied based on a combination of previous and present studies. In addition, small subunit ribosomal DNA (SSU rDNA) sequences are obtained from the clonal cultures. Phylogenetic analyses show that all three species are placed in the Spirostomum clade. However, isolates of the morphospecies S. minus are divided into two paraphyletic clades, while ‘populations’ of the nominal species, S. teres, are placed in at least four separate groups in the tree. After comparing morphological and molecular differences in closely related forms available, we hypothesized that S. minus and S. teres might represent species complexes. A key to the identification of the ten valid species of Spirostomum is also supplied.

Deciphering phylogenetic relationships and delimiting species boundaries using a Bayesian coalescent approach in protists: A case study of the ciliate genus Spirostomum (Ciliophora, Heterotrichea)

The ciliate genus Spirostomum comprises eight morphospecies, inhabiting diverse aquatic environments worldwide, where they can be used as water quality indicators. Although Spirostomum species are relatively easily identified using morphological methods, the previous nuclear rDNA-based phylogenies indicated several conflicts in morphospecies delineation. Moreover, the single locus phylogenies and previous analytical approaches could not unambiguously resolve phylogenetic relationships among Spirostomum morphospecies. Here, we attempt to investigate species boundaries and evolutionary history of Spirostomum taxa, using 166 new sequences from multiple populations employing one mitochondrial locus (CO1 gene) and two nuclear loci (rRNA operon and alpha-tubulin gene). In accordance with previous studies, relationships among the eight Spirostomum morphospecies were poorly supported statistically in individual gene trees. To overcome this problem, we utilised for the first time in ciliates the Bayesian coalescent approach, which accounts for ancestral polymorphisms, incomplete lineage sorting, and recombination. This strategy enabled us to robustly resolve deep relationships between Spirostomum species and to support the hypothesis that taxa with compact macronucleus and taxa with moniliform macronucleus each form a distinct lineage. Bayesian coalescent-based delimitation analyses strongly statistically supported the traditional morphospecies concept but also indicated that there are two S. minus-like cryptic species and S. teres is non-monophyletic. Spirostomum teres was very likely defined by a set of ancestral features of lineages that also gave rise to S. yagiui and S. dharwarensis. However, molecular data from type populations of the morphospecies S. minus and S. teres are required to unambiguously resolve the taxonomic problems.

Morphology and Phylogenetic Position of Gruberia lanceolata (Gruber 1884) (Ciliophora, Heterotrichea) from Rio de Janeiro, Brazil

Ciliates of the genus Gruberia are poorly studied. Consequently, most species lack detailed morphological descriptions, and all gene sequences in GenBank are not classified at the species level. In this study, a detailed morphological description of a population of G. lanceolata from Brazil is presented, based on live and protargol-stained organisms. We also present the 18S rRNA gene sequence and the phylogenetic position of this species. The primary characteristics of G. lanceolata from the Maricá Lagoon are as follows: an elongate fusiform body 280-870 × 40-160 μm in size; rosy cortical granules; a peristome occupying approximately 1/3-1/2 of body length; an adoral zone comprising 115-330 membranelles; a paroral membrane in 35-50 fragments; and a moniliform macronucleus with 11-16 nodules. Based on our observations and data from pertinent literature, we suggest G. beninensis to be a junior synonym of G. lanceolata.

New contributions to Gruberia lanceolata (Gruber, 1884) Kahl, 1932 based on analyses of multiple populations and genes (Ciliophora, Heterotrichea, Gruberiidae)

Gruberia Kahl, 1932 is a species-poor genus comprising only seven named species. Most of these species have not been reinvestigated since the original reports. In the present work, we investigated the taxonomy and phylogeny of Gruberia lanceolata (Gruber, 1884) Kahl, 1932 based on analyses of morphology and multiple gene sequences from four South Korean populations. This species is mainly characterized by a well-developed peristome region, segmented paroral membrane, and moniliform macronucleus. Some morphological features were not stable among the four populations investigated, such as body shape and size, cell color, and the ratio of oral length to body length. However, our molecular analyses of four different genetic markers - three nuclear DNA markers (18S rDNA, ITS1-5.8S-ITS2 region, D1D2 of 28S rDNA) and one mitochondrial (mt) marker (CO1 gene) - indicated that all Korean populations examined were the same species. Based on our present findings and historic works, we propose that G. calkinsi, G. aculeata, and G. beninensis are junior synonyms of G. lanceolata.

Biogeography and Character Evolution of the Ciliate Genus Euplotes (Spirotrichea, Euplotia), with Description of Euplotes curdsi sp. nov

Ciliates comprise a diverse and ecologically important phylum of unicellular protists. One of the most specious and best-defined genera is Euplotes, which constitutes more than 70 morphospecies, many of which have never been molecularly tested. The increasing number of described Euplotes taxa emphasizes the importance for detailed characterizations of new ones, requiring standardized morphological observations, sequencing of molecular markers and careful comparison with previous literature. Here we describe Euplotes curdsi sp. nov., distinguishable by the combination of the following features: 45–65 μm length, oval or elongated shape with both ends rounded, narrow peristome with 25–34 adoral membranelles, conspicuous paroral membrane, double-eurystomus dorsal argyrome type, 6–7 dorsolateral kineties and 10 frontoventral cirri. Three populations of the novel species have been found in brackish and marine samples in the Mediterranean and the White Sea. We provide the SSU rRNA gene sequences of these populations, and an updated phylogeny of the genus Euplotes. Using the molecular phylogenetic tree, we inferred aspects of the biogeographical history of the genus and the evolution of its most important taxonomic characters in order to provide a frame for future descriptions. Ultimately, these data reveal recurrent trends of freshwater invasion and highlight the dynamic, yet convergent, morphological evolution of Euplotes.

Expanded phylogenetic analyses of the class Heterotrichea (Ciliophora, Postciliodesmatophora) using five molecular markers and morphological data

Most studies of the molecular evolution of Heterotrichea have been based solely on the 18S-rDNA gene, which were inconsistent with morphological classification. Because of the limitations of single locus phylogenies and the recurring problem of lack of resolution of deeper nodes found in previous studies, we present hypotheses of the evolution of internal groups of the class Heterotrichea based on multi-loci analyses (18S-rDNA, 28S-rDNA, ITS1-5.8S-ITS2 region, COI and alpha-tubulin) and morphological data. Phylogenetic trees from protein coding gene data are presented for Heterotrichea for the first time. Phylogenetic analyses included Bayesian inference, maximum likelihood, maximum parsimony methods, and optimal trees were statistically compared to alternative topologies from the literature. Additionally, the Bayesian concordance approach (BCA algorithm) was used to assess the concordance factor between topologies obtained from isolated analyses. Because different loci may evolve at different rates, resulting in different gene topologies, we also estimated a species tree for Heterotrichea using the STAR coalescence-based method. The results show that: (1) single gene trees are inconsistent regarding the position of some heterotrichean families; (2) the concatenation of all data in a total-evidence tree improved the resolution of deep nodes among the heterotrichean families and genera; (3) the coalescent-based species tree is consistent with phylogenies based on the 18S-rDNA gene and shows Spirostomidae as the stem group of Heterotrichea; (4) however, the total-evidence tree suggests that the large Heterotrichea cluster is divided into nine lineages in which Peritromidae diverges at the base of the Heterotrichea tree.