Remarks on the Composition of the Large Ciliate Class Kinetofragmophora de Puytorac et al., 1974, and Recognition of Several New Taxa Therein,with Emphasis on the Primitive Order Primociliatida N. Ord.*

With the realization that new data (especially ultrastructural) and new ideas are making necessary a major revision of the scheme of classification of the Ciliophora, several groups of ciliatologists are preparing treatises on the subject. The present paper is concerned with the composition of the large new class of ciliates, Kinetofragmophora de Puytorac et al., 1974, established very recently by the French group. Several new taxa, at ordinal and subordinal levels, are proposed for inclusion in that class, with special emphasis on the new order to contain the most primitive of extant species. Actions taken here are incorporated in a major review and revisory work of the author which is being published elsewhere. The class Kinetofragmophora, by far the largest of the 3 classes now recognized as comprising the whole phylum Ciliophora, is itself considered to contain 4 sizeable subclasses and to embrace a total of 13 orders and 14 suborders. Two orders and 6 suborders are named and described here as new, enumerated and briefly identified as follows: Order Primociliatida n. ord., for the most “primitive” of gymnostomes, with three new suborders— Homokaryotina n. subord., for the homokaryotic genus Stephanopogon; Karyorelictina n. subord., for a number of mostly interstitial ciliates which, though heterokaryotic, possess nondividing, diploid macronuclei (e.g. Trachelocerca, Trachelonema, and Tracheloraphis); and Prorodontina n. subord., for a group of relatively specialized formerly “rhabdophorine” gymnostomes such as Coleps, Placus, and Prorodon and order Haptorida n. ord., for rapacious carnivorous forms, formerly lumped with the preceding groups as “rhabdophorines,” many with oral toxicysts and well developed thigmotactic ciliature (e.g. Actinobolina, Didinium, Dileptus, Enchelys, Spathidium, and Trachelius). All foregoing taxa are members of the 1st kinetofragmophoran subclass, the Gymnostomata. In the taxonomic conclusions drawn, new significance is placed on ultrastructural data, on macronuclear differences of evolutionary importance, and on habitat and behavior. A brief review of the literature on psammophilous ciliates is presented. In the subclass Vestibulifera is now located the order Entodiniomorphida Reichenow, a group formerly considered to be a spirotrich taxon. A suborder, Blepharocorythina n. subord., is proposed to contain the old “trichostome” family Blepharocorythidae, species commensalistic in horses and ruminants and now—with their syncilia, etc.—considered ancestral to the ophryoscolecids and relatives. In the subclass Hypostomata, order Nassulida, the suborder Paranassulina n. subord. is established to contain nassulids which appear more highly evolved than Nassula itself (e.g. Paranassula and Enneameron) in perioral ciliature, mode of stomatogenesis, etc. In the enigmatic and still vexatious order Rhynchodida, the suborder Aneistrocomina n. subord. is erected to embrace rhynchodid genera with an anteriorly located sucking tentacle (and other unique characteristics)—for example, Ancistrocoma, Crebricoma, Holocoma, and Sphenophrya. With the banishment of the bulk of the old “thigmotrichs” to the oligohymenophoran order Scuticociliatida, the ancistrocomines are left with the family Hypocomidae (and relatives) in the order Rhynchodida. It is not yet clear, however, how closely related the 2 suborders of rhynchodids should be considered. Special nomenclatural problems are also involved.     

The Zooflagellates Stephanopogon and Percolomonas are a Clade (Class Percolatea: Phylum Percolozoa)

ABSTRACT. The enigmatic marine protozoan Stephanopogon was first classified with ciliate protozoa because its pellicle also has rows of cilia. As ciliates have nuclear dimorphism with separate germline and somatic nuclei, Stephanopogon with several identical nuclei was regarded as a model for a hypothetical homokaryotic ancestor of ciliates. When electron microscopy revealed radical differences from ciliates this idea was abandoned, but its evolutionary position remains controversial, affinities with three other phyla being suggested. We sequenced 18S rDNA from Stephanopogon aff. minuta and actin genes from it and Stephanopogon apogon to clarify their evolutionary position. Phylogenetic analyses of 18S rRNA nest S. aff. minuta and Stephanopogon minuta securely within the protozoan phylum Percolozoa with zooflagellates of the genus Percolomonas, their closest relatives, comprising the clade Percolatea. This supports a previous grouping of Stephanopogon (order Pseudociliatida) with Percolomonas (order Percolomonadida) as a purely zooflagellate class Percolatea within Percolozoa, in contrast to the fundamentally amoeboid Heterolobosea, which are probably ancestral to Percolatea. Stephanopogon actins evolve exceptionally fast: actin trees place them as a long branch within bikont eukaryotes without revealing their sisters. We establish Percolomonadidae fam. n. for Percolomonas, excluding Pharyngomonas kirbyi g., sp. n. and Pharyngomonas (=Tetramastix=Percolomonas) salina comb. n., which unlike Percolomonas have two anterior and two posterior cilia and a pocket‐like pharynx, like “Macropharyngomonas”, now grouped with Pharyngomonas as a new purely zooflagellate class Pharyngomonadea, within a new subphylum Pharyngomonada; this contrasts them with the revised ancestrally amoeboflagellate subphylum Tetramitia. We discuss evolution of the percolozoan cytoskeleton and different body forms.     

Genome architecture drives protein evolution in ciliates

Studies of microbial eukaryotes have been pivotal in the discovery of biological phenomena, including RNA editing, self-splicing RNA, and telomere addition. Here we extend this list by demonstrating that genome architecture, namely the extensive processing of somatic (macronuclear) genomes in some ciliate lineages, is associated with elevated rates of protein evolution. Using newly developed likelihood-based procedures for studying molecular evolution, we investigate 6 genes to compare 1) ciliate protein evolution to that of 3 other clades of eukaryotes (plants, animals, and fungi) and 2) protein evolution in ciliates with extensively processed macronuclear genomes to that of other ciliate lineages. In 5 of the 6 genes, ciliates are estimated to have a higher ratio of nonsynonymous/synonymous substitution rates, consistent with an increase in the rate of protein diversification in ciliates relative to other eukaryotes. Even more striking, there is a significant effect of genome architecture within ciliates as the most divergent proteins are consistently found in those lineages with the most highly processed macronuclear genomes. We propose a model whereby genome architecture-specifically chromosomal processing, amitosis within macronuclei, and epigenetics-allows ciliates to explore protein space in a novel manner. Further, we predict that examination of diverse eukaryotes will reveal additional evidence of the impact of genome architecture on molecular evolution.     

Ciliate diversity and distribution across horizontal and vertical scales in the open ocean

Ciliates are globally distributed eukaryotic organisms inhabiting virtually all environments on Earth. Although ciliates range from 10 µm to a few millimetres in cell size, they are repeatedly reported in the pico‐sized fraction (<2–3 µm) of molecular surveys. Here, we used existing data sets (BioMarKs and Tara Oceans) with different size fractions to demonstrate that the ciliate pico‐sized signal, probably derived from cell breakage during filtration, is informative and reliable to study marine ciliate biodiversity and biogeography. We then used sequences from the pico‐eukaryotic fraction of two circumnavigation expeditions, Malaspina‐2010 and Tara Oceans, to give insights into the taxonomic composition and horizontal and vertical distribution of ciliates in the global ocean. The results suggested a high homogeneity of ciliate communities along the ocean surface from temperate to tropical waters, with ciliate assemblages dominated by a few abundant and widely distributed taxa. Very few taxa were found in a single oceanic region, therefore suggesting a high level of ciliate cosmopolitanism in the global ocean. In vertical profiles, ciliates were detected up to 4,000 m depth, and a clear vertical community structuring was observed. Our results provided evidence supporting ciliates as deeply integrated organisms in the deep‐sea trophic web, where they may play a relevant role as symbionts of metazoans and grazers of prokaryotes and small eukaryotes in the water column and in aggregates.     

WIDESPREAD PHAGOCYTOSIS OF CILIATES AND OTHER PROTISTS BY MARINE MIXOTROPHIC AND HETEROTROPHIC THECATE DINOFLAGELLATES1

An electron microscopic examination of large amorphous inclusions located in a variety of photosynthetic thecate dinoflagellates (Alexandrium ostenfeldii (Paulsen) Balech et Tangen, Gonyaulax diegensis Kofoid, Scrippsiella sp., Ceratium longipes (Bailey) Gran, and Prorocentrum micans Ehrenberg) and a nonphotosynthetic thecate species (Amylax sp.) revealed each inclusion to be a food vacuole, the majority of which were ingested ciliate prey. Recognizable features of these ciliates included linear arrays of basal bodies and cilia consistent with oligotrich polykinetid structure, characteristic macronuclei, chloroplasts (evidently kleptoplastids), cup-shaped starch plates, and cylindrical extrusomes. Three species contained (apparent) nonciliate prey: Scrippsiella sp., whose food vacuoles consistently contained unusual and complex extrusome-like cylindrical bodies having a distinctive six-lobed, multilayered structure; P. micans, which contained an unidentified encysted cell; and a single A. ostenfeldii cell, containing a Dinophysis sp. dinoflagellate cell. Several food vacuoles of ciliate origin had a red hue. This, together with the resemblance of A. ostenfeldii cells to planozygotes, suggests that similar structures previously identified as accumulation bodies may in fact be food vacuoles and that feeding may in some cases be associated with sexual processes.     

Evolution of Amitosis of the Ciliate Macronucleus: Gain of the Capacity to Divide

Ciliates exhibit nuclear dimorphism, i.e. they have a germline micronucleus and a somatic macronucleus. Macronuclei are differentiated from mitotic sisters of micronuclei. The macronuclei of "higher ciliates" are polyploid and divide acentromerically ("amitotically"); they differentiate once per life cycle. By contrast, Karyorelict (KR) ciliate macronuclei are nearly diploid and cannot divide; they must differentiate at every cell cycle. Diverse lines of evidence are presented to support the hypothesis that ancestral ciliate macronuclei were incapable of division (as in living karyorelict ciliates) and that higher ciliates gained, perhaps independently more than once, the ability to divide the macronucleus. Selective pressures that could have driven the evolution and macronuclear division and two plausible step-wise pathways for the evolution of macronuclear division are proposed. These hypotheses are relevant to our understanding of amitosis mechanisms, evolution of nuclear dimorphism, and phylogenetic classification of ciliates.     

Evolution of amitosis of the ciliate macronucleus: gain of the capacity to divide

Ciliates exhibit nuclear dimorphism, i.e. they have a germline micronucleus and a somatic macronucleus. Macronuclei are differentiated from mitotic sisters of micronuclei. The macronuclei of "higher ciliates" are polyploid and divide acentromerically ("amitotically"); they differentiate once per life cycle. By contrast, Karyorelict (KR) ciliate macronuclei are nearly diploid and cannot divide; they must differentiate at every cell cycle. Diverse lines of evidence are presented to support the hypothesis that ancestral ciliate macronuclei were incapable of division (as in living karyorelict ciliates) and that higher ciliates gained, perhaps independently more than once, the ability to divide the macronucleus. Selective pressures that could have driven the evolution and macronuclear division and two plausible step-wise pathways for the evolution of macronuclear division are proposed. These hypotheses are relevant to our understanding of amitosis mechanisms, evolution of nuclear dimorphism, and phylogenetic classification of ciliates.     

Effect of Exposure Period and Algal Concentration on the Frequency of Infection of Aposymbiotic Ciliates by Symbiotic Algae from Paramecium bursaria

The effect of exposure period and concentration of algae on the frequency of infection of aposymbiotic ciliates by algae obtained from the same clone of Paramecium bursaria syngen 2, was studied. The frequency of infection was roughly proportional to the algal concentration and to the exposure time of ciliates to algae. The relationship of algal concentration to infection frequency closely fitted the Poisson distribution curve for N = 1, suggesting that the minimum number of algae required to infect a single ciliate is 1. However, the data also strongly suggested that the average number of algae required to initiate infection of an average ciliate was ? 1,000. Three possible resolutions of this situation are: (a) the selection by the ciliate of a rare infective variant from a heterogeneous population: (b) the rare escape of an alga from digestion by the ciliate; and (c) the requirement for a large number of algae-ciliate contacts to induce susceptibility in the ciliate. Splitting the exposure of ciliates to algae into 2 periods of 0.5 h, separated by 5 h in the absence of algae, produced a much higher frequency of infection than a single l-h exposure, supporting the suggestion that the large number of algae is required to induce susceptibility in the ciliate which can then be infected by as few as a single algal cell.     

The Contribution of Ciliated Protozoa to Zooplankton Biomass in an Acidic, Subtropical Lake

Ciliated protozoa accounted for up to 50% of the mean daily zooplankton biomass in McCloud Lake, a small (5 ha), oligotrophic, acidic (pH 4.7) lake in north-central Florida. Food resources (algae and bacterioplankton) were limiting for crustacean and rotifer zooplankton during much of the year. Myxotrophic ciliates were a dominant component of the planktonic food web. Stentor niger , an uncommon species in the plankton of lakes, dominated the ciliate assemblage and usually comprised >90% of total ciliate biomass. Stentor niger always contained high densities of photosynthetic zoochlorellae and contributed an estimated 30% to the total autotrophic biomass.     

Size dependence of composition, photosynthesis and growth in the colony-forming freshwater ciliate, Ophrydium versatile

1. Ophrydium versatile is a symbiotic ciliate which forms gelatinous colonies up to several centimetres in diameter in transparent temperate lakes. The ciliates are evenly spaced at the colony surface and constitute a small proportion of the surface area (7%) and volume (3.1%) of the colony, but a large proportion of organic carbon (74%) and nitrogen content (82%) (exemplified for 1 cm³ colonies). The majority of the colony volume is formed by the jelly. The biomass proportion of ciliates scales inversely with colony size, following the decline of surface area to colony volume. The largest colonies found in Danish lakes in early summer contain almost 1 million ciliates, and assuming they derive from a single ciliate undergoing exponential division, they need twenty generations and, presumably, almost a year to reach maximum size. 2. The ciliates contain numerous symbiotic zoochlorellae that constitute about 10% of ciliate volume and more than half of the carbon content. Zoochlorellae dominate oxygen metabolism of the assemblage, resulting in low light compensation points, a large diel photosynthetic surplus, and a marked dependence on light for sustained growth and ciliate metabolism. Estimated gross photosynthesis (7ng C ciliate^?1 day^?1) of Ophnrydium from shallow, clear waters in June greatly exceeded the estimated carbon contained in filtered bacteria and small algae (1.9ng C cilicate^?1 day^?1). Nitrogen and phosphorus content of the prey, however, may provide the main nutrient source consistent with the correspondence between mass-specific rates of nutrient uptake and measured relative growth rates (average 0.067 day^?1, generation time 10 days). 3. The large Ophrydium colonies require increased allocation of photosynthetic carbohydrates with increasing colony size to maintain the jelly. The large colonies tend to become gas-filled, floating, mechanically destroyed and their ciliate inhabitants abandon them as swarmers. Colony formation, however, should offer protection against predators which may be more important for the natural abundance than the costs of growing in a colony.     

Molecular phylogeny of ciliates: What does it tell us about the evolution of the cytoskeleton and of developmental strategies?

An rRNA phylogeny of 22 species of ciliates belonging to seven of Small and Lynn's eight classes has been obtained by distance and parsimony methods. It displays good congruence with classical systematics at low taxonomic levels and several major surprises at higher levels: (1) The species analyzed group into five major branches, four of which emerge almost simultaneously: hypotrichs, oligohymenophorans, lito-stomes, and nassophoreans corresponding to four of Small and Lynn's classes. The simultaneous emergence of these groups contradicts the long accepted view that litostomes (a group with “simple”, symmetrical, apical oral apparatus) are “primitive,” while hypotrichs are “highly evolved.” (2) Heterotrichs group with a karyorelictid, together forming the first emerging branch. While this supports the view that karyorelictids may be early-emerging ciliates, it completely explodes the traditional “spirotrichs” taxon, which united heterotrichs and hypotrichs. Instead, this reinforces the concept of Postciliodesmatophora and suggests that asymmetric oral apparatuses (i.e., with distinct paroral and adoral ciliatures) may be primitive in ciliates. The global topology of the tree therefore does not fit with the classical views of ciliate evolution, from “simple” oral apparatus and stomatogenesis to “complex” ones. Instead, a rather striking agreement with the strategy adopted to construct the cortical framework was disclosed. We noted that the cytoskeletal elements used to strengthen the cell surface could be subdivided into four main types: epiplasm, filaments, continuous microtu-bules, or basal body derived fibers. These four types fitted quite well with the major evolutionary lines disclosed by the molecular phylogeny. We therefore discuss unorthodox hypotheses assuming an early explosive radiation of ciliates into a small number of major lineages differing essentially in the solution adopted to subtend the cell surface and anchor the infraciliature. © 1992 Wiley-Liss, Inc.     

Feeding response of a benthic copepod to ciliate prey type,prey concentration and habitat complexity

1. Protozoans are important consumers within microbial food webs and, in turn, they represent potential prey for small metazoans. However, feeding interactions within these food webs are rarely characterised and this is especially true for freshwater sediments. 2. We aimed to quantify the feeding links between a freshwater meiofaunal copepod and ciliates in two laboratory experiments. The first experiment addressed the response of Eucyclops serrulatus towards ciliate density and type (two ciliate species of the same genus differing in terms of body size). A second experiment assessed the effect of habitat structure on feeding rates by introducing different structural complexity into the feeding arena. In contrast to the first experiment, which was run only for one time period, this experiment also tested three different total feeding times (4, 7 and 9 h). 3. Eucyclops serrulatus exhibited high ingestion rates, with 3–69 ciliates copepod^?1 h^?1 consumed depending on food concentration, food type and habitat complexity. Copepods exhibited a preference for the smaller ciliate when total ciliate concentration was low, but selected both ciliates equally when food concentrations were medium or high. However, at very high food concentration, Eucyclops preferred the larger ciliate (which was 1/3 of its own body size), suggesting that the longer handling times of the larger prey are rewarding when the large prey is present in high numbers. In terms of total numbers consumed, copepods fed on more small ciliates, but in terms of carbon units both ciliates were selected equally when total prey concentration was low or medium. However, copepods derived more carbon from the larger prey at high and very high prey concentrations (up to 0.7 μgC out of a maximum of 1.1 μgC copepod^?1 h^?1). Habitat complexity influenced the feeding of copepods when it was observed over time. 4. The copepod–ciliate link is well known from the pelagic zone of both marine and freshwater habitats. We have shown its potential importance within the benthos, where it can be influenced by food identity, food quantity and possibly by habitat complexity.     

Vertical and Seasonal Dynamics of Planktonic Ciliates in a Strongly Stratified Hypertrophic Lake

Seasonal population dynamics and the vertical distribution of planktonic ciliates in a hypertrophic and strongly stratified temperate lake were studied from April to October in 2000 and from April to June in 2001. In the epi- and metalimnion the ciliate abundance peaked in spring and late summer, reaching maximum values in the metalimnion (86 cells ml⁻¹) on 7th August 2000. In the epilimnion, the highest biomass content (414 μg C l⁻¹) was observed on 8th May 2000. In the hypolimnion only a late summer peak occurred and the ciliate numbers were always lower than in the epi- and metalimnion. Five groups dominated the community of ciliates: Oligotrichida, Gymnostomatea, Prostomatida, Hymenostomata and Peritrichia, and the community composition varied greatly with depth. In the epilimnion the ciliate numbers were dominated by oligotrichs but small algivorous prostomatids, peritrichs and gymnostomes were also numerous. In the metalimnion these groups were gradually replaced by scuticociliates and mixotrophic Coleps spp. In the hypolimnion scuticociliates and species known as benthic migrants dominated. In the epilimnion and upper metalimnion in spring large herbivores and in summer small bacterivores were more numerous.     

On the origin of mitosing cells

A theory of the origin of eukaryotic cells ("higher" cells which divide by classical mitosis) is presented. By hypothesis, three fundamental organelles: the mitochondria, the photosynthetic plastids and the (9+2) basal bodies of flagella were themselves once free-living (prokaryotic) cells. The evolution of photosynthesis under the anaerobic conditions of the early atmosphere to form anaerobic bacteria, photosynthetic bacteria and eventually blue-green algae (and protoplastids) is described. The subsequent evolution of aerobic metabolism in prokaryotes to form aerobic bacteria (protoflagella and protomitochondria) presumably occurred during the transition to the oxidizing atmosphere. Classical mitosis evolved in protozoan-type cells millions of years after the evolution of photosynthesis. A plausible scheme for the origin of classical mitosis in primitive amoeboflagellates is presented. During the course of the evolution of mitosis, photosynthetic plastids (themselves derived from prokaryotes) were symbiotically acquired by some of these protozoans to form the eukaryotic algae and the green plants. The cytological, biochemical and paleontological evidence for this theory is presented, along with suggestions for further possible experimental verification. The implications of this scheme for the systematics of the lower organisms is discussed.     

八种纤毛虫减数分裂基因的分子进化研究

减数分裂是真核生物适应性进化的重要机制,以8种纤毛虫作为实验对象,通过生物信息学方法对其14个减数分裂基因进行了鉴定及分子进化研究。结果表明:(1)不同的纤毛虫种类存在一些特异性的减数分裂基因的丢失与复制现象;(2)减数分裂相关基因在纤毛虫中很保守;(3)纤毛虫减数分裂重要的同源重组过程是在真核生物中不常见的Ⅱ型。本研究表明,纤毛虫减数分裂可能代表了真核生物较原始的减数分裂方式,在进化的过程中很保守,为研究真核生物减数分裂起源与进化提供了重要线索。     

The role of C, N and P in dissolved and particulate organic matter as a nutrient source for phytoplankton growth, including toxic species

Phytoplankton have traditionally been regarded as strictly phototrophic, with a well defined position at the base of pelagic food webs. However, recently we have learned that the nutritional demands of a growing number of phytoplankton species can be met, at least partially, or under specific environmental conditions, through heterotrophy. Mixotrophy is the ability of an organism to be both phototrophic and heterotrophic, in the latter case utilizing either organic particles (phagotrophy) or dissolved organic substances (osmotrophy). This finding has direct implications for our view on algal survival strategies, particularly for harmful species, and energy- and nutrient flow in pelagic food webs. Mixotrophic species may outcompete strict autotrophs, e.g. in waters poor in inorganic nutrients or under low light. In the traditional view of the ‘microbial loop’ DOC is thought to be channeled from algal photosynthesis to bacteria and then up the food chain through heterotrophic flagellates, ciliates and mesozooplankton. Are mixotrophic phytoplankton that feed on bacteria also significantly contributing to this transport of photosynthetic carbon up the food chain? How can we estimate the fluxes of carbon and nutrients between different trophic levels in the plankton food web involving phagotrophic algae? These questions largely remain unanswered. In this review we treat evidence for both osmotrophy and phagotrophy in phytoplankton, especially toxic marine species, and some ecological implications of mixotrophy.     

SELECTIVE INHIBITION OF BLUE-GREEN ALGAL GROWTH BY ETHIONINE AND OTHER AMINO ACID ANALOG1,2

Some metabolic analogs, including azaguanine, azathymine, azauracil, caffeine, 4-azaleucine, dl -ethionine, and dl -p-fluorophenylanlanine, were examined for their ability to repress the multiplication of algae from natural aquatic sources grown in defined or semidefined media. dl -ethionine, dl -p-fluorophenylalanine, and 4-azaleucine, in that order, inhibited the multiplication of blue-green but not other algal groups. The purine and pyrimidine analogs were not inhibitory. In chemically defined axenic media, dl -ethionine was about 100 times more inhibitory to the blue-green algae Synechococcus cedrorum and Anabaena cylindrica than to the eucaryotic algae Ochromonas danica and Euglena gracilis. The ciliate Tetrahymena pyriformis was at least 100-fold more resistant to ethionine than the algae. The unusual sensitivity of blue-green algae to ethionine and other amino acid analogs represents an exceptional phylelic character and may be useful in the control of these algae when they become a nuisance. Amino acid analogs such as ethionine may also serve to remove blue-green algae from cultures one may desire to render axenic.     

Brandtia ciliaticola gen. et sp. nov. (Chlorellaceae,Trebouxiophyceae) a common symbiotic green coccoid of various ciliate species

Many freshwater protists harbor unicellular green algae within their cells and these host‐symbiont relationships slowly are becoming better understood. Recently, we reported that several ciliate species shared a single species of symbiotic algae. Nonetheless, the algae from different host ciliates were each distinguishable by their different genotypes, and these host‐algal genotype combinations remained unchanged throughout a 15‐month period of sampling from natural populations. The same algal species had been reported as the shared symbiont of several ciliates from a remote lake. Consequently, this alga appears to play a key role in ciliate‐algae symbioses. In the present study, we successfully isolated the algae from ciliate cells and established unialgal cultures. This species is herein named Brandtia ciliaticola gen. et sp. nov. and has typical ‘Chlorella‐like’ morphology, being a spherical autosporic coccoid with a single chloroplast containing a pyrenoid. The alga belongs to the Chlorella‐clade in Chlorellaceae (Trebouxiophyceae), but it is not strongly connected to any of the other genera in this group. In addition to this phylogenetic distinctiveness, a unique compensatory base change in the SSU rRNA gene is decisive in distinguishing this genus. Sequences of SSU‐ITS (internal transcribed spacer) rDNA for each isolate were compared to those obtained previously from the same host ciliate. Consistent algal genotypes were recovered from each host, which strongly suggests that B. ciliaticola has established a persistent symbiosis in each ciliate species.