Spirostomum spp. (Ciliophora, Protista), a suitable system for endocytobiosis research

Topological damping in an ultrafast giant cell

Cellular systems are known to exhibit some of the fastest movements in biology, but little is known as to how single cells can dissipate this energy rapidly and adapt to such large accelerations without disrupting internal architecture. To address this, we investigate Spirostomum ambiguum-a giant cell (1-4 mm in length) well-known to exhibit ultrafast contractions (50% of body length) within 5 ms with a peak acceleration of 15[Formula: see text]. Utilizing transmitted electron microscopy and confocal imaging, we identify an association of rough endoplasmic reticulum (RER) and vacuoles throughout the cell-forming a contiguous fenestrated membrane architecture that topologically entangles these two organelles. A nearly uniform interorganelle spacing of 60 nm is observed between RER and vacuoles, closely packing the entire cell. Inspired by the entangled organelle structure, we study the mechanical properties of entangled deformable particles using a vertex-based model, with all simulation parameters matching 10 dimensionless numbers to ensure dynamic similarity. We demonstrate how entangled deformable particles respond to external loads by an increased viscosity against squeezing and help preserve spatial relationships. Because this enhanced damping arises from the entanglement of two networks incurring a strain-induced jamming transition at subcritical volume fractions, which is demonstrated through the spatial correlation of velocity direction, we term this phenomenon "topological damping." Our findings suggest a mechanical role of RER-vacuolar meshwork as a metamaterial capable of damping an ultrafast contraction event.

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.

Segnochrobactrum spirostomi gen. nov., sp. nov., isolated from the ciliate Spirostomum yagiui and description of a novel family, Segnochrobactraceae fam. nov. within the order Rhizobiales of the class Alphaproteobacteria

A bacterial strain, designated Sp-1^T, was isolated from the heterotrich ciliate Spirostomum yagiui collected from a reservoir located in Ulsan, Republic of Korea. Cells of Sp-1^T were Gram stain-negative, rod-shaped, non-spore-forming, non-motile and contained poly-β-hydroxybutyrate granules. Phylogenetic analyses based on 16S rRNA gene sequences indicated that Sp-1^T constituted a distinct phylogenetic lineage within different families in the order Rhizobiales with a pairwise sequence similarity of 95 % to the species of the genus Ochrobactrum: Ochrobactrum anthropi ATCC 49188^T and Ochrobactrum cytisi ESC1^T (family Brucellaceae). The major cellular fatty acids were C_19 : 0 cyclo ω8c (44.4 %) and C_16 : 0 (32.1 %). The identified sole isoprenoid quinone was ubiquinone-10 (Q-10). The major polar lipids produced were phosphatidylcholine, phosphatidylglycerol, phosphatidylethanolamine, an unidentified aminolipid, two unidentified phospholipids and three unidentified lipids. The genome size was about 5.4 Mbp and the DNA G+C content was 68.2 mol%. Sp-1^T exhibited the highest average nucleotide identity value of 76.6 % and in silico DNA–DNA hybridization value of 22.1 % with Pseudoxanthobacter soli DSM 19599^T (family Xanthobacteraeae). This strain is distinguishable from closely related members of the order Rhizobiales by its differential phenotypic, chemotaxonomic, genomic and phylogenetic characteristics. On the basis of evidence from polyphasic taxonomic analysis, we concluded that Sp-1^T represents a novel species in a novel genus within the order Rhizobiales, for which the name Segnochrobactrum spirostomi gen. nov., sp. nov. is proposed. The type strain is Sp-1^T (=KCTC 62036^T=JCM 32162^T). We also describe a novel family, Segnochrobactraceae fam. nov., to encompass the proposed novel genus and species.

"Candidatus Hafkinia simulans" gen. nov., sp. nov., a Novel Holospora-Like Bacterium from the Macronucleus of the Rare Brackish Water Ciliate Frontonia salmastra (Oligohymenophorea, Ciliophora): Multidisciplinary Characterization of the New Endosymbiont and Its Host

We characterized a novel Holospora-like bacterium (HLB) (Alphaproteobacteria, Holosporales) living in the macronucleus of the brackish water ciliate Frontonia salmastra. This bacterium was morphologically and ultrastructurally investigated, and its life cycle and infection capabilities were described. We also obtained its 16S rRNA gene sequence and performed in situ hybridization experiments with a specifically-designed probe. A new taxon, "Candidatus Hafkinia simulans", was established for this HLB. The phylogeny of the family Holosporaceae based on 16S rRNA gene sequences was inferred, adding to the already available data both the sequence of the novel bacterium and those of other Holospora and HLB species recently characterized. Our phylogenetic analysis provided molecular support for the monophyly of HLBs and placed the new endosymbiont as the sister genus of Holospora. Additionally, the host ciliate F. salmastra, recorded in Europe for the first time, was concurrently described through a multidisciplinary study. Frontonia salmastra's phylogenetic position in the subclass Peniculia and the genus Frontonia was assessed according to 18S rRNA gene sequencing. Comments on the biodiversity of this genus were added according to past and recent literature.

Disentangling the Taxonomy of Rickettsiales and Description of Two Novel Symbionts ("Candidatus Bealeia paramacronuclearis" and "Candidatus Fokinia cryptica") Sharing the Cytoplasm of the Ciliate Protist Paramecium biaurelia

In the past 10 years, the number of endosymbionts described within the bacterial order Rickettsiales has constantly grown. Since 2006, 18 novel Rickettsiales genera inhabiting protists, such as ciliates and amoebae, have been described. In this work, we characterize two novel bacterial endosymbionts from Paramecium collected near Bloomington, IN. Both endosymbiotic species inhabit the cytoplasm of the same host. The Gram-negative bacterium “Candidatus Bealeia paramacronuclearis” occurs in clumps and is frequently associated with the host macronucleus. With its electron-dense cytoplasm and a distinct halo surrounding the cell, it is easily distinguishable from the second smaller symbiont, “Candidatus Fokinia cryptica,” whose cytoplasm is electron lucid, lacks a halo, and is always surrounded by a symbiontophorous vacuole. For molecular characterization, the small-subunit rRNA genes were sequenced and used for taxonomic assignment as well as the design of species-specific oligonucleotide probes. Phylogenetic analyses revealed that “Candidatus Bealeia paramacronuclearis” clusters with the so-called “basal” Rickettsiales, and “Candidatus Fokinia cryptica” belongs to “Candidatus Midichloriaceae.” We obtained tree topologies showing a separation of Rickettsiales into at least two groups: one represented by the families Rickettsiaceae, Anaplasmataceae, and “Candidatus Midichloriaceae” (RAM clade), and the other represented by “basal Rickettsiales,” including “Candidatus Bealeia paramacronuclearis.” Therefore, and in accordance with recent publications, we propose to limit the order Rickettsiales to the RAM clade and to raise “basal Rickettsiales” to an independent order, Holosporales ord. nov., inside Alphaproteobacteria, which presently includes four family-level clades. Additionally, we define the family “Candidatus Hepatincolaceae” and redefine the family Holosporaceae.

IMPORTANCE In this paper, we provide the characterization of two novel bacterial symbionts inhabiting the same Paramecium host (Ciliophora, Alveolata). Both symbionts belong to “traditional” Rickettsiales, one representing a new species of the genus “Candidatus Fokinia” (“Candidatus Midichloriaceae”), and the other representing a new genus of a “basal” Rickettsiales. According to newly characterized sequences and to a critical revision of recent literature, we propose a taxonomic reorganization of “traditional” Rickettsiales that we split into two orders: Rickettsiales sensu stricto and Holosporales ord. nov. This work represents a critical revision, including new records of a group of symbionts frequently occurring in protists and whose biodiversity is still largely underestimated.

Act together-implications of symbioses in aquatic ciliates

Mutual interactions in the form of symbioses can increase the fitness of organisms and provide them with the capacity to occupy new ecological niches. The formation of obligate symbioses allows for rapid evolution of new life forms including multitrophic consortia. Microbes are important components of many known endosymbioses and their short generation times and strong potential for genetic exchange may be important drivers of speciation. Hosts provide endo- and ectosymbionts with stable, nutrient-rich environments, and protection from grazers. This is of particular importance in aquatic ecosystems, which are often highly variable, harsh, and nutrient-deficient habitats. It is therefore not surprising that symbioses are widespread in both marine and freshwater environments. Symbioses in aquatic ciliates are good model systems for exploring symbiont-host interactions. Many ciliate species are globally distributed and have been intensively studied in the context of plastid evolution. Their relatively large cell size offers an ideal habitat for numerous microorganisms with different functional traits including commensalism and parasitism. Phagocytosis facilitates the formation of symbiotic relationships, particularly since some ingested microorganisms can escape the digestion. For example, photoautotrophic algae and methanogens represent endosymbionts that greatly extend the biogeochemical functions of their hosts. Consequently, symbiotic relationships between protists and prokaryotes are widespread and often result in new ecological functions of the symbiotic communities. This enables ciliates to thrive under a wide range of environmental conditions including ultraoligotrophic or anoxic habitats. We summarize the current understanding of this exciting research topic to identify the many areas in which knowledge is lacking and to stimulate future research by providing an overview on new methodologies and by formulating a number of emerging questions in this field.

"Candidatus Paraholospora nucleivisitans", an intracellular bacterium in Paramecium sexaurelia shuttles between the cytoplasm and the nucleus of its host

An intracellular bacterium was discovered in two isolates of Paramecium sexaurelia from an aquarium with tropical fish in Münster (Germany) and from a pond in the Wilhelma zoological-botanical garden, Stuttgart (Germany). The bacteria were regularly observed in the cytoplasm of the host, but on some occasions they were found in the macronucleus of the host cell. In these cases, only a few, if any, bacteria were observed remaining in the cytoplasm. The bacterium was not infectious to P. sexaurelia or other species of Paramecium and appeared to be an obligate intracellular bacterium, while bacteria-free host cells were completely viable. The fluorescence in situ hybridisation (FISH) and comparative 16SrDNA sequence analyses showed that the bacterium belonged to a new genus, and was most closely, yet quite distantly, related to Holospora obtusa. In spite of this relationship, the new bacteria differed from Holospora by at least two biological features. Whereas all Holospora species reside exclusively in the nuclei of various species of Paramecium and show a life cycle with a morphologically distinct infectious form, for the new bacterium no infectious form and no life cycle have been observed. For the new bacterium, the name Candidatus Paraholospora nucleivisitans is suggested. The host P. sexaurelia is usually known from tropical and subtropical areas and is not a species typically found in Germany and central Europe. Possibly, it had been taken to Germany with fish or plants from tropical or subtropical waters. Candidatus Paraholospora nucleivisitans may therefore be regarded as an intracellular neobacterium for Germany.