Apocarchesium rosettum and A. arndti were originally discovered in Japan (Lake Biwa) and Germany (River Rhine), respectively. We report the first record of A. arndti in the Danube and provide a detailed description of its colony development. Our findings support the theory of moderate endemicity and reveal a new, smaller zooid type in A. arndti. This zooid remains attached to the colony, connected to the stalk myoneme but lacks an aboral ciliary wreath. Unlike microzooids, it is incapable of leaving the colony. It exhibits a less spherical shape and arises from the fourth division of the colony-founder cell. Although its specific function is unknown, it is hypothesized to support the stalk dish. Our results have significant implications for understanding the systematics of vorticellids, suggesting their ancestral nature as colonial organisms characterized by a helically contracting stalk myoneme. Furthermore, the exclusive retention of the stalk myoneme by the parental cell after binary fission may serve as a synapomorphy for the Vorticellidae. We provide a descriptive analysis of the ecological environment and microhabitat of A. arndti in the Danube, revealing its preference for well-developed, detritus-rich biofilms during summer, absence in late winter and spring, emergence during peak summer, and subsequent decline until mid-winter.

Arcellinida is ascending in importance in protistology, but description of their diversity still presents multiple challenges. Furthermore, applicable tools for surveillance of these organisms are still in developing stages. Importantly, a good database that sets a correspondence between molecular barcodes and species morphology is lacking. Cytochrome oxidase (COI) has been suggested as the most relevant marker for species discrimination in Arcellinida. However, some major groups of Arcellinida are still lacking a COI sequence. Here we expand the database of COI marker sequences for Arcellinids, using single-cell PCR, transcriptomics, and database scavenging. In the present work, we added 24 new Arcellinida COI sequences to the database, covering all unsampled infra- and suborders. Additionally, we added six new SSUrRNA sequences and described four new species using morphological, morphometrical, and molecular evidence: Heleopera steppica, Centropyxis blatta, Arcella uspiensis, and Cylindrifflugia periurbana. This new database will provide a new starting point to address new research questions from shell evolution, biogeography, and systematics of arcellinids.

The study of endobiotic ciliates from the intestines of various zoo mammals is important for revealing the influence of various factors on endobiont communities. In this paper we describe the species diversity of endobiotic ciliates from the faeces of the eastern black rhinoceros Diceros bicornis michaeli (male and two females, mother and daughter) kept in the zoo. Seven species of the ciliates were found, among them Rhinozeta rhinozeta, R. triciliata and Prototapirella clypeata were observed in the rhinos only. The other two, Monoposthium sp. and Triplumaria sp., according to their morphology should have been identified as new ciliate species. Successful transmission of the endobionts from parents to the young rhino in the zoo was demonstrated. R. rhinozeta was investigated with the using of the methods of the light and immunofluorescence microscopy and molecular phylogeny. The composition and phylogeny of the family Cycloposthiidae were discussed.

Gregarine apicomplexans, a group of single celled organisms, inhabit the extracellular spaces of most invertebrate species. The nature of the gregarine-host interactions is not yet fully resolved, mutualistic, commensal and parasitic life forms have been recorded. In the extreme arid environment of the Atacama Desert, only a few groups of invertebrates hosting gregarines such as darkling beetles (Tenebrionidae) were able to adapt, providing an unparalleled opportunity to study co-evolutionary diversification. Here, we describe one novel gregarine genus comprising one species, Atacamagregarina paposa gen. et sp. nov., and a new species, Xiphocephalus ovatus sp. nov. (Apicomplexa: Eugregarinoridea, Stylocephalidae), found in the tenebrionid beetle genera Scotobius (Tenebrioninae, Scotobiini) and Psectrascelis intricaticollis ovata (Pimeliinae, Nycteliini), respectively. In the phylogenetic analysis based on SSU rDNA, Atacamgregarina paposa representing the new genus is basal, forming a separate clade with terrestrial gregarines specific for North American darkling beetles.

The protist genus Gromia was first described in 1835 by Dujardin and while gromiids are prominent in the marine environment, Gromia oviformis was, for a long time, the only valid species regularly recorded. To date, 16 species that are morphologically and/or genetically distinct have been described. While recent studies are documenting their diversity and their ecological importance, G. oviformis has been the sole gromiid species identified in the Black Sea, although unnamed Gromia species have also been recorded. We collected sediment samples from the Romanian continental shelf at varying depths (48 - 58m) to study the morphological and genetic diversity of gromiids in this part of the Black Sea. Three new species, Gromia bugnae sp. nov., Gromia dianae sp. nov. and Gromia fabi sp. nov., were identified based on an integrative taxonomic approach, thus bringing the total described gromiid species to 19. Analysis of partial SSU rRNA gene sequences confirms that these are distinct species. Additionally, an undescribed species is represented by a sequence from the northern part of the Black Sea (Sevastopol, Kazachya Bay). The study provides further evidence of the diversity of gromiids in the Black Sea and underlines the importance of this little-known group in marginal seas.

Coccidiosis, a serious intestinal parasitic disease caused by Eimeria spp., can result in huge annual economic losses to the poultry industry worldwide. At present, coccidiosis is mainly controlled by anticoccidial drugs. However, drug resistance has developed in Eimeria because of the long-term and unreasonable use of the drugs currently available. In our previous study, RNA-seq showed that the expression of methionine aminopeptidase1 (EtMetAP1) was up-regulated in diclazuril-resistant (DZR) and maduramicin-resistant (MRR) strains compared to drug-sensitive (DS) strain of Eimeria tenella. In this study, EtMetAP1 was cloned and expressed, and the function and characteristics of the EtMetAP1 protein were analyzed. The transcription and translation levels of EtMetAP1 in DS strain of E. tenella at different developmental stages were analyzed by qPCR and western blotting. We found that the transcription and translation levels of EtMetAP1 in second-generation merozoites (SM) were higher than those of the other three stages (unsporulated oocyst, sporulated oocyst, and sporozoites). Simultaneously, qPCR was used to analyze the mRNA transcription levels of EtMetAP1 in DS, DZR, MRR, and salinomycin-resistant (SMR) strain. The results showed that compared to the sensitive strain, the transcription levels of EtMetAP1 in DZR and MRR were up-regulated. There was no significant difference in transcription level in SMR. Indirect immunofluorescence localization showed that the protein was mainly localised in the cell membrane and cytoplasm of sporozoites and SM. An invasion inhibition test showed that anti-rEtMetAP1 polyclonal antibody could effectively inhibit the sporozoite invasion of host cells. These results suggest that the protein may be involved in the growth and development of parasites in host cells, the generation of drug resistance, and host cell invasion.

Two already known representatives of Holospora-like bacteria, "Candidatus Gortzia yakutica" from Paramecium putrinum and Preeria caryophila, originally retrieved from the Paramecium aurelia complex, were found in new hosts: Paramecium nephridiatum and Paramecium polycaryum, respectively. In the present study, these bacteria were investigated using morphological and molecular methods. For "Ca. G. yakutica", the first details of the electron microscopic structure in the main and new hosts were provided. Regarding Pr. caryophila, the ultrastructural description of this species was implemented by several features previously unknown, such as the so called "membrane cluster" dividing periplasm from cytoplasm and fine composition of infectious forms before and during its releasing from the infected macronucleus. The new combinations of these Holospora-like bacteria with ciliate hosts were discussed from biogeographical and ecological points of view. Host specificity of symbionts as a general paradigm was critically reviewed as well.

Maintaining high levels of biological diversity in various ecosystems is necessary for stable functioning of the Earth's biosphere. The article describes diversity and ecology of heterotrophic siliceous protists - rotosphaerids, colourless free-living thaumatomonad flagellates, and centrohelid heliozoans - in Arctic waters located of Asian Russia. Samples were collected in the mouths of the Olenyok, Yana, Indigirka, Kolyma Rivers - and small freshwater ponds near Settlement of Tiksi in Yakutia. Based on electron microscopy, 35 centrohelids (Heliozoa), 11 rotosphaerid species as well as four thaumatomonad flagellate species were found in the region. Seven species were recorded in Russia for the first time: Rabdiaster multicosta, Rabdiaster reticulata, Turriplaca denticulata, Choanocystis cf. cordiformis parvula, Raineriophrys pteromorphos, Pseudoraphidocystis glutinosa and Pseudoraphidiophrys formosa. For 43 species, the study area is the most northern location they were described. Morphological details of scales are discussed for selected species, particularly for widespread species of rotosphaerids - Pinaciophora fluviatilis. The literature on hydrochemical conditions was analyzed to find taxa with high sensitivity towards environmental changes. Such species could be further used for monitoring plankton, recovering evolutionary changes, and reconstructing past environments.

Heterotrophic flagellates (HF) represent an important protist group in marine ecosystem functioning. Characterised by high taxonomic diversity, identification and classification of HF is often difficult using classical methods of light microscopy (LM). Complementing LM with molecular methods, such as barcoding, enables reliable taxonomic identification of even small size nanoflagellates that share similar or unnoticeable morphological features. The order Bicosoecida is a group of heterotrophic nanoflagellates that are important part of protist plankton and benthic communities of the world oceans. Recently, on the basis of high-resolution light microscopy and barcoding, a new bicosoecid genus, Bilabrum, was described with B. latius sp. as a type species. Our study reports on identification of B. latius from co-culture with the diatom species Chaetoceros affinis isolated from fresh plankton samples collected in the southern Adriatic. This detection of the Adriatic B.latius represents first record of this species outside itś up to now known and described habitat (deep-sea sediment of the South - East Atlantic Ocean) and in diatom co-culture.