The endoplasmic reticulum (ER) is a large and highly dynamic component of the eukaryotic endomembrane system. In eukaryotic microalgae, it plays six distinct roles: (1) It envelopes the chromatin to form the nucleus. (2) It forms cisternae in the cytoplasm, some of which scaffold the synthesis of proteins destined for incorporation into membranes or for secretion. (3) It associates with Golgi cisternae to scaffold the synthesis of glycosylated proteins. (4) It associates with the plasma membrane to mediate the synthesis and secretion of hydrophobic molecules. (5) It mediates the synthesis of cytoplasmic lipid bodies. (6) In lineages harboring complex plastids of red algal ancestry, it forms the chloroplast ER, which envelops the primary chloroplast envelope. In this review, these systems are illustrated using the quick-freeze deep-etch electron microscopy (QFDEEM) technique, which lifts up the topological configurations adopted by this gossamer system. A key finding is that in all the complex microalgae examined except dinoflagellates, the inner nuclear envelope membrane associates directly with the plastid-contiguous membrane of the chloroplast ER at foci designated as chloroplast-nuclear junctions. These junctions may play a role in regulating the maintenance and physiology of the complex organelles.
Barthelona is a genus of anaerobic flagellates that forms a sister lineage to Fornicata along with Skoliomonas. Although "Barthelona spp." are known to separate into three distinct lineages, their detailed morphology has not been examined, and their taxonomic classification is incomplete. In this study, we investigated the morphology, especially ultrastructure, of three cultures of barthelonids, each representing one lineage. These flagellates possess an inconspicuous ventral groove and a highly elongated cytopharynx that originates near the posterior end of the cell and extends toward the anterior end. Their posterior flagellum lacks a vane. The flagellar apparatus shares some features with those of other excavate flagellates in possessing microtubular roots R1, split R2 and R3, but lacks a singlet root, B fiber and discernable C fiber. Notably, R2 splits near the posterior end of the cell. Cell size and the number of microtubules comprising R1 and R2 vary among strains. Based on these observations, we describe two new genera and species, Microbarthelona vorax gen. nov., sp. nov. and Parabarthelona vacuolata gen. nov., sp. nov. Additionally, we propose the following higher taxa for Barthelona, Microbarthelona, and Parabarthelona: Notopharyngea cl. nov., Barthelonida ord. nov., and Barthelonidae fam. nov.
Dinoflagellates and ciliates are important grazers of primary production in the Northeast Pacific but knowledge of their taxonomic composition and depth-distribution is limited. These organisms also display a variety of heterotrophic feeding modes including heterotrophy, mixotrophy, and parasitism. Here, we analyzed dinoflagellate and ciliate feeding modes and depth-distribution using high-throughput amplicon sequencing of the 18S rRNA V4 gene region on samples collected during the August-September 2018 EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) campaign near ocean station Papa. The parasitic dinoflagellate class Syndiniales comprised 52% of amplicon sequence variants (ASVs), only one of which could be identified to the genus level, highlighting the potential importance of unknown parasitic dinoflagellates to marine food webs. Frequent occurrences of carnivorous ciliates suggested grazer-on-grazer predation, driving carbon recycling through repeated trophic transfers. Over 90% of protist consumer ASVs were more likely to occur in either surface waters or below the mixed layer, highlighting significant vertical structure in the protistan consumer community. The activities of this diverse set of depth-stratified protist consumers likely modulate carbon flow in the upper ocean and may explain currently unattributed export production losses in the Northeast Pacific.
Radiolaria are unicellular marine organisms (protists) that have been drifting in oceanic plankton for hundreds of millions of years. These mineral architects can build extraordinarily complex skeletons, which fascinated and puzzled naturalists observing water samples through rudimentary microscopes. In the 19th century, the discovery and study of Radiolaria are associated with scientific voyages and human adventures. Naturalists who studied medicine and anatomy in European universities were captivated by the morphology of Radiolaria and expressed a profound wanderlust to collect them in the ocean. These intrepid and workaholic adventurers devoted their restless lives to studying microscopic life, while also actively engaging in teaching and sharing their observations and hypotheses with students. This article aims to retrace the discovery of Radiolaria through the lives of prominent naturalists and marine biologists, primarily Christian Gottfried Ehrenberg, Thomas Henry Huxley, Johannes Müller, and Ernst Haeckel. It also highlights the intellectual and geographic influences that shaped their research, including figures such as Johannes Wolfgang von Goethe and Alexander von Humboldt, as well as places like Jena, Helgoland, Villefranche-sur-Mer and Italy, which served as sampling locations and sources of romantic and artistic inspiration. Pioneering work on Radiolaria played a central role in shaping several emerging concepts (e.g., cell theory, individuality) and fields (e.g., taxonomy, evolution of morphology, symbiosis). The discovery of Radiolaria therefore reveals that even the most elusive marine microorganisms can deeply transform our understanding of life.
Cockroaches are known reservoirs for diverse bacterial microbiomes. However, comprehensive analyses of the eukaryotic communities within cockroaches remain limited. In this study, we selected three long-term laboratory-reared cockroach species (Blattella germanica, Periplaneta fuliginosa, and Periplaneta japonica) and performed metabarcoding of the 18S rRNA V9 region using the iSeq 100 platform. The nematode Blatticola blattae was identified in B. germanica, and Leidynema appendiculata was found in both P. fuliginosa and P. japonica. The amplicon sequence variant (ASV) of Nyctotherus (Ciliophora) was detected in all three species of cockroach, while Entamoeba sp. was detected in P. fuliginosa and P. japonica. Compared with the other two species, B. germanica exhibited a higher prevalence of the fungus Nephridiophaga. Our findings showed that laboratory-reared cockroaches belonging to different species harbored distinct commensal and parasitic eukaryotic taxa. While most ASVs were of reduced clinical concern for humans as opposed to studies using wild-caught specimens, the detection of commensals, arthropod parasites and potential human pathogens illustrates the complexity of the cockroach-associated eukaryome and the influence of host identity. Our study emphasizes the importance of host-specificity in the eukaryotic community of laboratory-reared cockroach models.
Wax esters (WEs) produced by Euglena gracilis under anaerobic conditions contain significant proportions of odd-chain fatty acids and fatty alcohols (30%-40%). Propionyl-CoA, the primer for odd-chain fatty acid synthesis, is generally thought to be generated via the methylmalonyl-CoA pathway; however, the enzymatic basis of this pathway in E. gracilis remains unclear. Here, we generated knockout (KO) mutants of candidate enzymes in the methylmalonyl-CoA pathway-succinyl-CoA synthetase (SCS), methylmalonyl-CoA mutase (MCM), methylmalonyl-CoA epimerase (MCE), and propionyl-CoA carboxylase (PCC)-using the CRISPR/Cas9 system and examined their roles in odd-chain fatty acid synthesis and propionate assimilation. The methylmalonyl-CoA pathway exhibited direction-dependent functions. KO of MCE and PCC did not affect the proportion of odd-chain WEs, indicating that these enzymes are not required for odd-chain WE synthesis and suggesting an alternative route converting methylmalonyl-CoA to propionyl-CoA. In contrast, both enzymes were required for propionate assimilation. Functional differentiation was observed between SCSβ isozymes: SCSβ1 contributed to odd-chain WE synthesis, whereas SCSβ2 primarily functioned in propionate assimilation. Partial impairment of propionate assimilation in SCSα or SCSβ2 KO strains suggests involvement of additional routes. These findings improve our understanding of direction-dependent roles in the methylmalonyl-CoA pathway in E. gracilis and support further studies of odd-chain lipid biosynthesis.
Thaumatomonads are common in soil and fresh water and play an important role in microbial communities. They use silicon for the intracellular synthesis of species-specific silica-scales that cover the surface of their cells. Their diversity is not well studied, and the combination of electron microscopy and molecular phylogeny of marker genes allowed the discovery of a new taxon. In this work, we established the Thaumatomonas strain IW08m from the Irkutsk Reservoir Bay (Eastern Siberia, Russia) into a monoculture and, using various microscopy methods, described its cell structure, including the stage of silica-scale formation. Based on combined data from the fine structure of the scales and phylogenetic analysis of the 18S rRNA gene, we showed congruence of morphological differences and genetic distances in the genus Thaumatomonas and described a new species, Thaumatomonas sibirica n. sp., and thus expanded our understanding of the diversity of thaumatomonads.
Single-celled protists, such as testate amoebae, have garnered significant attention as potential bioindicators which translate into their recognition of being ecologically sensitive. They are known for their use in ecological and paleoecological studies, yet their diversity and distribution remain poorly documented in blind spot regions of the Paleotropics. This study systematically consolidates all published species records of testate amoebae, evaluates spatial research bias, and assesses regional richness from Southeast Asian countries. A comprehensive literature search was conducted across digital databases and web-based repositories. In total, there are 497 testate amoebae belonging to 70 genera, including infra-specific species from the 46 studies scoped in this review. Species richness curves revealed steep trajectories in under sampled countries, indicating untapped diversity. Despite geographical proximity, the seeming overlap of species per country was limited, pointing toward predominance of species turnover than nested clustering of richness. This disjunct pattern in species composition suggests the influence of potential ecological specialization or, perhaps, a mere result of uneven research effort in the unexplored regions. The research for testate amoebae has been in a progressive state, yet strong emphasis needs addressing species exploration in response to intensified climate change and degradation of habitats as these would mean potential biodiversity loss.
A comprehensive field investigation was conducted on 300 specimens of common carp (Cyprinus carpio) from the Godavari Basin in Nanded and Parbhani districts between May 2024 and April 2025. The protozoan parasite Chilodonella uncinata was identified in 65 fish, resulting in a prevalence rate of 21.1%. Accurate identification was achieved using morphometric analysis under light microscopy, silver nitrate staining, and scanning electron microscopy (SEM), which revealed detailed ciliary structures confirming the species. Histopathological examination of infected gill and skin tissues showed marked pathological changes, including epithelial hyperplasia, lamellar fusion, and infiltration of inflammatory cells, indicating a potential impact on respiratory efficiency and general fish health. This represents the first confirmed report of C. uncinata infecting C. carpio in this region. The findings emphasize the necessity of routine parasite screening and targeted management strategies in aquaculture. Additionally, the study demonstrated that environmental factors, such as elevated water temperature, increased turbidity, reduced dissolved oxygen, and nutrient enrichment, significantly influence the prevalence of the parasite. These results underscore the importance of integrating environmental monitoring into fish health programs to mitigate the spread of protozoan infections and to support sustainable aquaculture development in freshwater ecosystems.
Cercozoa = Filosa (Rhizaria, SAR) is one of the largest rhizarian subgroups and consists of a diverse assemblage of amoeboid and flagellated protists. They are ecologically significant in microbial food webs, widely diverse, and even abundant in soils and deep marine sediments according to environmental sequencing. In spite of this, the cercozoan phylogeny remains poorly resolved by SSU rRNA gene analysis, and omics data are available for only a few well-characterized species. Here, we have sequenced the transcriptomes of three new gliding monadofilosan strains: the glissomonad RAM19S6, the marimonad CRO19P5, and the discocelid GT001. Because of its unusual morphology, we performed a thorough morphological characterization of the strain GT001 using light and electron microscopy and described a new species, Discocelia plataet sp. n. Transmission electron microscopy and expansion microscopy revealed the structure of the flagellar apparatus, allowing us to identify cercozoan microtubular root homologies and supplement our knowledge of the discocelid cell structure with new details. Unique features of the new species are the absence of body tip and velum tip, discoidal mitochondrial cristae, and presence of an acronema on the posterior flagellum. We discuss the phylogenetic position of the three strains within Monadofilosa and the evolutionary context of the order Discocelida.
The dinoflagellate order Prorocentrales comprises two genera: Prorocentrum, which has been extensively studied, and the little-known Mesoporos. We established a monoclonal culture of Mesoporos perforatus from Long Island Sound, USA. The high variability in cell size (12-27 μm) and outline, together with irregular chloroplast morphology, suggests that previously described congeneric species may represent synonyms. The principal generic diagnostic character, commonly referred to as a pore, is not a true perforation but a conical depression located at the center of the theca; its function remains unknown. Cell morphology (cell size and shape, thecal ornamentation, and short apical spine) suggests affinities with P. cordatum and related species. However, a first detailed study of the periflagellar structure reveals differences such as platelet 8 separating platelets 2 and 3, a feature unreported in Prorocentrum species. Our first report of molecular data (SSU, LSU rDNA, and ITS regions) places M. perforatus as a species nested within Prorocentrum sensu stricto, with P. nux as the closest relative. Accordingly, we consider Mesoporos a synonym of Prorocentrum, and this species should be referred to as Prorocentrum perforatum (Gran) Krachmalny 1993, despite the absence of a true thecal perforation.
Antimicrobial resistance (AMR) is becoming a major problem in the treatment of infectious diseases, increasing the need for new antimicrobial agents. Although bacteria and fungi have been extensively explored in antimicrobial discovery, protozoa remain comparatively underexplored despite their biological diversity and metabolic capabilities. Recent studies have reported that several protozoan species, including Acanthamoeba, Dictyostelium, Naegleria, and Entamoeba, are capable of exhibiting antimicrobial activity, particularly against multidrug-resistant (MDR) bacteria and fungi. These activities have been associated with a range of bioactive molecules, such as pore-forming peptides, proteases, and secondary metabolites produced by protozoa. The reported antimicrobial effects involve multiple mechanisms, including disruption of membrane integrity, induction of oxidative stress, and inhibition of biofilm formation. Despite these observations, research on protozoa-derived antimicrobial compounds remains limited due to challenges related to molecular characterization, large-scale cultivation, and the availability of in vivo data. This review summarizes current knowledge on bioactive molecules produced by protozoa, with emphasis on their ecological origins, mechanisms of action, and potential relevance to antimicrobial drug discovery. These findings suggest that protozoa-derived bioactive compounds may contribute to the ongoing search for novel and natural antimicrobial agents and may represent a promising complementary resource in efforts to combat antimicrobial resistance.
Telonemia is a fascinating and understudied group of microbial eukaryotes known to have a vast diversity that is still uncharacterized. In fact, although their phylogenetic position is still actively debated, their diversity and biology are largely unexplored: to date, there are only seven described species in three genera, although there are estimated to be hundreds more unknown lineages. Here, we describe the isolation and characterization of two new strains, including a new genus (Hyaliora molinica n. gen. n. sp.) and a new species (Telonema blandense n. sp.), and the re-isolation of a previously characterized telonemid, Telonema subtile, accompanied by new behavioral observations. We present morphological measurements highlighting differences among the isolates and a phylogenetic tree incorporating their 18S rRNA gene sequences. Furthermore, key aspects of their cell biology and structure are highlighted to provide insights into the evolution of their potential sister groups. Since they are relevant not only phylogenetically but also play a crucial role in food webs with some abundant representatives in aquatic ecosystems, the findings of this study provide further sampling and culturing of Telonemia to increase the knowledge of the hidden diversity and evolution of this mysterious group.
Nanoplanktonic diatoms (2-20 μm) are a significant yet historically understudied component of marine ecosystems. We investigated three recently isolated nanoplanktonic diatoms from the Northwest Atlantic Ocean (NWA): Minidiscus spinulatus, Mediolabrus comicus, and Minidiscus trioculatus. Using Oxford Nanopore sequencing, we assembled and annotated their complete chloroplast and mitochondrial genomes. Pangenome analyses revealed that Minidiscus species consistently clustered more closely with select Thalassiosira species, whereas M. comicus formed a sister clade with Skeletonema. Circularized chloroplast genomes allowed us to characterize the full-length 16S ribosomal RNAs for each isolate, thereby leading to higher resolution of these taxa in preexisting 16S metabarcoding data. During our study, M. spinulatus was primarily restricted to the Bedford Basin. In contrast, both M. trioculatus and M. comicus had larger geographic ranges extending to the Labrador Sea, and in the case of M. comicus, to the Canadian Arctic Gateway. Weekly metabarcoding from the coastal Bedford Basin, N.S., Canada (2014-2022), revealed a seasonal succession of nanoplanktonic taxa, with Minidiscus trioculatus dominating in the early months, followed by M. comicus and M. spinulatus. Our results highlight the critical value of phytoplankton isolations and organelle genomics for expanding our understanding of the diversity and biogeography of nanoplanktonic diatoms.
Cryptomonad protists are ubiquitously distributed over marine and freshwater habitats. As an exception to the colored cryptomonads, the heterotrophic cryptomonads of the genus Goniomonas have an ancestral phylogenetic position. They lack any kind of chloroplast and most likely represent a basal group to those cryptomonad groups having obtained their chloroplast by secondary endosymbiosis. Earlier studies have shown a deep divergence between freshwater and marine clades of goniomonads that comprise large genetic distances between members within the group and also between the two groups of marine and freshwater taxa. Still, marine and freshwater species carry the same genus name, and to date, only a few species have been described. We therefore restructured goniomonad systematics based not only on a separation of marine and freshwater taxa, but also, taking the large genetic distances into account, on several new genera that are described. Based on morphological as well as phylogenetic data (18S rDNA sequences), this leads to the formation of the freshwater genera Limnogoniomonas n. g., Goniomonas, and Aquagoniomonas n. g. and the marine genera Neptunogoniomonas n. g., Baltigoniomonas n. g., Marigoniomonas n. g., Thalassogoniomonas n. g., Poseidogoniomonas n. g., and Cosmogoniomonas n. g. To give the restructuring process a stable basis, we additionally propose a neotype for Goniomonas truncata.
Phosphorus (P) is a chemical element essential for life that can drive dynamics of microorganisms in marine environments. Nutrient acquisition by parasitic organisms is mainly done through infection. In the case of micro-parasites, how host diet limitations affect parasite stoichiometry and parasite-host interaction has not been thoroughly studied yet. This study focused on the free-living stage of Amoebophrya ceratii, a micro-parasite (Syndiniales, Dinoflagellates) of marine dinoflagellates. It aims to examine how host P limitation prior to infection affects parasite stoichiometry and infection cycle. To this end, Scrippsiella acuminata was used as a host and cultivated under P-replete or P-limiting conditions. Infection and parasite production were monitored, and elemental quotas and stoichiometry of the freshly produced dinospores were determined. In P-replete conditions, elemental quotas and ratios of the parasites were similar to their host, showing a strong need for P. While the host's stoichiometry was greatly impacted by P limitation, parasites were able to maintain stoichiometric homeostasis and elemental cell quotas. Finally, host P limitation mainly affected the dinospore production, exhibiting a trade-off between dinospores quotas, stoichiometric homeostasis, and production. These findings have implications for host-parasite interactions and biogeochemical cycling in coastal waters.
Rhizarians form an important component in ocean biogeochemistry, used as paleoproxies and significantly contribute to marine biodiversity. These organisms, which are fragile in nature, are poorly studied and understood. A rhizarian species, Litharachnium eupilium (Haeckel 1887) Takahashi 1991 (syn. Sethophormis eupilium Haeckel 1887), was recorded for the first time in the Indian Ocean in plankton samples of central Arabian Sea from 25 to 110 m depth. Another species Litharachnium tentorium (Haeckel 1862) was observed in samples collected from four different stations in multiple depth ranges (upto 1500 m). Additionally, L. tentorium was observed from the plankton samples collected from the Andaman Sea. Studies being scarce on Rhizarians in the Indian Ocean region, this work focuses to expand the knowledge on these protists in this region, which will be useful for future studies to understand their contribution to the biogeochemistry. Study expanding the known geographic range can help understand the factors controlling their distribution and abundance in the world oceans and their fate in a changing environment.
Free-living amoebae (FLA) such as Naegleria and Acanthamoeba are opportunistic pathogens increasingly linked to fatal and severe human infections, particularly in settings with limited hygiene, water sanitation, and diagnostic infrastructure. In this pilot study, we explored the diversity of potentially pathogenic FLA in sectors of the Kakum River Basin near Cape Coast, Ghana, using a combined approach of filtered, pelleted, and culture-enriched metabarcoding. Our results revealed a wide range of FLA from Amoebozoa and Heterolobosea clades, including several of clinical relevance, such as Acanthamoeba, Vermamoeba, Balamuthia, and Paravahlkampfia. Importantly, various FLA, including Naegleria and most of the opportunistic amoebae species, were also detected and morphologically confirmed. This raises public health concerns, given the favorable environmental conditions for their proliferation in the sampling sites. The use of culture-enriched metabarcoding was particularly valuable in recovering organisms that may encyst and be missed by direct methods. This study underscores the importance of integrative and sensitive molecular approaches for detecting neglected pathogens in vulnerable communities. Our findings provide a foundation for larger epidemiological studies that incorporate clinical data and support the development of cost-effective monitoring tools for FLA-associated public health threats in sub-Saharan Africa.
This study employs advanced three-dimensional electron microscopy techniques, including Transmission Electron Microscopy (TEM) tomography and freeze-fracture imaging via scanning electron microscopy (SEM), to investigate the ultrastructural organization of Cyrilia lignieresi-infected red blood cells (iRBCs) in the host fish Synbranchus marmoratus. The analysis focuses on the parasitophorous vacuole (PV) and reveals a highly complex intravacuolar membranous network (IVN) composed of vesicles, tubules, and interconnected membranous structures. These elements exhibit considerable diversity in size, morphology, and electron density, suggesting dynamic functional roles in the parasite-host interaction. The electron tomography and three-dimensional reconstructions data provide unprecedented insights into the spatial organization and potential functional significance of these membranous systems. These findings not only enhance our understanding of the cellular adaptations of C. lignieresi but also contribute to a broader knowledge of apicomplexan parasitism and host-pathogen interactions.
Copepods, dominant marine zooplankton, are hosts to microbial eukaryotic symbionts, but the copepod eukaryome remains largely unexplored. We used 18S rRNA gene primers with reduced metazoan amplification to identify microbial eukaryotes in a culture of Calanus finmarchicus (Copepoda). Samples were taken from the inlet water (99.5% of reads from non-copepod sources) and the contents of the culture, which included ambient water (99.7%), bulk (many crushed copepods, 60.2%), individual copepods (1%-41%, mean = 7.4), and bulk fecal pellets (74%). The microbial eukaryotic community in the culture differed from the inlet water. The culture contained saprotrophs and bacterivores typical of eutrophic aquacultures and known parasites of copepods. Individual copepod eukaryomes varied in richness (8-33 operational taxonomic units, mean = 16.1) and revealed variation in non-copepod read yields related to specific taxa. Perkinsea, not previously reported in copepods, as well as Ascomycota and Basidiomycota (Fungi), formed the core eukaryome (found in > 90% of individuals), indicating potentially important symbiosis. The small eukaryome, relative to reported microbiomes in C. finmarchicus, suggests that ecological inferences from microbiomes, which largely address bacteria, are not readily applicable to the eukaryotic microbes. The study underpins the need for investigations of eukaryomes.