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Marine protist diversity inventories have largely focused on planktonic environments, while benthic protists have received relatively little attention. We therefore hypothesize that current diversity surveys have only skimmed the surface of protist diversity in marine sediments, which may harbor greater diversity than planktonic environments. We tested this by analyzing sequences of the hypervariable V4 18S rRNA from benthic and planktonic protist communities sampled in European coastal regions. Despite a similar number of OTUs in both realms, richness estimations indicated that we recovered at least 70% of the diversity in planktonic protist communities, but only 33% in benthic communities. There was also little overlap of OTUs between planktonic and benthic communities, as well as between separate benthic communities. We argue that these patterns reflect the heterogeneity and diversity of benthic habitats. A comparison of all OTUs against the Protist Ribosomal Reference database showed that a higher proportion of benthic than planktonic protist diversity is missing from public databases; similar results were obtained by comparing all OTUs against environmental references from NCBI's Short Read Archive. We suggest that the benthic realm may therefore be the world's largest reservoir of marine protist diversity, with most taxa at present undescribed. Sampling RNA in near-shore marine environments along the European coast shows that benthic protist communities are extremely diverse with many species currently unknown. Graphical Abstract Figure. Sampling RNA in near-shore marine environments along the European coast shows that benthic protist communities are extremely diverse with many species currently unknown.

Toxoplasmosis is a disease of worldwide distribution, causing high morbidity and mortality in humans, as well as heavily impacting animal health and the economy. Toxoplasma gondii, the causative agent, is an intracellular parasite with a complex life cycle whose completion entails asexual, pre-sexual, and sexual stage conversions. Pre-sexual and sexual differentiation take place only within the intestinal epithelium of felines. Recently, several transcriptional factors and epigenetic components crucial to trigger parasite stage transitions within the cat have been identified, allowing, through precise genetic manipulation, obtaining pre-sexual stages known as merozoites in vitro. Through conditional depletion of two pre-sexual stage-specific gene silencing transcription factors, AP2XII-1 and AP2XII-2, we have characterized the interplay between cell division and the sequence of events leading up to differentiation of tachyzoites into merozoites. We explored genome duplication, assembly of daughter cells, karyokinesis, and cytokinesis, characterizing the differential cell division modes and kinetics undergone by critical structures along the differentiation axis. Building onto the pre-existing body of knowledge, primarily describing the underpinnings of these forms of division by transmission electron microscopy, our work contributes previously unexplored temporal and spatial resolution to the transitions between endodyogeny and endopolygeny, providing a conceptual framework for understanding and exploring T. gondii’s route of sexual differentiation.IMPORTANCESexual development in Toxoplasma gondii is essential for transmission, but remains poorly understood, largely because pre-sexual stages are restricted to the feline intestine and have only recently become experimentally accessible. Here, we leverage an in vitro differentiation system to resolve how parasites transition toward merozoite formation at the cellular level. By combining expansion microscopy, stage-specific markers, and quantitative analyses, we define the temporal sequence of nuclear division and daughter cell assembly during merogony, addressing longstanding ambiguity regarding division modes in these stages. Our findings reveal that parasites can adopt alternative division strategies emerging from a polyploid intermediate, highlighting an unexpected degree of flexibility in how cell division is executed during differentiation. Beyond refining this developmental framework, this work establishes a foundation for future mechanistic studies of pre-sexual biology and provides broader insight into the diversity of eukaryotic cell division strategies.

Heterotrophic soil protists encompass lineages that are both evolutionarily ancient and highly diverse, providing an untapped wealth of scientific insight. Yet the diversity of free-living heterotrophic terrestrial protists is still largely unknown. To contribute to our understanding of this diversity, we present a checklist of heterotrophic protists currently reported from terrestrial Antarctica, for which no comprehensive evaluation currently exists. As a polar continent, Antarctica is especially susceptible to rising temperatures caused by anthropogenic climate change. Establishing a baseline for future conservation efforts of Antarctic protists is therefore important. We performed a literature search and found 236 taxa identified to species and an additional 303 taxa identified to higher taxonomic levels in 54 studies spanning over 100 years of research. Isolated by distance, climate and the circumpolar vortex, Antarctica is the most extreme continent on Earth: it is not unreasonable to think that it may host physiologically and evolutionarily unique species of protists, yet currently most species discovered in Antarctica are considered cosmopolitan. Additional sampling of the more extreme intra-continental zones will probably result in the discovery of more novel and unique taxa.

Phagotrophic protists (formerly protozoa) are a highly diverse, polyphyletic grouping of generally unicellular, heterotrophic eukaryotes that are key regulators of the soil microbiome. The biodiversity and ecology of soil phagotrophic protists are still largely uncharacterized, especially in the Antarctic, which possesses some of the harshest terrestrial environments known and potentially many physiologically unique and scientifically interesting species. Antarctic soil systems are also highly limited in terms of moisture, temperature, and carbon, and the resulting reduced biological complexity can facilitate fine-tuned investigation of the drivers and functioning of microbial communities. To facilitate and encourage future research into protist biodiversity and ecology, especially in context of the broader functioning of Antarctic terrestrial communities, I review the biodiversity, distribution, and ecology of Antarctic soil phagotrophic protists. Biodiversity appears to be highly structured by region and taxonomic group, with the Antarctic Peninsula having the highest taxonomic diversity and ciliates (Ciliophora) being the most diverse taxonomic group. However, richness estimates are likely skewed by disproportionate sampling (over half of the studies are from the peninsula), habitat type bias (predominately moss-associated soils), investigator bias (toward ciliates and the testate amoeba morphogroup), and methodological approach (toward cultivation and morphological identification). To remedy these biases, a standardized methodology using both morphological and molecular identification and increased emphasis on microflagellate and naked amoeba morphogroups is needed. Additionally, future research should transition away from biodiversity survey studies to dedicated ecological studies that emphasize the function, ecophysiology, endemicity, dispersal, and impact of abiotic drivers beyond moisture and temperature.

Domoic acid (DA) is a potent neurotoxin produced by marine micro- and macroalgae problematic to fisheries and toxic to humans and animals. Our study elucidates the molecular mechanisms underlying DA production in the widespread Western Pacific benthic diatom, Nitzschia navis-varingica . Genomic and biochemical insights add information to our understanding of the evolution of toxin production across diverse phyla and also fill a gap in the knowledge of secondary metabolism in marine diatoms. These findings provide a genetic framework for identifying toxin production and its impacts in the benthos of vulnerable, coastal ecosystems.

The plant-like organelle named apicoplast is essential for malaria parasites and is a major antimalarial drug target. For more than a decade, scientists have believed that malaria parasites in the blood stages could dispense with the apicoplast if they were supplied with a critical metabolite made by the organelle, leading to the idea of “apicoplast-minus” parasites. Our results challenge this long-standing view. We find that even when the apicoplast is disrupted, the organelle remains in a highly reduced form. This apicoplast-derived organelle is inherited as parasites continue their life cycles, suggesting that it contains essential functions even when the organelle is disrupted. Our data reveal an unexpected level of complexity in apicoplast biology and open new doors for future identification of essential apicoplast-derived pathways that cannot be easily bypassed.

The Brazilian Atlantic Forest and its associated ecosystems are highly biodiverse but still understudied, especially with respect to eukaryotic microbes. Protists represent the largest proportion of eukaryotic diversity and play important roles in nutrient cycling and maintenance of the ecosystems in which they occur. However, much of protist diversity remains unknown, particularly in the Neotropics. Understanding the taxonomic and functional diversity of these organisms is urgently needed, not only to fill this gap in our knowledge, but also to enable the development of public policies for biological conservation. This is the first study to investigate the taxonomic and trophic diversity of the major protist groups in freshwater systems and brackish coastal lagoons located in fragments of the Brazilian Atlantic Forest by DNA metabarcoding, using high-throughput sequencing of the gene coding for the V4 region of the 18S rRNA gene. We compared α and β diversity for all protist communities and assessed the relative abundance of phototrophic, consumer, and parasitic taxa. We found that the protist communities of coastal lagoons are as diverse as the freshwater systems studied in terms of α diversity, although differed significantly in terms of taxonomic composition. Our results still showed a notable functional homogeneity between the trophic groups in freshwater environments. Beta diversity was higher among freshwater samples, suggesting a greater level of heterogeneity within this group of samples concerning the composition and abundance of OTUs.Ciliophora was the most represented group in freshwater, while Diatomea dominated diversity in coastal lagoons.

Trichomonas vaginalis , the most common non-viral sexually transmitted parasite, relies on adherence to host epithelial cells to establish infection. Our previous work highlighted the importance of N6-methyladenine (6mA) DNA methylation in the regulation of transcription and three-dimensional chromatin structure. Now, our study integrates RNA-seq, MeDIP-seq, and assay for transposase-accessible chromatin sequencing data to reveal how 6mA and chromatin accessibility modulate gene expression during T. vaginalis interaction with human host cells. We identified over 3,600 differentially expressed genes upon parasite contact with prostate cells, including pathogenesis-related genes. Moreover, we identified transcriptionally active and repressive regions flanked by 6mA that remain largely stable during the process of host interaction. We mapped genome-wide chromatin accessibility and uncovered differentially accessible regions upon host cell contact associated with a subset of genes involved in adhesion. These results suggest that local chromatin accessibility has a major role in modulating gene expression of key virulence genes during host interaction.

The ancient uptake and transformation of free-living bacteria into eukaryotic organelles involved extensive structural, physiological, and genetic changes. More recently established endosymbioses offer a unique opportunity to observe intermediate stages in the complex process by which a prokaryote becomes genetically integrated into a eukaryotic cell. Hence, studying the molecular mechanisms that govern host-endosymbiont interactions holds the potential for uncovering the scenarios and molecular processes behind organelle formation. The trypanosomatid Angomonas deanei has been recently reported to manifest nuclear control over its endosymbiont’s division. In this study, we identified and characterized a new nucleus-encoded component of the endosymbiont division machinery. This study further supports that a novel intermediate between endosymbiont and organelle evolved in A. deanei and provides new leverage to entangle the evolution of its fascinating nucleus-controlled endosymbiont division machinery.

Intestinal protists represent an underappreciated yet functionally significant component of the human gut microbiome. Historically dismissed as parasites or transient contaminants, many of these microbial eukaryotes, particularly spp., are now recognized as integral to gut ecosystem function and host immune homeostasis. This review examines the complex roles of spp. in the mammalian gut, positioning them as dynamic microbiome \"landscapers\" that influence host-pathogen interactions, immune tone, and microbial diversity. We explore the evolutionary adaptation of to the gut's anaerobic and immunologically active environment, highlighting both pathogenic ( ) and non-pathogenic species ( , ) and their distinct immunomodulatory strategies. Special attention is given to the host immune responses shaped by , including inflammasome activation, macrophage polarization, and suppression of protective type-2 responses. The review also details 's interactions with the gut microbiota, emphasizing their capacity for selective bacterial predation, disruption or enhancement of microbial community structure, and synergistic or antagonistic relationships with commensals and pathogens alike. Methodological challenges in protist detection, genome annotation, and cultivation are discussed, alongside promising advances in sequencing, host DNA depletion, and animal modeling. Taken together, current evidence reframes spp. not as mere pathogens but as key ecological players whose presence can signal resilience or susceptibility within the gut ecosystem. Understanding the context-dependent functions of intestinal protists may offer new insights into microbial therapeutics, immune modulation, and disease prevention strategies.