Journal of Eukaryotic Microbiology最新文献

The oxymonads are anaerobic protist symbionts of animals with spectacular morphological complexity and variability, but because only a handful of species have been cultured, very little data are available for the group. This includes molecular data, with only a couple of species having sequenced genomes or transcriptomes. Oxymonads are divided into five families in a classification that has remained more or less unchanged for 35 years, but the relationships of these families are unknown due to a lack of molecular data from three of them. Here, we used single cell transcriptomics on six species from five genera (Oxymonas, Streblomastix, Pyrsonympha, Saccinobaculus, and Laeohelix) to carry out the first phylogenomic analysis of oxymonads that includes all five families. We find a major division between Polymastigidae and Streblomastigidae on one side, and Saccinobaculidae, Oxymonadidae, and Pyrsonymphidae on the other, with Oxymonadidae and Pyrsonymphidae branching as sisters in the latter clade. The phylogenomic tree largely confirms the SSU rRNA tree but provides stronger support at all nodes and suggests a different root. This tree represents a strong starting point to reconstruct the evolution of oxymonads many unusual morphological and molecular characters and to test whether new lineages might represent new families.

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.

Euglenids are a diverse group of flagellated protists that include phagotrophic, osmotrophic, and phototrophic lineages. Understanding the phylogenetic relationships of phagotrophic euglenids is crucial in understanding euglenid evolution. Yet many relationships within euglenids remain unclear, and further resolution requires extensive sampling, particularly from the deep-branching, paraphyletic group known as "ploeotids". Improved resolution of evolutionary relationships among ploeotid taxa is necessary to elucidate the origin and diversification of complex ultrastructural traits (e.g., pellicle and feeding apparatus). Here, we isolated, cultivated and characterized a novel freshwater "ploeotid" species named Hokulea waialensis n. gen. et sp. using light and scanning electron microscopy, single-cell sequencing, and phylogenomic analyses. This new species is relatively small (~10 μm long) compared to related euglenids and shares several morphological traits with related species of Alistosa. Both single and multigene phylogenetic reconstructions from single amplified genome data show that Hokulea waialensis n. gen. et sp. is closely related to several environmental small subunit ribosomal DNA (SSU rDNA) sequences, and more broadly to Lentomonas and Decastava.

Plastids originated through endosymbiosis and enabled the emergence of photosynthetic eukaryotes. However, in several eukaryotic lineages, plastids are not permanently integrated but are instead acquired transiently through predation, a phenomenon known as kleptoplasty. Although kleptoplasty has been documented across diverse eukaryotic groups, the extent to which prey identity constrains plastid acquisition and subsequent maintenance remains poorly defined. In particular, it is unclear whether kleptoplastic organisms exploit plastid donors broadly or depend on highly selective host-prey interactions. Here, we investigated prey responses and feeding behavior in the kleptoplastic euglenoid Rapaza viridis using a phylogenetically diverse set of Tetraselmis species. Through comparative analysis of ingestion behavior, behavioral interactions, and postingestion chloroplast fate among candidate chloroplast donors, we show that R. viridis exhibits strong prey specificity at the species level. Although R. viridis has been shown to depend on Tetraselmis as a source of kleptoplasts, our results demonstrate that ingestion, functional retention, and growth support are restricted to species belonging to the subgenus Tetraselmis. These findings indicate that chloroplast acquisition in R. viridis is constrained by fine-scale host-prey compatibility, rather than by prey availability alone, and highlight the importance of selective prey recognition and post-ingestion processing in regulating kleptoplasty.

Chrysophyceae (Stramenopiles) are a diverse group of protists widely distributed in various aquatic habitats, including alpine lakes, where they play key ecological roles. Their nutritional modes-phototrophy, heterotrophy, and mixotrophy-enable them to adapt to the unique light and temperature conditions found across altitudinal gradients in mountain environments. This study investigates the growth responses of three mixotrophic and three phototrophic chrysophyte strains, isolated from alpine and pre-alpine lakes, to varying light intensities and temperatures. Our results show that both temperature and light intensity exert strong, species-specific effects on the growth of phototrophic and mixotrophic Chrysophyceae. Despite their contrasting original habitats, most strains shared similar growth optima, with peak performance generally observed between 15°C and 19°C and reduced growth at both lower and higher temperatures. All strains exhibited negative growth in darkness, confirming their phototrophic dependence. Growth rates typically increased with light availability up to a species-specific optimum (12-35 μE m^-2 s^-1), beyond which either a plateau or a decline was observed. Despite environmental differences among their habitats of origin, temperature and light optima were remarkably consistent across strains.

Relatively large Acanthocystis turfacea Carter 1863 are the type species of the centrohelid genus Acanthocystis Carter 1863, characterized by bifurcated spine scales. In this study, several isolates and strains identified as A. turfacea are re-evaluated based on morphological, morphometric, and molecular evidence. The Len strain corresponds to A. turfacea sensu stricto, while the Luga and Kos isolates, previously considered A. turfacea paucilituatus Nicholls 2023, are elevated to species level as Acanthocystis paucilituatus n. stat. A distinct Sin strain, with a smaller cell diameter and lacking symbiotic algae, is described here as a new species, Acanthocystis ladogensis n. sp. Molecular phylogenetic analysis based on 18S rRNA gene sequences confirms that these taxa form a well-supported clade within the genus Acanthocystis, and supports the hypothesis that bifurcations at the distal ends of spine scales represent an ancestral feature. Observations under long-term culture demonstrate that A. turfacea maintains a stable symbiotic association with symbiotic algae. The taxonomic status of the A. turfacea species complex and correlations between molecular and structural data are discussed.

Freshwater and sediment environments host diverse microeukaryotic communities that differ in structure and composition, yet exhibit taxonomic overlap. Using 18S V9 rRNA gene sequencing, we compared communities from these habitats in controlled 10-day mesocosm experiments, examining diversity, overlap, and responses to treated wastewater (TWW). Habitat type was the strongest determinant of community composition: sediments displayed higher diversity and greater temporal stability than freshwater communities. While many taxa were shared, highly dominant OTUs were mostly rather habitat-specific, whereas taxa occurring evenly across both habitats were generally rare. Distinct trophic structures further distinguished the habitats, with sediments showing relatively balanced assemblages of phototrophs, mixotrophs, consumers, and parasites, while freshwater communities were dominated by consumers. TWW exposure induced pronounced but transient changes in freshwater communities, including an initial increase in richness from allochthonous taxa, followed by partial convergence toward controls, whereas sediment communities remained largely unaffected. We identified 14 taxa associated with TWW, nine of which have not previously been linked to wastewater, highlighting their potential as bioindicators. Our findings reveal contrasting sensitivity and resilience of freshwater microeukaryotic habitats and emphasize the importance of integrating both water column and sediment communities in monitoring and assessing the ecological impacts of treated wastewater.

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.