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Thraustochytrium aureum ssp. strugatskii, a marine protist belonging to the class Labyrinthulea, exhibits a complex life cycle characterized by alternating motile and vegetative phases. Using an integrative multimodal microscopy approach, we reconstructed its full developmental cycle and analyzed the coordination between cellular morphology, subcellular architecture, and population-level behavior. Transmission and scanning electron microscopy, combined with fluorescence and time-lapse imaging, revealed the dynamics of nuclear division, organelle rearrangement, and zoospore formation. Morphometric analysis of serial ultrathin sections demonstrated distinct changes in mitochondrial distribution, Golgi apparatus, and lipid droplet abundance during transitions between stages. We have shown that vegetative cells undergo synchronized karyokinesis coupled with stable nuclear-to-cytoplasmic ratios, leading to the emergence of multinucleate stages prior to zoospore formation. The integration of ultrastructural and dynamic data enabled us to propose a systems-level model linking metabolic state, morphogenesis, and population structure. This model highlights feedback regulation between nutrient availability, biomass accumulation, and developmental synchronization. Our results establish that T. aureum ssp. strugatskii has good potential to serve as a tractable model organism for systems-level studies of protists and provide an initial framework for predictive modeling of its life cycle under controlled conditions.

The thraustochytrids are a group of marine protists known for their significant ecological roles as decomposers and parasites as well as for their potential biotechnological applications, yet their evolutionary and structural diversity remains poorly understood. Our review critically examines the phylogeny of this taxa, utilizing available up-to-date knowledge and their taxonomic classifications. Additionally, advanced imaging techniques, including electron microscopy, are employed to explore the ultrastructural characteristics of these organisms, revealing key features that contribute to their adaptive capabilities in varying marine environments. The integration of this knowledge with available omics data highlights the huge biotechnological potential of thraustochytrids, particularly in producing ω-3 fatty acids and other bioactive compounds. Our review underscores the importance of a systems biology approach in understanding thraustochytrids biology and highlights the urgent need for novel, accurate omics research to unlock their full biotechnological potential. Overall, this review aims to foster a deeper appreciation of thraustochytrids by synthesizing information on their evolution, ultrastructure, and practical applications, thereby providing a foundation for future studies in microbiology and biotechnology.

Eicosapentaenoic acid (EPA) from freeze-dried biomass of Nannochloropsis oceanica microalgae resists ruminal biohydrogenation in vitro, but in vivo demonstration is needed. Therefore, the present study was designed to test the rumen protective effects of N. oceanica in lambs. Twenty-eight lambs were assigned to one of four diets: Control (C); and C diets supplemented with: 1.2% Nannochloropsis sp. oil (O); 12.3% spray-dried N. oceanica (SD); or 9.2% N. oceanica (FD), to achieve 3 g EPA /kg dry matter. Lambs were slaughtered after 3 weeks and digestive contents and ruminal wall samples were collected. EPA concentration in the rumen of lambs fed FD was about 50% higher than lambs fed SD or O diets. Nevertheless, the high levels of EPA in cecum and faeces of animals fed N. oceanica biomass, independently of the drying method, suggests that EPA was not completely released and absorbed in the small intestine. Furthermore, supplementation with EPA sources also affected the ruminal biohydrogenation of C18 fatty acids, mitigating the shift from the t 10 biohydrogenation pathways to the t 11 pathways compared to the Control diet. Overall, our results demonstrate that FD N. oceanica biomass is a natural rumen-protected source of EPA to ruminants.

The recently discovered Provora supergroup has primarily been examined to determine their phylogenomic position in the eukaryotic tree. Their morphology is more poorly studied, and here we focus on their cellular organization and how it compares with that of other supergroups. These small eukaryovorous flagellates exhibit several ultrastructural features that are also found in a subset of taxa from a wide variety of deep-branching lineages (Stramenopiles, Alveolata, Hemimastigophora, Malawimonadidae, Discoba and Metamonada), including vesicles beneath the plasmalemma, two opposing vanes on the flagella, a ventral feeding groove and a fibrillar system resembling the excavate type. Additionally, we identified four main microtubular roots (r1–r4) and a singlet root between r1 and r2, which support the strong feeding apparatus resembling ‘jaws’. Their unique extrusive organelles (ampulosomes) have a similar organization to Hemimastigophora extrusomes, but most of their cell characteristics most closely resemble features of the TSAR + Haptista grouping. We also describe a new species, Nibbleromonas piranha sp. nov., and highlight features of its feeding behaviour, which can be so aggressive as to result in cannibalism.

Improved methods for the extraction of eicosapentaenoic acid (EPA), an essential and economically important polyunsaturated fatty acid, are urgently required. However, lipid extraction rates using food-grade solvents such as ethanol are usually low. To improve the ethanol-based extraction rate, and to elucidate the relevant mechanisms, we used cellulase and laccase to treat powdered Nannochloropsis, one of the most promising microalgal sources of EPA. Cellulase and laccase synergistically increased lipid yields by 69.31% and lipid EPA content by 42.63%, by degrading the amorphous hemicellulose and cellulose, improving crystallinity, and promoting the release and extraction of lysodiacylglyceryltrimethylhomoserine. Scanning electron microscopy showed that cell morphology was substantially altered, with cell-wall rupture, loss of cell boundaries, and the release of intracellular substances. In conclusion, Nannochloropsis lipid yields may be directly linked to cell-wall hemicellulose structure, and enzymatic treatment to alter this may improve lipid yields.

Chlorella vulgaris, is a kind of single-celled green algae, which could serve as a potential source of food and energy because of its photosynthetic efficiency. In our study, a pathogenic organism targeting C. vulgaris was discovered. The algae-lytic activity relates to a fraction from lysates of infected C. vulgaris that was blocked upon filtration through a 3 µm filter. 18S rRNA gene sequence analysis revealed that it shared 99.0% homology with the protist Pseudobodo tremulans. Scanning electron microscope analysis showed that Pseudobodo sp. KD51 cells were approximately 4-5 µm long, biflagellate with an anterior collar around the anterior part of the cell in unstressed feeding cells. Besides the initial host, Pseudobodo sp. KD51 could also kill other algae, indicating its relatively wide predatory spectrum. Heat stability, pH and salinity tolerance experiments were conducted to understand their effects on its predatory activities, and the results showed that Pseudobodo sp. KD51 was heat-sensitive, and pH and salinity tolerant.

Thraustochytrids are marine protists that naturally accumulate triacylglycerol with long chains of polyunsaturated fatty acids, such as ω3-docosahexaenoic acid (DHA). They represent a sustainable response to the increasing demand for these “essential” fatty acids (FAs). Following an attempt to transform a strain of Aurantiochytrium limacinum, we serendipitously isolated a clone that did not incorporate any recombinant DNA but contained two to three times more DHA than the original strain. Metabolic analyses indicated a deficit in FA catabolism. However, whole transcriptome analysis did not show down-regulation of genes involved in FA catabolism. Genome sequencing revealed extensive DNA deletion in one allele encoding a putative peroxisomal adenylate transporter. Phylogenetic analyses and yeast complementation experiments confirmed the gene as a peroxisomal adenylate nucleotide transporter (AlANT1), homologous to yeast ScANT1 and plant peroxisomal adenylate nucleotide carrier AtPNC genes. In yeast and plants, a deletion of the peroxisomal adenylate transporter inhibits FA breakdown and induces FA accumulation, a phenotype similar to that described here. In response to this metabolic event, several compensatory mechanisms were observed. In particular, genes involved in FA biosynthesis were upregulated, also contributing to the high FA accumulation. These results support AlANT1 as a promising target for enhancing DHA production in Thraustochytrids.

This study investigated the taxonomic affiliation of the algal strain nak-9, which has been reported to absorb radioactive cesium with high efficiency, using light and electron microscopy, and molecular phylogenetic analysis based on 18S ribosomal RNA gene (rDNA) sequences. This alga is spherical and coccoid, with a smooth cell wall, large vacuole, crystalline structure, reddish globule, and refractile granules (lamellate vesicles). The cells possess one to several greenish parietal chloroplasts with a bulging pyrenoid surrounded by lamellate vesicles. The chloroplasts include orderly thylakoid lamellae but no girdle lamella. Molecular phylogenetic analysis suggests that strain nak-9 is a member of the eustigmatophycean clade, which includes Goniochloris, Pseudostaurastrum, and Trachydiscus. On the basis of these results, we propose that strain nak-9 (NIES-2860) comprises a new species and new genus of the Eustigmatophyceae, Vacuoliviride crystalliferum gen. et sp. nov.

Thraustochytrids are single cell marine eukaryotes that produce large amounts of polyunsaturated fatty acids such as docosahexaenoic acid. In the present study, we report the visualization of endoplasmic reticulum (ER) and mitochondria in a type strain of the thraustochytrid, Aurantiochytrium limacinum ATCC MYA-1381, using the enhanced green fluorescent protein (EGFP) with specific targeting/retaining signals. We expressed the egfp gene with ER targeting/retaining signals from A. limacinum calreticulin or BiP/GRP78 in the thraustochytrid, resulting in the distribution of EGFP signals at the perinuclear region and near lipid droplets. ER-Tracker™ Red, an authentic fluorescent probe for the visualization of ER in mammalian cells, also stained the same region. We observed small lipid droplets generated from the visualized ER in the early growth phase of cell culture. Expression of the egfp gene with the mitochondria targeting signal from A. limacinum cytochrome c oxidase resulted in the localization of EGFP near the plasma membrane. The distribution of EGFP signals coincided with that of MitoTracker® Red CMXRos, which is used to visualize mitochondria in eukaryotes. The ER and mitochondria of A. limacinum were visualized for the first time by EGFP with thraustochytrid cell organelle-specific targeting/retaining signals. These results will contribute to classification of the intracellular localization of proteins expressed in ER and mitochondria as well as analyses of these cell organelles in thraustochytrids.

Most swarmers (swimming cells) of the stramenopile group, ranging from unicellular protist to giant kelps (brown algae), have two heterogeneous flagella: a long anterior flagellum (AF) and a relatively shorter posterior flagellum (PF). These flagellated cells often exhibit phototaxis upon light stimulation, although the mechanism by which how the phototactic response is regulated remains largely unknown. A flavoprotein concentrating at the paraflagellar body (PFB) on the basal part of the PF, which can emit green autofluorescence under blue light irradiance, has been proposed as a possible blue light photoreceptor for brown algal phototaxis although the nature of the flavoprotein still remains elusive. Recently, we identified helmchrome as a PF-specific flavoprotein protein in a LC-MS/MS-based proteomics study of brown algal flagella (Fu et al. 2014). To verify the conservation of helmchrome, in the present study, the absence or presence and the localization of helmchrome in swarmers of various algal species were investigated. The results showed that helmchrome was only detected in phototactic swarmers but not the non-phototactic ones of the stramenopile group. Electron microscopy further revealed that the helmchrome detectable swarmers bear a conserved PFB-eyespot complex, which may serve as structural basis for light sensing. It is speculated that all three conserved properties: helmchrome, the PFB structure, and the eyespot apparatus, will be essential parts for phototaxis of stramenopile swarmers.