Identification of euglenophycin--a toxin found in certain euglenoids

Ecology of freshwater harmful euglenophytes: A review

A diverse array of aquatic ecosystems are inhabited by the euglenophytes, a group of autotrophic and eukaryotic organisms. In inland waterbodies, the red bloom is caused by a rapid development or accumulation of euglenophytes. Recent studies have designated euglenophytes as bioindicator of organic pollution. The ecology of euglenophytes is influenced by the changes in the intensity of sunlight, temperature, nutrient cycles, and seasons. Most of the species of euglenophytes grow prolifically with the increase of water temperature. Nitrogen and phosphorus are often thought to be the main nutrients that influence the cellular growth of toxic euglenophytes. A high concentration of nutrients is required for the euglenophytes to grow and to form bloom. Heavy bloom of euglenophytes in the summer season is the characteristic of eutrophic ponds. Inland waterbodies in many countries suffer from euglenophyte blooms, which shade submerged vegetation, deplete the dissolved oxygen and disrupt the aquatic food webs. Dense bloom of euglenophytes clog the gills of fishes, cause breathing difficulties and in extreme cases results mortality. Red blooms of the deadly toxin producing Euglena sanguinea negatively affect the water quality resulting massive mortality of fishes. Consequently, aquaculture systems and fisheries are facing a serious threat from the predicted outbreak of toxic red blooms of euglenophytes worldwide. To ensure sustainability in the fisheries and aquaculture industry, it is essential to analyze the ecology of euglenophytes. Again, interesting research on euglenophycin, a Euglena-derived natural product, has shown that it can be utilized as a potential anti-cancer drug. This paper comes up with a thorough review of the latest research in this area, revealing new insights and solutions that can help mitigate the negative impact of the freshwater harmful euglenophytes. By implementing considerable management strategies, the health of the valuable aquatic ecosystems and the future of the aquaculture and fisheries can also be secured.

Gravity sedimentation of eukaryotic algae Euglena gracilis accelerated by ethanol cultivation

Euglena gracilis (E. gracilis) is a unicellular microalga with various applications in medicine, agriculture, aquaculture, health supplement, and jet fuel production. Euglena possibly solves population growth and exhaustion of fossil resources. Efficient cell harvesting is needed for the industry, and the gravity sedimentation method is low cost and does not require any equipment, although it has low efficiency. This study showed that the gravity sedimentation of E. gracilis cells is improved by cultivation in the presence of ethanol (EtOH). The gravity sedimentation of E. gracilis cells cultivated under 0.5% or 1.0% EtOH conditions was faster than that cultivated without EtOH. The mean calculated cell diameter was also found to be largest in cells cultivated under 0.5% or 1.0% EtOH conditions compared to that in cells cultivated without EtOH. Intracellular paramylon content, cell shapes, and motility differed between cells cultivated under 0.5% or 1.0% EtOH conditions and in the absence of EtOH. The results suggest that E. gracilis cultivation with EtOH leads to increased cell productivity, paramylon production, and efficient cell harvesting. KEY POINTS: • Euglena gracilis is an edible microalga producing value-added metabolites. • Ethanol addition upregulates E. gracilis growth and paramylon accumulation. • Gravity sedimentation is accelerated by ethanol-grown E. gracilis cells.

Euglenatides, Potent Antiproliferative Cyclic Peptides Isolated from the Freshwater Photosynthetic Microalga Euglena gracilis

By limiting the nitrogen source to glutamic acid, we isolated cyclic peptides from Euglena gracilis containing asparagine and non-proteinogenic amino acids. Structure elucidation was accomplished through spectroscopic methods, mass spectrometry and chemical degradation. The euglenatides potently inhibit pathogenic fungi and cancer cell lines e.g., euglenatide B exhibiting IC50 values of 4.3 μM in Aspergillus fumigatus and 0.29 μM in MCF-7 breast cancer cells. In an unprecedented convergence of non-ribosomal peptide synthetase and polyketide synthase assembly-line biosynthesis between unicellular species and the metazoan kingdom, euglenatides bear resemblance to nemamides from Caenorhabditis elegans and inhibited both producing organisms E. gracilis and C. elegans. By molecular network analysis, we detected over forty euglenatide-like metabolites in E. gracilis, E. sanguinea and E. mutabilis, suggesting an important biological role for these natural products.

Nuisance phytoplankton transport is enhanced by high flow in the main river for drinking water in Uruguay

Eutrophication, climate change, and river flow fragmentation are the main cause of nuisance algal blooms worldwide. This study evaluated the conditions that trigger the growth and occurrence of nuisance phytoplankton in the Santa Lucía River, a subtropical floodplain lotic system that supplies drinking water to 60% of the population of Uruguay. The main variables that explained phytoplankton biovolume were extracted from generalized linear models (GLM). The potential impact of nuisance organism advection on water utility was estimated by the phytoplankton biovolume transport (BV_TR, m³ day^-1), an indicator of biomass load. Santa Lucía River had a wide flow range (0.7×10⁵-1438×10⁵ m³ day^-1) and eutrophic conditions (median, TP: 0.139 mg L^-1; TN: 0.589 mg L^-1). GLMs indicated that phytoplankton biomass increased with temperature and soluble reactive phosphorus. Contrary to expectations, the presence of cyanobacteria was positively associated with periods of high flow that result in high cyanobacterial biovolume transport, with a probability of 3.35 times higher when flow increased by one standard deviation. The cyanobacterial biovolume transported (max: 9.5 m³ day^-1) suggests that biomass was subsidized by allochthonous inocula. Biovolume from other nuisance groups (diatoms, cryptophytes, and euglenophytes) was positively associated with low-flow conditions and high nutrient concentrations in the main river channel, thereby indicating that these conditions boost eukaryote blooms. The evaluation of BV_TR allows a better understanding of the dynamics of fluvial phytoplankton and can help to anticipate scenarios of nuisance species transport.

Toxic or Otherwise Harmful Algae and the Built Environment

This article gives a comprehensive overview on potentially harmful algae occurring in the built environment. Man-made structures provide diverse habitats where algae can grow, mainly aerophytic in nature. Literature reveals that algae that is potentially harmful to humans do occur in the anthropogenic environment in the air, on surfaces or in water bodies. Algae may negatively affect humans in different ways: they may be toxic, allergenic and pathogenic to humans or attack human structures. Toxin-producing alga are represented in the built environment mainly by blue green algae (Cyanoprokaryota). In special occasions, other toxic algae may also be involved. Green algae (Chlorophyta) found airborne or growing on manmade surfaces may be allergenic whereas Cyanoprokaryota and other forms may not only be toxic but also allergenic. Pathogenicity is found only in a special group of algae, especially in the genus Prototheca. In addition, rare cases with infections due to algae with green chloroplasts are reported. Algal action may be involved in the biodeterioration of buildings and works of art, which is still discussed controversially. Whereas in many cases the disfigurement of surfaces and even the corrosion of materials is encountered, in other cases a protective effect on the materials is reported. A comprehensive list of 79 taxa of potentially harmful, airborne algae supplemented with their counterparts occurring in the built environment, is given. Due to global climate change, it is not unlikely that the built environment will suffer from more and higher amounts of harmful algal species in the future. Therefore, intensified research in composition, ecophysiology and development of algal growth in the built environment is indicated.

Euglenozoa: taxonomy, diversity and ecology, symbioses and viruses

Euglenozoa is a species-rich group of protists, which have extremely diverse lifestyles and a range of features that distinguish them from other eukaryotes. They are composed of free-living and parasitic kinetoplastids, mostly free-living diplonemids, heterotrophic and photosynthetic euglenids, as well as deep-sea symbiontids. Although they form a well-supported monophyletic group, these morphologically rather distinct groups are almost never treated together in a comparative manner, as attempted here. We present an updated taxonomy, complemented by photos of representative species, with notes on diversity, distribution and biology of euglenozoans. For kinetoplastids, we propose a significantly modified taxonomy that reflects the latest findings. Finally, we summarize what is known about viruses infecting euglenozoans, as well as their relationships with ecto- and endosymbiotic bacteria.

Toxin Analysis of Freshwater Cyanobacterial and Marine Harmful Algal Blooms on the West Coast of Florida and Implications for Estuarine Environments

Recent marine and freshwater algal and cyanobacterial blooms in Florida have increased public concern and awareness of the risks posed by exposure to these organisms. In 2018, Lake Okeechobee and the Caloosahatchee river, on the west coast of Florida, experienced an extended bloom of Microcystis spp. and a bloom of Karenia brevis in the coastal waters of the Gulf of Mexico that coincided in the Fort Myers area. Samples from the Caloosahatchee at Fort Myers into Pine Island Sound and up to Boca Grande were collected by boat. High concentrations of microcystin-LR were detected in the cyanobacterial bloom along with brevetoxins in the marine samples. Furthermore, β-N-methylamino-L-alanine (BMAA) and isomers N-(2-aminoethyl)glycine (AEG) and 2,4-diaminobuytric acid (DAB) were detected in marine diatoms and dinoflagellates, and cyanobacteria of freshwater origin. High freshwater flows pushed the cyanobacterial bloom to barrier island beaches and Microcystis and microcystins could be detected into the marine environment at a salinity of 41 mS/cm. For comparison, in 2019 collections of Dapis (a new generic segregate from Lyngbya) mats from Sarasota showed high concentrations of BMAA, suggesting the possibility of long-term exposure of residents to BMAA. The findings highlight the potential for multiple, potentially toxic blooms to co-exist and the possible implications for human and animal health.

Safety of dried whole cell Euglena gracilis as a novel food pursuant to Regulation (EU) 2015/2283

Following a request from the European Commission, the EFSA Panel on Nutrition, Novel Foods and Food Allergens (NDA) was asked to deliver an opinion on the safety of dried whole cell Euglena gracilis as a novel food (NF) pursuant to Regulation (EU) 2015/2283. E. gracilis is a single-cell microalga which occurs widely in nature and is commonly found in freshwater habitats. The NF, the dried biomass of E. gracilis, is produced by fermentation and its major constituent (> 50%) is a β-glucan polysaccharide. The applicant proposed to use the NF in food supplements, in foods for total diet replacement for weight control and as a food ingredient added to a number of food products. The target population proposed by the applicant is the general population, except for food supplements and for foods for total diet replacement for which the target population is the general population from 12 months of age onwards. In 2019, E. gracilis was attributed the qualified presumption of safety (QPS)-status with the qualification 'for production purposes only', which includes food products based on microbial biomass of the microalga. Based on the information provided, E. gracilis is not expected to survive the manufacturing process. The submitted toxicity studies did not raise safety concerns. No adverse effects were observed in the subchronic toxicity study, up to the highest dose tested, i.e. 3,300 mg NF/kg body weight, considered as the no observed adverse effect level (NOAEL). The margins of exposure between this dose and the high (95th percentile) intake estimates, range from 33 for infants to 192 for adults. The Panel considers that in view of the QPS status of the source of the NF, supported by the compositional data and lack of toxicity observed in the 90-day study, the margins of exposure are sufficient. The Panel considers that the NF, i.e. dried whole cell Euglena gracilis, is safe at the proposed uses and use levels.

Mechanism Underlying Flow Velocity and Its Corresponding Influence on the Growth of Euglena gracilis, a Dominant Bloom Species in Reservoirs

The effects of hydrodynamics on algae growth have received considerable attention, and flow velocity is one of the most frequently discussed factors. For Euglena gracilis, which aggregates resources and is highly resistant to environmental changes, the mechanism underlying the impact of flow velocity on its growth is poorly understood. Experiments were conducted to examine the response of algae growth to different velocities, and several enzymes were tested to determine their physiological mechanisms. Significant differences in the growth of E. gracilis were found at different flow velocities, and this phenomenon is unique compared to the growth of other algal species. With increasing flow velocity and time, the growth of E. gracilis is gradually inhibited. In particular, we found that the pioneer enzyme is peroxidase (POD) and that the main antioxidant enzyme is catalase (CAT) when E. gracilis experiences flow velocity stress. Hysteresis between total phosphorus (TP) consumption and alkaline phosphatase (AKP) synthesis was observed. Under experimental control conditions, the results indicate that flow velocities above 0.1 m/s may inhibit growth and that E. gracilis prefers a relatively slow or even static flow velocity, and this finding could be beneficial for the control of E. gracilis blooms.

Toxicological assessment of Euglena gracilis strain Eu029 shows no adverse effects in vivo and in vitro

Euglena gracilis is a single-celled organism capable of photosynthesis and heterotrophy. Euglena sp. have long been studied in the laboratory for its metabolic pathways, cell motility, and ease of culture. The safety of E. gracilis strain eu029 (EG029) for use as a food ingredient was assessed in a bacterial reverse mutagenesis assay (Ames), rec assay, in vivo micronucleus assay, acute toxicity study in mice, 13-week toxicology in rats, and a teratology study in mice and rats. EG029 was not genotoxic. The No Observed Adverse Effect Level (NOAEL) in the 13-week study was greater than 1000 mg/kg/day, the highest dose tested. Teratogenicity studies did not find any defects in fetal development or effects to maternal health in rats at 1000 mg/kg/day, the highest dose tested.

PCR identification of toxic euglenid species Euglena sanguinea

Euglena sanguinea Ehrenberg is the only known species of euglenids which forms toxic blooms causing tangible losses to fish farms. Euglena sanguinea produces euglenophycin, a toxin similar in structure to solenopsin, an alkaloid found in fire ant venom. It was proved that euglenophycin exhibits not only ichthyotoxic but also herbicidal and anticancer activity. Recently, a specific mass spectrometric method of identification and quantitation of euglenophycin was developed to facilitate monitoring of that toxin in freshwater ponds. Despite the recent taxonomic verifications, proper identification of E. sanguinea is still difficult, especially for less experienced researchers. Herein, we describe a simple method based on nested PCR amplification of the nSSU rDNA fragments to identify a single E. sanguinea cell and its detection in a sample of water. The method will further facilitate monitoring of water reservoirs, especially estimating the risk of toxic blooms.

Therapeutic effects of the euglenoid ichthyotoxin, euglenophycin, in colon cancer

Colorectal cancer (CRC) remains one of the most commonly diagnosed cancers and the 3^rd leading cause of cancer-related mortality. The emergence of drug resistance poses a major challenge in CRC care or treatment. This can be addressed by determining cancer mechanisms, discovery of druggable targets, and development of new drugs. In search for novel agents, aquatic microorganisms offer a vastly untapped pharmacological source that can be developed for cancer therapeutics. In this study, we characterized the anti-colorectal cancer potential of euglenophycin, a microalgal toxin from Euglena sanguinea. The toxin (49.1-114.6 μM) demonstrated cytotoxic, anti-proliferative, anti-clonogenic, and anti-migration effects against HCT116, HT29, and SW620 CRC cells. We identified G1 cell cycle arrest and cell type - dependent modulation of autophagy as mechanisms of growth inhibition. We validated euglenophycin’s anti-tumorigenic activity in vivo using CRL:Nu(NCr)Foxn1^nu athymic nude mouse CRC xenograft models. Intraperitoneal toxin administration (100 mg/kg; 5 days) decreased HCT116 and HT29 xenograft tumor volumes (n=10 each). Tumor inhibition was associated with reduced expression of autophagy negative regulator mechanistic target of rapamycin (mTOR) and decreased trend of serum pro-inflammatory cytokines. Together, these results provide compelling evidence that euglenophycin can be a promising anti-colorectal cancer agent targeting multiple cancer-promoting processes. Furthermore, this study supports expanding natural products drug discovery to freshwater niches as prospective sources of anti-cancer compounds.

Euglenophycin is produced in at least six species of euglenoid algae and six of seven strains of Euglena sanguinea

Euglena sanguinea is known to produce the alkaloid toxin euglenophycin and is known to cause fish kills and inhibit mammalian tissue and microalgal culture growth. An analysis of over 30 species of euglenoids for accumulation of euglenophycin identified six additional species producing the toxin; and six of the seven E. sanguinea strains produced the toxin. A phylogenetic assessment of these species confirmed most taxa were in the Euglenaceae, whereas synthesis capability apparently has been lost in the Phacus, Eutreptiella, and Discoplastis branches.

Identification of a new-to-science cyanobacterium, Toxifilum mysidocida gen. nov. & sp. nov. (Cyanobacteria, Cyanophyceae)

Cyanobacteria occupy many niches within terrestrial, planktonic, and benthic habitats. The diversity of habitats colonized, similarity of morphology, and phenotypic plasticity all contribute to the difficulty of cyanobacterial identification. An unknown marine filamentous cyanobacterium was isolated from an aquatic animal rearing facility having mysid mortality events. The cyanobacterium originated from Corpus Christi Bay, TX. Filaments are rarely solitary, benthic mat forming, unbranched, and narrowing at the ends. Cells are 2.1 × 3.1 μm (width × length). Thylakoids are peripherally arranged on the outer third of the cell; cyanophycin granules and polyphosphate bodies are present. Molecular phylogenetic analysis in addition to morphology (transmission electron microscopy and scanning electron microscopy) and chemical composition all confirm it as a new genus and species we name Toxifilum mysidocida. At least one identified Leptolyngbya appears (based on genetic evidence and TEM) to belong to this new genus.

Evolutionary Origin of Euglena

Euglenids (Excavata, Discoba, Euglenozoa, Euglenida) is a group of free-living, single-celled flagellates living in the aquatic environments. The uniting and unique morphological feature of euglenids is the presence of a cell covering called the pellicle. The morphology and organization of the pellicle correlate well with the mode of nutrition and cell movement. Euglenids exhibit diverse modes of nutrition, including phagotrophy and photosynthesis. Photosynthetic species (Euglenophyceae) constitute a single subclade within euglenids. Their plastids embedded by three membranes arose as the result of a secondary endosymbiosis between phagotrophic eukaryovorous euglenid and the Pyramimonas-related green alga. Within photosynthetic euglenids three evolutionary lineages can be distinguished. The most basal lineage is formed by one mixotrophic species, Rapaza viridis. Other photosynthetic euglenids are split into two groups: predominantly marine Eutreptiales and freshwater Euglenales. Euglenales are divided into two families: Phacaceae, comprising three monophyletic genera (Discoplastis, Lepocinclis, Phacus) and Euglenaceae with seven monophyletic genera (Euglenaformis, Euglenaria, Colacium, Cryptoglena, Strombomonas, Trachelomonas, Monomorphina) and polyphyletic genus Euglena. For 150 years researchers have been studying Euglena based solely on morphological features what resulted in hundreds of descriptions of new taxa and many artificial intra-generic classification systems. In spite of the progress towards defining Euglena, it still remains polyphyletic and morphologically almost undistinguishable from members of the recently described genus Euglenaria; members of both genera have cells undergoing metaboly (dynamic changes in cell shape), large chloroplasts with pyrenoids and monomorphic paramylon grains. Model organisms Euglena gracilis Klebs, the species of choice for addressing fundamental questions in eukaryotic biochemistry, cell and molecular biology, is a representative of the genus Euglena.

The transcriptome of Euglena gracilis reveals unexpected metabolic capabilities for carbohydrate and natural product biochemistry

Euglena gracilis is a highly complex alga belonging to the green plant line that shows characteristics of both plants and animals, while in evolutionary terms it is most closely related to the protozoan parasites Trypanosoma and Leishmania. This well-studied organism has long been known as a rich source of vitamins A, C and E, as well as amino acids that are essential for the human diet. Here we present de novo transcriptome sequencing and preliminary analysis, providing a basis for the molecular and functional genomics studies that will be required to direct metabolic engineering efforts aimed at enhancing the quality and quantity of high value products from E. gracilis. The transcriptome contains over 30,000 protein-encoding genes, supporting metabolic pathways for lipids, amino acids, carbohydrates and vitamins, along with capabilities for polyketide and non-ribosomal peptide biosynthesis. The metabolic and environmental robustness of Euglena is supported by a substantial capacity for responding to biotic and abiotic stress: it has the capacity to deploy three separate pathways for vitamin C (ascorbate) production, as well as producing vitamin E (α-tocopherol) and, in addition to glutathione, the redox-active thiols nor-trypanothione and ovothiol.

Genotoxicity and subchronic toxicity evaluation of dried Euglena gracilis ATCC PTA-123017

Euglena gracilis is a microalga capable of synthesizing various nutrients of interest in human and animal nutrition. When cultivated aerobically in the dark, Euglena synthesize paramylon, a storage polysaccharide comprised of high molecular weight beta-1,3-D-glucose polymers organized in cytoplasmic granules. Beta-glucans have been shown to have immune modulation effects, including anti-microbial, anti-tumor, and anti-oxidant properties, and metabolic effects, such as regulation of cholesterol and blood sugar levels. Preparations of E. gracilis and paramylon may therefore have potential utility as functional food ingredients for human and animal nutrition. A battery of toxicological studies was conducted on a dried preparation of E. gracilis and paramylon to support their safe food use. The dried alga was not genotoxic in a bacterial reverse mutation test and mammalian micronucleus test. In the subchronic toxicity study, rats were provided E. gracilis in the diet at levels of 0, 12,500, 25,000 or 50,000 ppm. Paramylon was provided at a concentration of 50,000 ppm. No effects that could be attributable to treatment were observed in clinical observations, body weight, food consumption, ophthalmology, hematology and clinical chemistry, urinalysis, and macroscopic and microscopic findings. A NOAEL of 50,000 ppm in the diet was determined for both ingredients.

Augmenting anti-cancer natural products with a small molecule adjuvant

Aquatic microbes produce diverse secondary metabolites with interesting biological activities. Cytotoxic metabolites have the potential to become lead compounds or drugs for cancer treatment. Many cytotoxic compounds, however, show undesirable toxicity at higher concentrations. Such undesirable activity may be reduced or eliminated by using lower doses of the cytotoxic compound in combination with another compound that modulates its activity. Here, we have examined the cytotoxicity of four microbial metabolites [ethyl N-(2-phenethyl) carbamate (NP-1), Euglenophycin, Anabaenopeptin, and Glycolipid 652] using three in vitro cell lines [human breast cancer cells (MCF-7), mouse neuroblastoma cells (N2a), and rat pituitary epithelial cells (GH4C1)]. The compounds showed variable cytotoxicity, with Euglenophycin displaying specificity for N2a cells. We have also examined the modulatory power of NP-1 on the cytotoxicity of the other three compounds and found that at a permissible concentration (125 µg/mL), NP-1 sensitized N2a and MCF-7 cells to Euglenophycin and Glycolipid 652 induced cytotoxicity.

Quantitative mass spectrometric analysis and post-extraction stability assessment of the euglenoid toxin euglenophycin

Euglenophycin is a recently discovered toxin produced by at least one species of euglenoid algae. The toxin has been responsible for several fish mortality events. To facilitate the identification and monitoring of euglenophycin in freshwater ponds, we have developed a specific mass spectrometric method for the identification and quantitation of euglenophycin. The post-extraction stability of the toxin was assessed under various conditions. Euglenophycin was most stable at room temperature. At 8 °C there was a small, but statistically significant, loss in toxin after one day. These methods and knowledge of the toxin's stability will facilitate identification of the toxin as a causative agent in fish kills and determination of the toxin's distribution in the organs of exposed fish.

Antiproliferative activities of lesser galangal (Alpinia officinarum Hance Jam1), turmeric (Curcuma longa L.), and ginger (Zingiber officinale Rosc.) against acute monocytic leukemia

Acute monocytic leukemia (AML M5 or AMoL) is one of the several types of leukemia that are still awaiting cures. The use of chemotherapy for cancer management can be harmful to normal cells in the vicinity of the target leukemia cells. This study assessed the potency of the extracts from lesser galangal, turmeric, and ginger against AML M5 to use the suitable fractions in neutraceuticals. Aqueous and organic solvent extracts from the leaves and rhizomes of lesser galangal and turmeric, and from the rhizomes only of ginger were examined for their antiproliferative activities against THP-1 AMoL cells in vitro. Lesser galangal leaf extracts in organic solvents of methanol, chloroform, and dichloromethane maintained distinctive antiproliferative activities over a 48-h period. The turmeric leaf and rhizome extracts and ginger rhizome extracts in methanol also showed distinctive anticancer activities. The lesser galangal leaf methanol extract was subsequently separated into 13, and then 18 fractions using reversed-phase high-performance liquid chromatography. Fractions 9 and 16, respectively, showed the greatest antiproliferative activities. These results indicate that the use of plant extracts might be a safer approach to finding a lasting cure for AMoL. Further investigations will be required to establish the discriminatory tolerance of normal cells to these extracts, and to identify the compounds in these extracts that possess the antiproliferative activities.

A redescription of morphologically similar species from the genus Euglena: E. laciniata, E. sanguinea, E. sociabilis, and E. splendens(1)

Euglena sanguinea (Ehrenberg 1831) was one of the first green euglenoid species described in the literature. At first, the species aroused the interest of researchers mainly due to the blood-red color of its cells, which, as it later turned out, is not a constant feature. Complicated chloroplast morphology, labeled by Pringsheim as the "peculiar chromatophore system", made the correct identification of the species difficult, which is the reason why, throughout the 20th century, new species resembling E. sanguinea were continually being named due to a lack of suitable diagnostic features to distinguish E. sanguinea. Interest in E. sanguinea has returned in recent years, following findings that the species can produce ichthyotoxins. This was followed by the need to classify E. sanguinea correctly, which was achieved through the verification of morphological and molecular data for all species similar to E. sanguinea. As the result of the analysis, the number of species sharing some morphological similarities with E. sanguinea could be reduced from 12, as described in the literature, to four, with established epitypes and updated diagnostic descriptions. The most important diagnostic features included: the presence of mucocysts (i.e., whether they were visible before and/or after staining), the number of chloroplasts, the size of the double-sheathed pyrenoids, and the presence of the large paramylon grain in the vicinity of the stigma. Moreover, sequence analysis revealed the presence of unusually long SSU rDNA sequences in E. sanguinea. Previously, SSU rDNA sequences of such length were known to be present in primary osmotrophic euglenoids.