Frustule Morphology Research Articles

Effects of salinity and temperature on growth performance, biochemical composition, and biosilification process of Cyclotella cryptica

Microalgae are valuable resources for producing high-value compounds, but large-scale cultivation in open raceway ponds (ORPs) faces challenges due to salinity and temperature fluctuations, which affect biomass yield and quality. Developing strains with high productivity and stable quality across varying salinity and temperature levels offers a promising approach to overcoming these challenges. Cyclotella cryptica, a marine diatom species, is known for its robustness under diverse salinity conditions, but its biochemical composition and frustule morphology in respond to salinity remains largely unknown. Moreover, the responses of C. cryptica to temperature fluctuations are largely unexplored, posing a barrier to its industrial application in ORPs. In this study, C. cryptica was cultivated under six salinity levels (19–34 ‰ at 3 ‰ intervals) and five temperature regimes (17–33 °C at 4 °C intervals) to investigate the effects of these environmental factors on growth performance, macronutrient composition, fatty acid (FA) profile, and carotenoid content. Additionally, we examined the biosilica content, frustule morphology, and the transcriptional levels of five frustule biogenesis related genes (CcSin1, CcSin2, and CcSAP1–3) to assess the impact of salinity and temperature on the biosilification process. Our findings revealed that salinity exerts minimal effects on growth, macronutrient composition, FA profile, and carotenoid content, whereas it induces significant variations in frustule morphology and biosilica deposition. In contrast, temperature markedly influences all evaluated parameters. These insights into the adaptive mechanisms of C. cryptica to salinity and temperature variations are crucial for optimizing the scale-up cultivation strategy of this species in ORPs.

First Account of Epibiotic Diatom Taxa from the Carapaces of Green Swimming Crab Callinectes bellicosus (Stimpson 1859) (Decapoda, Portunidae)

Diatoms are among the most common epibionts and have been recorded on the surfaces of various living substrates, either plants or animals. However, studies on them are still scarce in view of the many substrata available. In this study, epibiotic diatoms living on Callinectes bellicosus were identified for the first time from a subtropical coastal lagoon in Northwest Mexico. We tested the null hypothesis that the diatom flora living on the carapaces of C. bellicosus would not be similar to that recorded for mangrove sediments, its typical habitat. The epibiotic diatoms were brushed off from the carapaces of two specimens, acid-cleaned, mounted in synthetic resin, and identified based on frustule morphology. This way, 106 taxa from 46 genera were recorded, including 25 singletons, and 6 new records for the Mexican northwest region. The best-represented genera were Nitzschia (10 taxa), Mastogloia (9), Diploneis (8), Navicula (7), Amphora (5), Cocconeis (5), Tryblionella (4), and Gyrosigma (4). Species composition included 93% of local taxa, thus refuting the proposed hypothesis and supporting the alternate one. Although the estimated species richness was lower than that in sediments, it deems the green crab carapace a favorable substrate for the growth of benthic diatoms.

Transcriptional responses to salinity-induced changes in cell wall morphology of the euryhaline diatom Pleurosira laevis.

Diatoms are unicellular algae with morphologically diverse silica cell walls, which are called frustules. The mechanism of frustule morphogenesis has attracted attention in biology and nanomaterials engineering. However, the genetic regulation of the morphology remains unclear. We therefore used transcriptome sequencing to search for genes involved in frustule morphology in the centric diatom Pleurosira laevis, which exhibits morphological plasticity between flat and domed valve faces in salinity 2 and 7, respectively. We observed differential expression of transposable elements (TEs) and transporters, likely due to osmotic response. Up-regulation of mechanosensitive ion channels and down-regulation of Ca2+-ATPases in cells with flat valves suggested that cytosolic Ca2+ levels were changed between the morphologies. Calcium signaling could be a mechanism for detecting osmotic pressure changes and triggering morphological shifts. We also observed an up-regulation of ARPC1 and annexin, involved in the regulation of actin filament dynamics known to affect frustule morphology, as well as the up-regulation of genes encoding frustule-related proteins such as BacSETs and frustulin. Taken together, we propose a model in which salinity-induced morphogenetic changes are driven by upstream responses, such as the regulation of cytosolic Ca2+ levels, and downstream responses, such as Ca2+-dependent regulation of actin dynamics and frustule-related proteins. This study highlights the sensitivity of euryhaline diatoms to environmental salinity and the role of active cellular processes in controlling gross valve morphology under different osmotic pressures.

Enhancement of hemostatic properties of Cyclotella cryptica frustule through genetic manipulation

BackgroundThe silicified cell wall of diatoms, also known as frustule, shows huge potential as an outstanding bio-nanomaterial for hemostatic applications due to its high hemostatic efficiency, good biocompatibility, and ready availability. As the architectural features of the frustule determine its hemostatic performance, it is of great interest to develop an effective method to modify the frustule morphology into desired patterns to further improve hemostatic efficiency.ResultsIn this study, the gene encoding Silicalemma Associated Protein 2 (a silicalemma-spanning protein) of Cyclotella cryptica (CcSAP2) was identified as a key gene in frustule morphogenesis. Thus, it was overexpressed and knocked down, respectively. The frustule of the overexpress lines showed no obvious alteration in morphology compared to the wild type (WT), while the size, specific surface area (BET), pore volume, and pore diameter of the knockdown strains changed greatly. Particularly, the knockdown frustules achieved a more pronounced coagulation effect and in vivo hemostatic performance than the WT strains. Such observations suggested that silicalemma proteins are ideal genetic encoding targets for manipulating frustule morphology associated hemostatic properties. Furthermore, the Mantel test was adopted to identify the key morphologies associated with C. cryptica bleeding control. Finally, based on our results and recent advances, the mechanism of frustule morphogenesis was discussed.ConclusionThis study explores a new strategy for enhancing the hemostatic efficiency of the frustule based on genetic morphology modification and may provide insights into a better understanding of the frustule morphogenesis mechanism.

Diatoms Versus Copepods: Could Frustule Traits Have a Role in Avoiding Predation?

Predation is one of the strongest selection pressures phytoplankton has evolved strategies to cope with. Concurrently, phytoplankton growth must deal with resource acquisition. Experiments on mono- and mixed cultures of morphologically different diatoms exposed to copepods were performed to assess if size and shape were primary drivers in avoiding predation. Additionally, frustule silicification was investigated as a potential factor affecting prey selection by copepods. Thalassiosira pseudonana, Conticribra weissflogii, Cylindrotheca closterium, and Phaeodactylum tricornutum were exposed to the presence of Temora longicornis, a calanoid copepod. The physiological response in terms of growth, elemental composition and morphology was determined. The power of Image Flow Cytometry allowed functional single-cell analyses of mixed cultures in the presence and absence of copepods. Results highlighted that T. pseudonana although the most eaten by copepods in monospecific cultures, was not the preferred prey when the bigger C. weissflogii was added to the culture. When pennates were co-cultured with centric diatoms, their growth was unaffected by predators. Our data suggested that the frustule morphology contributes to long-term prey-predator interaction since the elongated thinner frustule, which evolved more recently, benefited cells in escaping from predators also when facing competition for resources.

Morphological variability and genetic analysis of Thalassiosira tenera (Bacillariophyta), a dominant phytoplankton species from the northwestern Sea of Japan

ABSTRACT Thalassiosira species significantly contribute to phytoplankton communities, forming algal blooms in temperate and polar regions, and are broadly used as model organisms for nanotechnology and as important food sources in the aquaculture industry. Despite being widespread in the world’s oceans, data on abundance and seasonal patterns of individual species of Thalassiosira are sparse. In the present study, we have identified Thalassiosira tenera from the northwestern Sea of Japan, using light and scanning electron microscopy (SEM), and molecular genetic analysis of 28S rDNA sequence. Based on field and cultured material, morphological variability of T. tenera was studied; a detailed diagnostic description of the species is provided, and its morphometry is compared with that of the morphologically similar T. exigua. Growth peculiarities of the species were investigated in laboratory culture; variations in frustule morphology occur at different growth stages. The degree of frustule silicification is assumed to depend on the stage of cell development. By SEM observations of field samples, the seasonal dynamics of the species was examined and data on its abundance are provided for the first time. Findings indicate that T. tenera is present year-round in the phytoplankton assemblage in the northwestern Sea of Japan. The species dominates the community from November to February, accounting for more than 60% of the total phytoplankton abundance. The highest numbers, 7,281 cells l−1, were recorded under the sea ice in January, and the lowest numbers, less than 100 cells l−1, occurred in April. In addition, the study gives information on the species ecology.

Aulacoseira chockii sp. nov., an early freshwater centric diatom from the Eocene bearing a unique morphology

Aulacoseira is a freshwater diatom genus found today in numerous lakes, ponds and rivers worldwide, and inhabiting a wide range of environmental conditions. The genus is one of the oldest diatom lineages known to colonize freshwater environments, dating to the late Cretaceous and radiating over much of the Cenozoic. The purpose of this paper is to describe a new species, Aulacoseira chockii, from an early Eocene locality situated near the Arctic Circle in northern Canada. The exquisitely preserved specimens have allowed for a detailed examination of frustule morphology of this early Cenozoic taxon. Three characteristics clearly separate A. chockii from all other known fossil and modern species in the genus. First, the spines of A. chockii are formed by coalescence of extensions from three to five mantle costae and multiple ribs that originate on the valve face. This type of spine design has never been reported and represents a potentially ancient trait in the Aulacoseiraceae lineage. Second, the valve has multiple sessile rimoportulae that are each connected to the end of a tube or canal that runs parallel to, and inside, the mantle wall. Rimoportulae with this structure are rare, and found primarily on extinct species of Aulacoseira. Third, the mantle striae are sinistrorse, and represent another rarely observed character within the genus. Because frustules of A. chockii possessed only separation spines, existed largely as single cells and not in long filaments, and were found in associated with remains of numerous heliozoans as well as testate euglyphids and sponges, it is believed this species grew in the littoral zone of a shallow waterbody.

Description and phylogenetic position of three new species of Stauroneis Ehrenberg (Bacillariophyceae: Stauroneidaceae) from the Indian Subcontinent

ABSTRACT Stauroneis Ehrenberg is a species-rich genus that is common and widespread with some variation in frustule morphology. Recently, studies have explored the molecular diversity of the Stauroneis from the temperate region, but there are no molecular data for tropical species. In the present study, four species of Stauroneis were investigated using morphological and molecular data. Morphological characters were analysed using light microscopy (LM) and scanning electron microscopy (SEM), and the features of each taxon were compared with similar species within the genus. For the molecular analyses, Stauroneis strains were isolated, cultured, DNA was extracted, and sequences from rbcL and 18S genes were analysed to determine phylogenetic relationships. Our analysis describes three new species and records one previously known species. The new species are: (1) Stauroneis lateritica Wadmare, Kociolek & B.Karthick, characterized by small frustules with elliptical lanceolate valves and short subrostrate apices, broad central bow-tie shaped stauros, and few moderate striae centrally becoming strongly radiate towards the poles; (2) Stauroneis sholaii Wadmare, Kociolek & B.Karthick, which has broad lanceolate valves with short rectangular stauros and radiate striae; and (3) Stauroneis bartii Wadmare, Kociolek & B.Karthick, which has large, lanceolate valves with rounded apices, a stauros with 4–7 shortened striae and radiate striae. Ultrastructure and molecular data from India for the previously known, widespread species Stauroneis gracilis Ehrenberg are presented. The combined morphological and molecular approach supports recognition of S. lateritica, S. sholaii and S. bartii as new species. This study is the first-ever attempt at molecular species discovery of diatoms from the Indian subcontinent.

Light-Emitting Biosilica by In Vivo Functionalization of Phaeodactylum tricornutum Diatom Microalgae with Organometallic Complexes

In vivo incorporation of a series of organometallic photoluminescent complexes in Phaeodactylum tricornutum diatom shells (frustules) is investigated as a biotechnological route to luminescent biosilica nanostructures. [Ir(ppy)2bpy]+[PF6]−, [(2,2′-bipyridine)bis(2-phenylpyridinato)iridium(III) hexafluorophosphate], [Ru(bpy)3]2+ 2[PF6]−, [tris(2,2′-bipyridine)ruthenium(II) hexafluorophosphate], AlQ3 (tris-(8-hydroxyquinoline)aluminum), and ZnQ2 (bis-8-hydroxyquinoline-zinc) are used as model complexes to explore the potentiality and generality of the investigated process. The luminescent complexes are added to the diatom culture, and the resulting luminescent silica nanostructures are isolated by an acid-oxidative treatment that removes the organic cell matter without altering both frustule morphology and photoluminescence of incorporated emitters. Results show that, except for ZnQ2, the protocol successfully leads to the incorporation of complexes into the biosilica. The spontaneous self-adhering ability of both bare and doped Phaeodactylum tricornutum cells on conductive indium tin oxide (ITO)-coated glass slides is observed, which can be exploited to generate dielectric biofilms of living microorganisms with luminescent silica shells. In general, this protocol can be envisaged as a profitable route to new functional nanostructured materials for photonics, sensing, or biomedicine via in vivo chemical modification of diatom frustules with organometallic emitters.

FEM exploration of the potential of silica diatom frustules for vibrational MEMS applications

Numerical simulations were carried out using the Finite Element Method (FEM) to determine the frequency characteristics of mechanical vibration of diatom silica frustules under the conditions and at frequencies that are not readily accessible to experimental measurement. The results revealed the influence of the frustule morphology on the natural frequency spectra. The effect of frustule density, stiffness, dimensions, pore size, and wall thickness on the eigenfrequencies and the corresponding modal shapes were studied in detail. Diatom frustules have natural frequencies in the range between several MHz and tens of MHz that make them a promising candidate for future MEMS applications. Eigenfrequencies depend linearly on the speed of sound in the frustule wall and decrease parabolically with the diameter and the pore size to diameter ratio. Dimensional analysis allowed obtaining functional correlations that encapsulate the various dependencies in compact analytical form. The satisfactory nature of our calculations and correlations derived from them is confirmed through an agreement with the analytical solutions from the literature.

Assessment of five live-cell characteristics in periphytic diatoms as a measure of copper stress

Metal pollution of fluvial systems remains a major problem and biomonitoring can be a useful tool for assessing the metal contamination. To assess their potential as new bioindicators of copper stress, we treated a field-collected live periphytic diatom community (dominated by Amphora, Navicula, and Nitzschia) with dissolved Cu under optimal growth conditions. We studied the effects of Cu on five live-cell attributes: motility, protoplasmic content, lipid body number and biovolume, and frustule morphology. In all three genera, motility and protoplasmic content decreased, whereas the LB number, biovolume and deformity increased when Cu and exposure time increased. The sensitivity to Cu was highest for % MF, % CPC and % BCLB in Navicula and the LB number and deformity in Nitzschia. Amphora appeared to be more tolerant to Cu in comparison with other genera. The five cell attributes were inter-related. A heatmap showed that a recommended indicator for rapid screening of Cu toxicity was % BCLB for Amphora and % MF for Navicula and Nitzschia. % MF might be the most common representative indicator that can be applied to all three genera to evaluate the lethal effects of Cu stress if only one of the five cell attributes must be selected.

Species of the diatom taxa <i>Aulacodiscus</i> and <i>Trinacria</i> with biostratigraphic utility in Palaeogene and Neogene North Sea sediments

Abstract. Species of Aulacodiscus and Trinacria, two important marine diatom genera with biostratigraphic utility in offshore North Sea exploration and onshore correlation, are identified, described and emended and the North Sea microfaunal zonation scheme is revised accordingly. Occurring mainly as pyritised diatom moulds or steinkerns, detailed scanning electron microscope (SEM) analysis of several specimens, formerly in open nomenclature, has allowed the correct taxonomic identification of pyritised morphologies found to belong to the genus Aulacodiscus, including A. allorgei, A. heterostictus, A. insignis, A. singilewskyanus, A. subexcavatus and A. suspectus. The important marker species Trinacria regina is emended. SEM studies, using specimens preserved in pyrite and original silica, have shed further light on the varying forms and frustule morphology of Trinacria regina so that valves and frustules formerly thought to represent separate species are now found to be grouped within this taxon; SEM studies have shown that many of these variations represent different valves within a chain, whilst others may signify ecophenotypic variants. Emendments are therefore made to clarify the taxonomic status of different variants within T. regina, important in the Palaeocene–Eocene boundary interval onshore and offshore such as the Sele and Balder formations and the Fur Formation diatomite of Jutland, Denmark. Species of taxa formerly in open nomenclature are now assigned to Aulacodiscus insignis, which are important offshore markers in offshore late Oligocene to early Miocene sediments in northwest Europe.

Molecular phylogeny of the diatom genera Amphora and Halamphora (Bacillariophyta) with a focus on morphological and ecological evolution.

The taxonomic history of the diatom genus Amphora is one of a broad early morphological concept resulting in the inclusion of a diversity of taxa, followed by an extended period of revision and refinement. The introduction of molecular systematics has increased the pace of revision and has largely resolved the relationships between the major lineages, indicating homoplasy in the evolution of amphoroid symmetry. Within the two largest monophyletic lineages, the genus Halamphora and the now taxonomically refined genus Amphora, the intrageneric morphological and ecological relationships have yet to be explored within a phylogenetic framework. Critical among this is whether the range of morphological features exhibited within these diverse genera are reflective of evolutionary groupings or, as with many previously studied amphoroid features, are nonhomologous when examined phylogenetically. Presented here is a four-marker molecular phylogeny that includes 31 taxa from the genus Amphora and 77 taxa from the genus Halamphora collected from fresh, brackish, and salt waters from coastal and inland habitats of the United States and Japan. These phylogenies illustrate complex patterns in the evolution of frustule morphology and ecology within the genera and the implications of this on the taxonomy, classification, and organization of the genera are discussed.

Thalassiosira mala (Bacillariophyta), a potentially harmful, marine diatom from Chilka Lake and other coastal localities of Odisha, India: Nomenclature, frustule morphology and global biogeography

Our examination of net phytoplankton collected from coastal localities in Odisha on the east coast of India, including Chilka Lake, Chandrabhaga Beach and Puri, in December 2015, revealed the overwhelming dominance of Thalassiosira mala, a gelatinous colony-forming, potentially harmful, marine planktonic diatom. The large numbers of cells allowed us to observe details of the cingulum not previously reported. The epicingulum is composed of four open bands including an areolated valvocopula, an areolated copula and two non-areolated pleurae. The immature hypocingulum includes at least two bands. Openings of alternate bands are arranged in a dextral pattern. Based on previous reports from the west coast and our current findings, Thalassiosira mala appears to be a common, widely distributed primary producer in Indian coastal waters. The presence of morphologically similar species, especially those <20 μm in diameter, underscores the importance of reliable species-level taxonomy using appropriate techniques for meaningful ecological and biogeographic considerations and for monitoring potentially harmful algae in India’s economically important coastal waters. Published reports suggest that Thalassiosira mala is widely distributed in temperate and tropical waters, present in 26 of 232 ecoregions and 18 of 62 provinces recognized in a recent classification of coastal marine ecoregions.

Morphology and geographic distribution of three Kobayasiella species (Bacillariophyta) in northeastern European Russia

Based on scanning electron microscopy, frustule morphology of the diatoms Kobayasiella subtillissima, K. parasubtillissima and K. okadae from dystrophic weakly mineralized water bodies in northeastern European Russia (the Komi Republic and the Vologda Region) was studied. The ranges of variation in valve length and width and stria density for K. subtillissima and K. parasubtillissima coincided with published data on these species and are different for K. okadae due to interpopulation variability. Taxonomic assessment of these characters was also made. New records of K. subtillissima, K. parasubtillissima and K. оkada allowed us to clarify their geographic ranges.

The effect of different light regimes on diatom frustule silicon concentration

Significant changes in frustule morphology of Coscinodiscus granii induced by different light regimes have previously been observed, but whether these changes are companied by similar changes in cellular silicon concentration or are solely due to reorganization is unknown. The influence of six different light regimes, blue (B), green (G), yellow (Y), red-orange (RO), red (R) and white (W), at two intensities (100 and 300μmolphotonsm−2s−1) on cellular silicon content was therefore assessed for C. granii. Both the cellular Si content and the Si concentration per surface area were higher at 300 than at 100μmolphotonsm−2s−1 for all tested wavelengths, except Y light. At the same light intensity, cells grown at B light had the highest cellular Si content (except W at 300μmolphotonsm−2s−1) and Si concentration per surface area. At R, OR, G, B and W light, high intensity led to higher cellular Si content although the mean frustule size was smaller (opposite for Y light). As the higher Si content was not due to a larger cell size, we hypothesize that the effect is due to changes in valve thickness. The underlying adaptive significance of this phenomenon might be that at low irradiance (100μmolphotonsm−2s−1), diatom buoyancy is modulated by accumulating less silicon in the cells, as the less silicified cells may have lower sinking rate and thus remain longer at the higher light in the upper part of the water column, which could partly contribute to the ecological success of the diatom.

Response of the freshwater diatom Halamphora veneta (Kützing) Levkov to copper and mercury and its potential for bioassessment of heavy metal toxicity in aquatic habitats

This study investigates the effects of copper and mercury on growth rate, chlorophyll a content, superoxide dismutase (SOD) activity, SOD mRNA gene expression, and frustule morphology of the benthic freshwater diatom Halamphora veneta (Kützing) Levkov and the potential utility of each for toxicity assessment in aquatic habitats. Results showed the following: (1) Compared to mercury, exposure to copper resulted in greater growth inhibition of H. veneta even at low concentrations and after short durations of exposure; (2) high accumulation of chlorophyll a in H. veneta is a stress response to the presence of heavy metals; (3) SOD activity and SOD gene expression varied in H. veneta according to the concentration, exposure time, and type of heavy metal; and (4) exposure to mercury resulted in deformity in the shape and an increase in size of the frustule of H. veneta. Growth rate, chlorophyll a content, SOD activity and gene expression, and frustule morphology of H. veneta are all potential candidates for the toxicological assessment of copper and mercury in aquatic habitats.

Long-term cultivation of the diatom Coscinodiscus granii at different light spectra: effects on frustule morphology

The effect on diatom frustule morphology of long-term cultivation at mono-spectral light (six wavelengths combined with two light intensities) was investigated. The results showed that the size of Coscinodiscus granii frustules and the foramen size increased after 10-month cultivation for all light treatments, compared to results after short-term cultivation. The main difference between the light intensities was in the foramen density and similar to the short-term treatment, the lowest density was found at high-intensity red light. Thus, the mean foramen density and foramen diameter at red light 300 μmol photons m−2 s−1 and white light 100 μmol photons m−2 s−1 were statistically similar for short- and long-term tests. The foramen density was significantly reduced at all colored wavelengths at 100 μmol photons m−2 s−1, and blue and white light at 300 μmol photons m−2 s−1 after long-term compared to short-term cultivation. After both short- and long-term experiments, the foramen diameter showed a general trend of higher light intensity resulting in smaller foramen diameter, except for Y light. This study shows that C. granii can be maintained at mono-spectral light over long time periods and indicates that the most important morphological differences are maintained with the range of variability in some of these parameters being more constrained compared to short-term cultivation. This has important implications for mass cultivation of diatoms at different light conditions for applied purposes.

Study of the type material of two Platessa Lange-Bertalot species (formerly Cocconeis brevicostata Hust. and Cocconeis cataractarum Hust. (Bacillariophyta)

After their original description in the late 1930s by F. Hustedt, Cocconeis brevicostata Hust. and Cocconeis cataractarum Hust. have rarely been cited and illustrated. In the early 2000s, both taxa were transferred to Platessa Lange-Bertalot. At the time, no study of the type material was presented. In order to clarify the ultrastructure of these two taxa and their correct assignment to Platessa, their type material held at the Hustedt Diatom Study Centre (Alfred-Wegener-Institute, Bremerhaven, Germany) was studied under light and electron microscopy. Observations on the frustule morphology and morphometric data for each taxon are provided. Frustule features of Platessa brevicostata (Hust.) Lange-Bertalot and Platessa cataractarum (Hust.) Lange-Bertalot (type material) match those of Platessa and their transfer was correct. The following frustule features distinguish between the two taxa: valve outline, raphe structure, stria type and density, and the presence of tiny marginal areolae (poroids).

Taxonomy, frustular morphology and systematics of Platichthys, a new genus of canal raphe bearing diatoms within the Entomoneidaceae

Platichthys, a new genus of canal raphe bearing diatoms, is formally described following analysis of the frustule morphology using LM and EM examination. The genus type originates from a thermal spring in the temperate climate zone of Chile, and an example of the second member of this new genus is observed from a coastal lagoon on the Galápagos Islands (Isabela Island). Taken together, the morphological features of these two newly described taxa demonstrate a unique set of characteristics observed within Thalassiophysaceae, Bacillariaceae and Entomoneidaceae. Platichthys gen. nov. contains both a canal raphe and fibulae, and the compressed valve face, absence of distinct valve mantle and numerous open porous copulae found in Platichthys gen. nov. are typical of Entomoneidaceae. However, unlike species belonging to Entomoneis, the valves of the new genus do not show frustule twisting and the striae are composed of single simple areolae instead of double rows. The most characteristic feature of the new genus is raphe positioned on an elevated keel, robust tubular fibulae, the strongly developed, steep valve face and an indistinct valve mantle in a form of a narrow structureless strip. The relationships of Platichthys gen. nov. to Nitzschia subgenus Nitzschia and to other sections of Nitzschia sensu lato, Entomoneis and to Hamatusia are discussed.