ABSTRACT Despite being one of the most consequential events in the evolutionary history of diatoms, the emergence of pennates remains poorly understood both in terms of its timing and their ancestry. Here, we re-examined several Late Cretaceous (Campanian) diatom-rich samples from the Alpha Ridge in the Central Arctic Ocean using scanning electron microscopy with a focus on pennate taxa. Greater than anticipated species richness was recovered, increasing known Cretaceous araphid diatom flora by 25%. No raphid pennates were encountered. The majority of taxa found have heteropolar valves, with either field pores, ocelli and/or ocellulimbi, simple pores making up striae, and one rimoportula. Only one species found was previously known to science. One new genus (Cesaria) and three species (Cesaria palaiospennata, Kannoa archaea and K. simswilliamsorum) are formally proposed. For six others, the considered taxonomic affinity ranges from species to family and remains tentative. The plasticity of the structural characters of Cesaria frustules may serve as an example of a valve design that could have been advantageous among the earliest araphid pennates. Extinct Kannoa species show overall structural similarity to extant Asteroplanus and Asterionellopsis. Our unnamed species show some affinity to the genera Sceptroneis, Lancineis, Neodelphineis, and possibly other broadly defined Rhaphoneidaceae. All our diatoms are relatively small and delicate and can easily go unnoticed among the abundance of accompanying large and heavily silicified polar and non-polar centrics. If our assumptions regarding their taxonomic affinity based on morphological similarities are correct, the molecular phylogenies place them all among the earliest divergences of the pennates.

The examination of the phytoplankton communities of the Tonle Sap Lake, particularly the diatom flora, has highlighted the unique microflora of the lake. One new genus (Praecyclostephanos Tudesque, Le Cohu & Van de Vijver) and several new species have recently been described (Aulacoseira micropunctata Tudesque, Le Cohu & Van de Vijver, A. spiralorimoportula Tudesque, Le Cohu & Van de Vijver, Cyclotubicoalitus tonlesapensis (Tudesque, Le Cohu & Van de Vijver) K.Schultz, Tudesque & Van de Vijver, and Urosolenia anlongreangensis Tudesque, Le Cohu & Van de Vijver), highlighting a high level of endemism. A recent survey conducted in 2022 investigated the taxonomy and distribution of the planktonic diatom flora and recorded the presence of a novel Aulacoseira species. Detailed light and scanning electron microscope observations show that this species possesses a unique combination of taxonomic characteristics not found in any other known taxon. In particular, this short-chained species, which lacks linking spines and has small and barely visible rimoportulae, exhibits a characteristic pronounced thickening on the mantle – also known as the Müller step on one of its valves. This article provides high-resolution micrographs and a diagnosis of the new species, examines its distinctiveness, and further underscores the taxonomic singularity of the diatom flora of the Tonle Sap Lake.

A comprehensive study of diatom algae from the Oligocene–Miocene Maykop deposits of the Yashma section, located in the South Caspian region, has been conducted for the first time. The main objectives of the work were to determine the species composition of the microflora, perform stratigraphic interpretation, and reconstruct paleoenvironmental conditions based on micropaleontological data. More than 20 samples were collected and processed using standard methods, including acid cleaning, slide preparation, and examination by light and scanning electron microscopy. The composition and preservation state of the microflora made it possible to reconstruct the paleoenvironmental conditions. For the first time, the presence of silicoflagellates was also established, indicating marine and warm depositional conditions.Specific paleoecological conclusions were obtained for the sections: in Yashma, a transition was revealed from freshwater fauna to subtropical marine fauna with high productivity in the Late Miocene. The obtained data refine the stratigraphy of the region, demonstrate fluctuations in the water level and salinity of the Eastern Paratethys basin, and highlight the diagnostic value of diatoms.

Natural and anthropogenic changes can alter the surface water quality, and diatoms can be used to assess or track these alterations with effective biomonitoring. This requires consistent and reliable identification. The diatom flora in the present study is intended to serve as an image voucher of epilithic diatom taxa from the Mula-Mutha River Basin, a tropical river system located in the Western Ghats of India. These samples were collected seasonally from 19 stations during 2022 and 2023. 170 diatom taxa were documented and identified in this study. The morphological range of each taxon was imaged. Nitzschia (24), Navicula (20), and Gomphonema (17) commonly occurred across sites in the river basin. Approximately 4.11% (7 taxa) of the documented diatoms are potentially novel and have been provisionally numbered. There is a distinct and significant seasonal trend with seven taxa indicative of pre-monsoon conditions, eight taxa associated with monsoon flows, and a single post-monsoon indicator. To our knowledge, there are no previous studies that have comprehensively documented diatom diversity across the entire river basin. This study will serve as a reliable diatom identification resource and contribute towards improving taxonomic consistency and enabling comparisons for future monitoring efforts.

Background: Species checklists are a way in which local biodiversity can be monitored. There is no readily accessible database or checklist of diatom flora of South Africa. This publication gives an account of the diatom taxa encountered during two survey projects (2010–2012 and 2015–2017) within the Eastern Karoo. Objectives: This list has been compiled to allow for: (1) comparisons with future research in the geographical area; (2) comparison with ecological data from other countries; (3) monitoring the occurrence of new species; and (4) documentation of the disappearance of ‘clean water’ indicator species. Methods: Sampling took place in the area known as the Eastern Karoo during the two projects. Samples were scrubbed, cleaned and checked for live cells. Permanent slides were made and diatoms identified using light and electron microscopy. Each species was assigned a four-letter code from the software Omnidia version 6. Results: A total of 474 taxa were encountered, some of which are, as yet, undescribed. This list contains taxonomic rank currently assigned as well as the Omnidia codes as this software is commonly used throughout the world and in South Africa for diatom assessment protocols. Conclusion: The National Environmental Management: Biodiversity Act does not cover the protection of diatom species, which can only realistically be conserved if the habitats in which they are found are also conserved. This species checklist can serve as a catalyst for a move towards legislation accepting the use of diatoms as bioindicators for freshwater within South Africa.

ABSTRACT Nitzschia intermedia is often misidentified due to a lack of good knowledge of its type population and the former grouping of species as synonyms in the early 1970s. The present contribution analyses and illustrates the type material of N. intermedia from Dresden and compares the observed results with published images of all presumed synonyms. In addition, a recently collected population from Flanders (Belgium), identified as N. intermedia, is also studied using light and scanning electron microscopy. Comparison with the type of N. intermedia showed that the Flemish should be separated from the type population and be described as a new species, N. intermediafalsa, based on differences in stria density, valve outline, areola structure on the keel and fibula density. For both species, the accompanying diatom flora is used to determine their ecological preferences.

Microorganisms perform essential functions in Arctic terrestrial ecosystems. Yet, their ecology and biogeography are poorly understood, despite being necessary to predict microbial responses to future climate change. Here, we provide the first large‐scale floristic and biogeographic study of the moss diatom flora in the tundra regions of the North Atlantic sector of the Arctic. Diatom communities in 284 terrestrial moss samples from herbarium and recent collections (1962–2023) were analysed by light and scanning electron microscopy. Moss diatoms show clear regionalisation across the Arctic, with strong compositional differences between the three biogeographic regions (High Arctic, Low Arctic and Subarctic), reflecting contemporary microclimatic conditions and historical processes. We identified an inverse latitudinal diversity gradient with species richness increasing towards the High Arctic, likely driven by a temperature–moisture gradient. Nearly half of all taxa in our study are currently only observed in and known from the Arctic, and 44% were confined to a single biogeographic region, indicating a high degree of potential endemism. Our results serve as a foundation for future studies on polar diatoms and highlight their potential use as bioindicators for reconstructing and monitoring past, present, and future climate change.

Diatoms are microscopic yellow-brown algae widely distributed in aquatic environments, where they play a key role in trophic networks and primary productivity. This study aims to better characterize the current diatom assemblages of Lake Iro, located in southern Chad, a region previously unexplored from a limnological perspective. A taxonomic inventory was conducted based on seven samples collected from different substrates (sediments, plankton, macrophytes), complemented by in situ measurements of hydrochemical parameters (temperature, pH, conductivity, and dissolved oxygen). The results reveal low species diversity, dominated by Aulacoseira granulata, a typical freshwater species that thrives in alkaline, well-mixed waters. The highest diversity indices were observed in samples collected from macrophytes. As the first study of Lake Iro, this work highlights the need for additional, especially seasonal, investigations to better understand the ecological dynamics of this Sahelian lake system.

During a study of Iberian benthic diatom flora in disconnected Mediterranean pools, some small populations of Planothidium lacustre were found in Andalusia and Extremadura (Spain). Specifically, in seven pools of temporary rivers in the Guadiana and Tajo basins. Since its discovery in a small pond in Doñana (Andalusia, Spain) in 2013, there have been no new clear records of this species. Our detailed morphological analysis based on optical microscopy and scanning electron microscopy allowed us to show the morphological variability of the taxon. The specimens of P. lacustre are illustrated with microscopic images, and information on their distribution and ecology is provided. Our findings also suggest that P. engelbrechtii could be a species closely related to P. lacustre. They allow a better understanding of this distinctive Iberian taxon and highlight the need to protect Mediterranean temporary pools and ponds, often forgotten habitats.

ABSTRACT Regular monitoring of the diatom flora as part of the biomonitoring of French rivers has highlighted the remarkable species richness of Gomphonema in the Nouvelle-Aquitaine region, southwestern France, and especially in the north, in the former Limousin region. This taxonomic study is based on detailed analyses using light and scanning electron microscopy. It describes six new species of Gomphonema and also highlights three other species rarely observed in French rivers and lakes. The emergence of new species should be explained both by the focus on understudied lentic environments and by the misidentification of species included in Gomphonema species complexes during routine exercises. We compare the new taxa with the most morphologically similar species of Gomphonema and demonstrate unambiguously that these new species possess a unique set of taxonomic features not observed in any other taxon. The assignment to a complex of Gomphonema species is also mentioned, including, more specifically, the species complex of G. clevei, G. entolejum, G. gracile and G. parvulum. Particular attention is paid to the description of the cingulum of Gomphonema dordoniense sp. nov. whose structure is highly unusual. In addition, we provide detailed ecological characteristics for each of these new species, including their sensitivity to major physico-chemical descriptors.

ABSTRACT During a survey of the diatom flora in a sediment core sampled from a volcanic lake (Lac d’Issarlès), located in central France, two unusual Staurosirella species were discovered. Whereas one could be identified as Staurosirella coutelasiana, a rare species recently described from Switzerland, the second species could not be identified based on the currently available literature. The most striking feature of the latter is the formation of long, ribbon-like colonies using dendritic linking spines, a feature only rarely observed in the genus Staurosirella. Detailed light and scanning electron microscopy observations and comparison with the handful of known colony-forming Staurosirella species, justified the description of the latter taxon as a new species: Staurosirella constrictalonga. The new species is separated from the three known colony-forming species by its weakly constricted central part, the total absence of apical pore fields and the protracted, broadly rostrate apices.

The unique geomorphological and hydrological characteristics along with the stable abiotic parameters have shaped springs into suitable aquatic habitats for a high number of microorganisms. Increasing anthropogenic impact, such as groundwater pollution, alteration or destruction of the eucrenal spring area and high demand for pristine water, are affecting the crenic biodiversity. The present study is the first to focus on taxa belonging to the Bacillariophyta phylum and includes diatom samples taken from 30 karstic springs located in the Apuseni Mountains, Romania. Because diatoms often display distinct preferences for specific substrates, 15 samples were collected from each spring: five from each of the three substrate types (bryophytes, stones, and fine sediments) within the eucrenal area. For qualitative assessment, three mixed samples were subsequently prepared for each substrate type, resulting in a total of 99 analysed samples. A total of 216 taxa were identified in the analysed samples, with 16% found only on bryophytes and 15% occurring only on sand. Two groups of frequent taxa, distinguished by their preferred substrate type and present in at least 25 springs have been observed: Caloneis fontinalis (Grunow) A. Cleve, Cocconeis lineata Ehrenberg and Meridion circulare (Greville) C. Agardh in epibryon, followed by Gomphonema parvulum (Kützing) Kützing and Planothidium dubium (Grunow) Round & Bukhtiyarova in epipsammon; three taxa Achnanthidium minutissimum (Kützing) Czarnecki, Amphora pediculus (Kützing) Grunow and Cocconeis placentula Ehrenberg stand out as a dominant group across all three types of substrates. Moreover, in the epilithon these were the only taxa present in all samples. Microhabitat-specific richness was shaped primarily by light conditions and canopy cover, with oxygen availability contributing as a secondary factor. Gomphonema elegantissimum E. Reichardt & Lange-Bertalot and Caloneis fontinalis, along with other 13 taxa have been identified for the first time in Romania. Keywords: Bacillariophyta, Carpathian Mountains, epibryon, epilithon, epipsammon.

This second paper on the seaweed-associated diatoms of Dokdo (South Korea) documents 26 araphid pennate taxa in the subclasses Fragilariophycidae and Urneidophycidae, collected from macroalgal hosts at 5–15 m water depth. Needle-like synedroid taxa of the family Ulnariaceae (hyalosynedroids) were particularly species-rich; many are stalk-forming, secreting mucilage from apical pore fields. Using coordinated LM and SEM with explicit comparative morphometrics, we provide voucher images and report four species new to science – Divergita lanceolata, Hyalosynedra obtusa, H. parva, and Stricosus dokdoensis; together with four species newly recorded for Korean waters – Glyphodesmis acus, Cyclophora castracanei, Pteroncola inane, and Divergita decipiens. Since several hyalosynedroid taxa are difficult to separate under LM alone, SEM-dependent characters (e.g. rimoportula position, apical pore-field structure, areolar pattern) were essential for reliable identification. These data refine the baseline for Dokdo's epiphytic diatom flora and provide a reproducible foundation for future ecological and biogeographical assessments.

A new brackish-water diatom species, Navicula subphyllepta Contamine, Beauger & Van de Vijver sp. nov., is described from a managed canal in the Marais Poitevin (Vendée, western France). This taxon was initially misidentified as Navicula phyllepta Kützing, but detailed morphological analysis using light and scanning electron microscopy revealed significant differences in valve dimensions, stria density, and ultrastructural features of the central area and raphe system. The new species was dominant in a pelagic sample from a moderately alkaline site characterized by high conductivity and elevated sulfate and bromide concentrations, reflecting a strong brackish influence. Navicula subphyllepta is compared to similar species, including 1205527114 N. phyllepta, N. phylleptosoma, N. salinarum, and N. lindae, from which it is clearly separated by a combination of morphometric and structural features. The new species is observed seasonally in both planktonic and benthic assemblages. Its description contributes to the growing recognition of taxonomic diversity in brackish diatom floras, which remain understudied in Western Europe.

Diatom algae in phytoplankton of the Argazi Reservoir and lakes Arakul, Elovoe, Irtyash, Itkul, Bolshiye Kasli, Kirety, Bolshoi Kisegach, Sinara, Sungul, and Silach are studied for the first time using electron microscopy. Data on the species composition of the diatom flora in waters of Chelyabinsk oblast (79 taxa) are significantly updated. In total, 168 species and intraspecific taxa from 55 genera, including 9 new for the flora of Russia and 26 algae identified only at the genus level, were found in the water bodies under study. According to the generalized literature, databases, and original data, the highest taxonomic diversity of diatom algae is recorded in lakes Silach (174), Bolshoi Kisegach (159), and Sinara (146).

Diatom-based monitoring offers a cost-effective and reliable tool for assessing water quality in developing countries. As natural bio-indicators, diatoms respond sensitively to environmental changes, making them ideal for long-term ecological monitoring. The current paper aims to examine the potential use of a diatom-based tool at the genus level (generic diatom index (GDI)), when flora at the species level is poorly known, for water quality assessment in the Cambodian river systems. Nineteen monitoring sites within the Sangker River catchment have been chosen for the study, while eleven physicochemical parameters and trophic diatom index (TDI) values have been used to predict the response of GDI values. Basic statistical tests and linear regression (LM) were used to describe the spatio-temporal variation of water quality based on GDI classification. A total of 78 diatom genera were recorded, dominated by biraphid taxa (67%), and 13 genera accounted for ~ 90% of total abundance. GDI classification indicated that 10% of sites were of good quality, 53% moderate, and 37% poor, with water quality declining downstream and in areas influenced by urban and agricultural activities. The linear model revealed that GDI was significantly associated with nutrient- and oxygen-related parameters (orthophosphate, dissolved oxygen, and chloride) and correlated strongly with TDI (adjusted R2 = 0.96). Seasonal variation was also significant, highlighting the importance of temporal dynamics in tropical rivers. The current findings provided clear evidence of potential uses of diatom indices to assess the water quality in the Sangker River and other Cambodian river networks even at the genus level of determination. More importantly, diatom indices could be potential tools for long-term biomonitoring in developing countries like Cambodia, where the data and resources are limited. Consequently, the study suggested further investigation on the ecological sensitivity of diatom flora to localize their values, improving GDI classification performance within the tropical environment.

Encyonema liujiangensis sp. nov., a new freshwater diatom species was found on stones of Liujiang River, Guangxi Zhuang Autonomous Region, China. The morphological features of this new species are presented based on light and scanning electron microscopy. E. liujiangensis sp. nov. exhibits the typical characteristics of the genus, such as dorsiventral valves and the direction of deflection of the external distal raphe ends relative to the dorsal margin. The new species closely resembles E. malaysianum and E. chebalingense and shares certain morphological features with some other members in this genus. However, a distinct combination of morphological features justifies identifying E. liujiangensis sp. nov. as a new species: 1) the outline of valves being distinctly dorsiventral with acutely rounded ends, 2) the narrower valve width, 3) the higher stria density at the ends. Brief notes on the accompanying diatom flora and its ecological characteristics are included.

The study involved an analysis of planktonic and benthic diatom samples collected in October 2024 from the high-mountain Adyl-Su River (Kabardino-Balkarian Republic). From the obtained material, 10 pure cultures were isolated and subsequently processed for genotyping.

A rich diatom flora (181 species and intraspecific taxa), including a significant number of aberrant forms, was found in the sediments of Amursky Bay (Sea of Japan) accumulated over the past 2 thousand years (core LV66-1). The increase in their number in sediments formed before the development of industry in the adjacent territory indicates natural factors that led to anomalies in the morphology of diatom valves. Possible causes of these anomalies are heavy metals supplied with river runoff or gas emissions (methane), characteristic of the sediments of Amursky Bay.

The paper presents the results of a microfossil study of Holocene sediments in the Yana River flow zone in the southeastern part of the Laptev Sea. A rich diatom flora (242 species and intraspecific taxa, of which 177 species are freshwater) was revealed; additionally, five species of marine tintinnids (planktonic ciliates) and 15 species of freshwater testate amoebae (testacean) were discovered for the first time in the sea sediments. Three assemblages of microfossils reflecting the phases of environmental changes during the Holocene transgression are distinguished in the studied sediments of core LV83-32. Assemblage 1 was formed under terrestrial conditions (assemblage of diatoms Eunotia-Pinnularia and testacean Difflugia-Cylindrifflugia-Centropyxis), assemblage 2 in the zone of mixing of sea and fresh waters (assemblages of diatoms Cyclotella striata-Aulacoseira, Thalassiosira hyperborea-Chaetoceros and T. hyperborea-Aulacoseira, testacean Cyclopyxis kahli, tintinnids Tintinnopsis fimbriata), and assemblage 3 reflects modern conditions in the inner shelf of the Laptev Sea under the strong influence of river runoff (assemblage of diatoms T. hyperborea-Aulacoseira-M. arctica and tintinnids Tintinnopsis ventricosoides). Changes in the natural environment in the coastal part of the Laptev Sea shelf during the Holocene, established by microfossil assemblages, are confirmed by geochemical data.