A new species, Pseudostaurosiropsis hartziae sp. nov., is described from coastal lakes in Rio Grande do Sul, southern Brazil. This taxon has been consistently identified in previous studies as either related to Pseudostaurosiropsis or other small 'staurosiroid' diatoms (i.e. Fragilaria sp. or Staurosira sp.). We also provide the first detailed observation on the type material of F. construens var. javanica Hustedt, a morphologically similar taxon from Indonesian tropical coastal lakes of Java, and transfer it to the genus Pseudostaurosiropsis. Both species are classified within Pseudostaurosiropsis based on fine structural characteristics, including stria areola configuration (number per stria), spine positioning along the striae, and the presence of closing plates. Furthermore, we employed statistical morphometric analyses to discriminate between the two species, further supporting their morphological separation. In addition, we describe the ecological preferences of P. hartziae. Given its high abundance and occurrence in the study region, this taxonomic clarification has significant implications for ecological research in southern Brazilian coastal lake systems.

This paper describes Encyonema macieliarum sp. nov., collected from epilithic substrates in lotic environments of the Ibicu & iacute; river catchment (southern Brazil, Pampa Biome). The material was examined with the LM and SEM. Morphological analysis and PCA helped to separate the new species from the most similar taxon: Encyonema sprechmannii. The new species E. macieliarum can be differentiated from the others within the genus by a set of characteristics, which includes its valve outline, stria pattern, and absence of intermission. The species is found in oligotrophic to mesotrophic environments, in streams with more stable flow. This finding contributes to the morphological understanding of Encyonema and emphasizes the taxonomic relevance of the underexplored Pampa Biome.

Mayamaea sardiniensis sp. nov. is described from periphytic diatom samples collected in a small thermo-mineral spring in Sardinia (Italy), based on a morphological approach. In light and scanning electron microscopy, the species revealed the presence of typical features of the genus Mayamaea, as reported in the original diagnosis, including elliptical to oval valve outline, broadly rounded apices, and uniseriate striae composed of round areolae occluded by external hymenes. Mayamaea sardiniensis shares strong similarities with M. cavernicola and M. petersenii. Differences with these species mainly include shape of the valve, pattern and density of striae. The presence of M. sardiniensis in the type locality seems to be linked to the ecotonal nature of the spring environment.

Throughout history, mineral springs have been utilized worldwide for various purposes. In the French Massif Central, numerous mineral springs, such as the CO2-rich Ceix Spring, have been historically significant. The Ceix spring was formerly used for the manufacture of statuettes by petrifying objects. In December 2019, the spring was restored to create a natural area downstream the point of emergence (water catchment that remains confined within a small, enclosed building), including the formation of spring-associated travertines. From January 2020 to March 2021, the evolution of diatom communities was monitored during the implementation of the travertine deposit following the restoration. Four sampling sites for diatoms and water quality were selected: two points near the emergence point of the restored area (P1, P2) and two at the foot of the travertine deposit (P3, P4). A significant difference was observed between the points closest to the emergence and those on the travertines, including changes in mineralization, increased pH, and higher dissolved oxygen content. Regarding diatoms, the community composition between points P1, P2 (e.g. Amphora copulata, Diploneis krammeri, Fallacia pygmaea) and points P3, P4 (e.g. Craticula halophila, Cymbella hantzschiana, Mastogloia lacustris) showed a statistically significant difference, indicating that water quality primarily controlled the diatom community composition.

A new fossil diatom species of Encyonema K & uuml;tzing was collected from the Pleistocene sediments of Col & ocirc;nia basin, State of S & atilde;o Paulo, Southeast Brazil, as part of an ongoing survey of its sedimentary record, aiming at exploring the long-term evolution of the Southern Hemisphere lowland rainforest. Morphology of the new species was examined using both light and scanning electron microscopy, and compared with the morphologically closest species in the genus, Encyonema latum Krammer and E. macedonicum Z.Levkov, D.Metzeltin and S.Krstic. The new species is characterized by a distinctly dorsiventral valve outline, a nearly straight ventral margin, and a relatively low length-to-width ratio, along with a higher areolae number. Preliminary diatom analysis of the Col & ocirc;nia basin sedimentary record indicates that the new species occurred associated with high organic matter input and shallow lake conditions, during a cold and humid period. The study contributes to the understanding of the Col & ocirc;nia basin biodiversity and will assist future ecological and evolutionary studies in the region.

Starting from 2007, ring tests on benthic diatom taxonomy and related indices has been spreading throughout Europe. Indeed, in the framework of the WFD application and the use of diatoms as indicators for the ecological status assessment of rivers, interlaboratory harmonization has been recognised as an important tool to assure the reliability and comparability of results. In the present work we show the results of the third Italian ring test, carried out in 2023-2024, involving 39 diatomists from different Italian laboratories and 2 international auditors. Two sampling sites were chosen, both classified as belonging to the Central (lowland) river type and having their origin in lakes. The two sampling localities were chosen to represent different ecological status, respectively good and moderate. The Ticino sampling site is located just downstream the Lago Maggiore (Lombardy, Northern Italy), the second largest lake in Italy; in the chosen stretch, riverbed is 50- 80m large and the main human pressure is represented by wastewater treatment plants. The Chiese various human pressures, including the presence of residential and industrial areas, as well as water extraction for agricultural and hydroelectric purposes. The participants' results, encompassing taxonomic composition and the diatom index calculated using the Italian ICMi, were assessed using the Bray-Curtis similarity index (with a 60% threshold) and z-scores. Important differences could be observed among participants. Most of these differences arose from taxonomic misidentifications, mainly concerning the genera Achnanthidium (in particular the A. minutissimum complex and A. delmontii) and Fragilaria (in particular the F. rinoi and F. vaucheriae complex) and/or to the overlooking of small taxa, such as Fistulifera saprophila and Mayamaea permitis. Even though long experience in diatom identification or the number of samples counted per year are important, these do not guarantee the similarity of results between participants and auditors. In this context, the main objectives of ring tests, training courses, or workshops-problem sharing and knowledge transfer-are the best tools for reducing differences in results.

The diatom genus Punctastriata (Staurosiraceae) includes small-celled taxa often indistinguishable from Staurosirella under light microscopy, complicating their identification in ecological and monitoring studies. Examination of a historic sample from Frahan (Belgium, Delogne collection) revealed two distinct Punctastriata species. One, with heteropolar clavate valves and variable spine positioning on both virgae and vimines, is described as Punctastriata davidwilliamsii Van de Vijver & C.E.Wetzel sp. nov. The second corresponds to P. glubokoensis, for which an emended description is provided. Both species were abundant in the Frahan material, and additional observations confirm they are more widespread in Europe than previously recognized. Frequent misidentifications with Staurosirella or poorly defined Punctastriata taxa underline the need for scanning electron microscopy in resolving this genus. The recognition of P. davidwilliamsii refines species concepts within Punctastriata and improves our understanding of the diversity and biogeography of small-celled araphid diatoms in European freshwaters.

Chamaepinnularia is a small and diverse genus comprises of almost 70 species, whose origin remains uncertain, including taxa previously classified as Navicula and others Pinnularia. A new Chamaepinnularia species (Bacillariophyeceae) was found for Neotropical region, which still harbors significant taxonomic gaps for the genus. Named Chamaepinnularia mirim sp. nov it is distinguished from other Chamaepinnularia species by its wider central inflation along the median portion of the valve, extending towards the apices, and by a more radiant striation pattern.This new species occurs in epiphytic habitats in subtropical coastal lakes of Brazil. Morphological details were documented using both light and scanning electron microscopy, and insights into its differences from closely related species are discussed.

During routine biomonitoring in Europe, an unknown Fragilaria taxon was frequently observed in western Europe, that was initially identified as Fragilaria pectinalis, a species described in the late eighteenth century from the UK and often misidentified. The epitype material of Conferva pectinalis collected in 1802 by Dillwyn was analysed using both light and scanning electron microscope observations. Additional historic material from the Rabenhorst and Grunow collections was likewise investigated and compared to the unknown French Fragilaria populations identified in Wetzel & Ector (2015) as F. pectinalis. While the Rabenhorst material was identified as F. pectinalis, the taxon described by Grunow as Synedra capitellata f. striis-distantioribus was transferred to the genus Fragilaria and raised to species level. The French populations showed sufficient morphological differences to be separated as a new species, Fragilaria lucectorii, based on valve outline, shape of the apices and stria density. Comparisons are made with similar taxa and brief notes on their ecology are added.

Amphora copulata is a widely reported diatom name in water quality surveys throughout Europe. Following a survey of 1 000 floristic lists from the French part of the Rhine-Meuse basin (2000-2021), seven samples with the highest abundance of this species were selected for detailed examination-based on light (LM) and scanning electron microscopy (SEM) observations. In addition, samples from the Netherlands and Germany with a high abundance of the Amphora copulata species complex were also studied. The specimens previously identified as A. copulata sensu lato could not be assigned to an existing species based on the currently available literature, and proved to belong to two taxa described here as new: Amphora vanderwerffiana sp. nov. and Amphora lucectoreana sp. nov. The two new species are illustrated using both light and electron microscopy and compared to morphologically similar species. As such, type materials of A. eileencoxiae and A. neglectiformis, and recent material from the type locality of A. macedoniensis are also illustrated and discussed.

Type specimens of Synedra hamata W.Sm. have been examined and identified as a species of the genus Tabularia. This re-evaluation is discussed.

During the examination of a historical sample collected by Friedrich Hustedt, an unidentified taxon belonging to the genus Humidophila was observed. This taxon could not be matched with any known species based on the currently available literature. Following a thorough morphological analysis and comparison, the taxon is described as Humidophila vandevijveri sp. nov. Its morphology is documented and illustrated using both light microscopy (LM) and scanning electron microscopy (SEM). Humidophila vandevijveri can be distinguished from closely related species by its stria density and distinctive ultrastructural features. A detailed morphological comparison with similar taxa is provided, along with information on the associated diatom assemblage.

Thermo-mineral springs are specific ecosystems with extreme environmental conditions that constitute interesting models for studying metacommunity processes. Since these springs can be considered as islands within a terrestrial context, we first aimed to test some predictions of the theory of island biogeography and the distance-decay hypothesis on benthic diatom communities. Then, we aimed to quantify the influence of physical, chemical, climatic and spatial factors on species assemblages. We evaluated the species-area relationship for all springs and for several group of springs classified according to their environmental and hydrochemical composition. The influence of environmental variables on α-diversity was tested. We also investigated whether β-diversity was related to spring isolation or changes in environmental conditions. Finally, we determined the importance of environmental and spatial variables in shaping diatom communities using ordination and variation partitioning. We did not find any species-area relationship whatever the group of springs investigated, and no geographical distance-decay pattern was observed. We found a significant effect of physical and chemical parameters on α-diversity and composition. Thus, diatoms communities of thermo-mineral springs did not seem to be influenced by dispersal processes at the scale of our study but appeared to be patterned by physical and chemical factors.

Background and aims – Euodia frauenfeldii has the characteristic ‘semi-circular’ or ‘crescent-shaped’ valves found in most species of Euodia, but as it also has a pair of conspicuous transapical costae, it was later transferred to Eunotogramma. Examination of the structure of its valves and girdle has led to a reassessment of its taxonomic position. Material and methods – We discuss various aspects of terminology used for the parts of the valve, their abbreviations used throughout the literature and provide a detailed taxonomic background to both Euodia and Eunotogramma. Key results – A new genus, Moralesonia is described and its relationships discussed.

A new Planothidium species discovered in epiphytic samples from southern Brazil’s coastal lakes is described as new using light and scanning electron microscopy. Named Planothidium minuanum sp. nov., it is distinguished from the most similar species by its elliptical, slightly asymmetrical valves with pronounced short rostrate ends and coarse striae consisting of one to two rows of areolae. This taxon bears resemblance to the cavum-bearing species in the “Planothidium rostratum sensu lato” group and the recently described Planothidium xinguense from the Brazilian Amazon. This study compares the morphology and ecology of this new species with related taxa and provides insights into its occurrence in southern Brazilian coastal lakes.

The original description and illustrations of Nitzschia amphibia by Grunow in 1862 are insufficiently clear for the accurate identification of contemporary specimens, which may have contributed to misidentifications among N. amphibia, N. amphibioides, and N. semirobusta commonly found in the literature. This study aims to clarify the morphological differences between these three species by examining both historical and recent samples, along with the original description of N. amphibia. The species are compared morphologically, and a lectotype for N. amphibia is designated. This work also includes the first report of scanning electron microscopy images of N. amphibioides. While fibula morphology clearly distinguishes N. amphibia from N. semirobusta, measurements such as valve length, width, and the densities of striae and fibulae do not separate these two species. In contrast, N. amphibioides is distinctly different, characterized by longer, wider, and more robust valves with coarser striation. Additionally, this study reassesses various previously published records and revises some

Cocconeis ectorii and C. azteca are described as new species based on unialgal strains, cultivated from samples collected in streams of Mexico, California, and Germany. For C. ectorii, strains from Mayotte Island and Spain were re -analysed. Although these two new species belong morphologically within the C. placentula group, genetic distances for the nuclear -encoded 18S -V4 and the plastid gene rbcL show that these species are distinct from other species with available sequencing data. Morphologically, these two taxa can be distinguished from similar species by a combination of characters such as length, width, striae and areolae count, size of hyaline rim, number of submarginal areolae, by closed or open valvocopulae of both valves and their additional features fimbriation and smoothness. These characters are difficult to assess in environmental samples containing several taxa from this group especially when only small specimens are identified with light microscopy. Despite the disjunct origin of the 13 analysed C. ectorii strains, encompassing different climatic zones, their diagnostic morphological features are broad but stable, and share identical sequences for the two markers, indicative of a cosmopolitan distribution. Lastly, we provide occurrence notes for freshwater species of the placentuloid group based on eDNA metabarcoding data from subpolar, temperate, and tropical zones, further supporting the global distribution of C. ectorii and the probable endemic occurrence of C. azteca in North America.

In connection with its geological history, many thermo-mineral springs are present in the Massif Central (France). The physicochemical characteristics of these springs (temperature, mineralization, gas content) result both from their deep circulation and from the geological formations or surface waters with which they interact during their ascent. In mineral springs, diatoms are well represented. To date, no study has focused on coupling the origin of water with diatom communities, whether in the Massif Central or in other parts of the world. The purpose of this study is to cross the information obtained on the water and the diatoms in order to analyse the biodiversity and improve knowledge of the species autecology. Twelve mineral springs with some of which have high levels of radioactivity were studied in the Massif Central. As results, the geochemical facies, the origin of the water and the Total Dissolved Solids influence the taxonomic composition but not the specific richness, Shannon index and Evenness. Furthermore, Rn-222 influences diatom richness and community structure. Planothidium frequentissimum, Crenotia angustior and Sellaphora labernardierei were associated with high values of Rn-222 activity. The water temperature was a factor that influences diatom species distribution. Some species appears to be more sensitive to the composition of water, such as Planothidium frequentissimum which attests to a mixing between deep and surface waters.

Two species of Synedra that were first described in the Erbario Crittogamico Italiano (1858-1885) are re-described and transferred to Ulnaria.

Nanofrustulum fogedii sp. nov. is described based on a historic sample from Spitsbergen collected by Niels Foged. The new species, originally identified by Foged in 1964 as Nanofrustulum ( Melosira) cataractarum, can be separated from the latter by its larger valve diameter, longer striae composed of 1-6 transapically elongated areolae and a higher valve mantle. The species was observed in several samples collected from thermal springs in the Arctic Svalbard Archipelago. Based on its major morphological features, such as the quasifract girdle bands, the small valve dimensions and the serrated linking spines, the transfer of another small-celled araphid species, formerly described as Pseudostaurosira bardii Beauger et al. to the genus Nanofrustulum is justified.