Diatom cultures of the G. parvulum species complex were established from seven different sites in the Faroe Islands, Sweden, Germany, Mexico and Korea, and were studied in detail. Eight morphodemes were identified which corresponded to the descriptions of the cosmopolitan taxon G. parvulum (Kützing) Kützing sensu lato: its nominate variety (var. parvulum), G. parvulum var. exilissimum Grunow and G. parvulum f. saprophilum Lange-Bertalot & Reichardt, G. [parvulum var.] lagenula Kützing plus four unidentifiable morphodemes. The concatenated analysis of the sequences of the markers 18SV4, rbcL, and ITS as well as morphological data resulted in a separation of four taxa based on their biogeography in Mexico, Korea, central Continental Europe and Northern Atlantic Europe. Mantel tests showed a significant correlation between molecular and geographical distances. The diagnoses of two taxa, G. parvulum sensu stricto, and G. lagenula, were emended, G. saprophilum elevated to species rank and epitypes designated. One species was newly described.

In order to understand the taxonomy and phylogeny of the genus Gomphonema, the type of the name of the genus, G. acuminatum, and related species complexes are studied with an integrative taxonomic approach. Sixty-seven unialgal Gomphonema strains were established from 34 different water bodies in Croatia, Faroe Islands, Finland, Iceland, Germany, Mexico, Spain, Switzerland and Korea, which provided molecular (18S and rbcL), morphometric and ultrastructural data. Our study was complemented by molecular data from INSDC, summing up the studied data to 82 freshwater Gomphonema strains from the G. acuminatum and related species complexes which are here defined as the Gomphonema core group. Except for outline variations which occur even within one clone, their micro-morphologies are indistinguishable. The molecular data of the two markers clustered the taxa of the core group into one clade, well differentiated from other Gomphonema taxa such as G. augur and G. parvulum. For differentiation within the clade, the plastid marker rbcL provided better species resolution comparable to traditionally recognized taxa. The combined analysis of the molecular as well as morphological data resulted in the defining of only 7 subclades in contrast to 20 currently assigned names for the taxa of the Gomphonema core group in this study. The Gomphonema core group has been overdescribed due to the use of outline as the main criterion for species delimitation. A polyphasic approach, combining molecular and micromorphological data for taxonomy, nomenclatural evaluation, and observations from clonal cultures can reveal the full intricacies of evolutionary relations.

Background and aims – Within the project “German Barcode of Life – Diatoms” common diatoms of German waters were routinely isolated and cultivated. In order to understand the taxonomy and phylogeny of the genus Gomphonema, one of the most common taxa of Central Europe, known currently either under the name Gomphonema olivaceum in Europe or Gomphoneis olivacea in America, was studied. Methods – Twenty unialgal strains were established from five different water bodies in Germany and one from Lake Balaton, Hungary, which supplied molecular data (18S V4 and rbcL) besides morphometric and ultrastructural data. In addition, on eight populations from different water bodies including the type from Denmark, morphometric and micromorphological studies by light and scanning electron microscopy were performed. Key results – Molecular and micromorphological data show that the target taxon neither belongs to Gomphonema Ehrenb. nor to Gomphoneis Cleve. By reinstating the genus name Gomphonella Rabenh., the nomenclatural and taxonomic enigma of this taxon is solved, and with the presentation of the type by Hornemann the authorship of the epithet is clarified. Molecular data for the unialgal strains and several environmental clones show that there is more diversity in the Gomphonella olivacea clade than can be identified morphologically. In addition, the establishment of the new species Gomphonella coxiae and Gomphonella acsiae is supported. The molecular data classified Gomphonella species as belonging to the Cymbellales but not to the Gomphonemataceae. In addition, molecular data put Gomphoneis tegelensis R.Jahn & N.Abarca also into Gomphonella. In order to make the genera Gomphoneis and Gomphonema monophyletic, their astigmate members are transferred to Gomphonella. Conclusions – The results clarify that the gomphonemoid outline is not restricted to the family Gomphonemataceae but seem to be distributed across the entire order Cymbellales. This is shown in this paper for the revived genus Gomphonella, which contains the astigmate group of Gomphoneis and Gomphonema besides the longly disputed G. olivacea. Only a polyphasic approach, combining molecular and micromorphological data for taxonomy, nomenclatural evaluation, and observations from clonal cultures can reveal the full intricacies of evolutionary relations.

Temperature and salinity are some of the most influential abiotic parameters shaping biota in aquatic ecosystems. In recent decades, climate change has had a crucial impact on both factors—especially around the Antarctic Peninsula—with increasing air and water temperature leading to glacial melting and the accompanying freshwater increase in coastal areas. Antarctic soft and hard bottoms are typically inhabited by microphytobenthic communities, which are often dominated by benthic diatoms. Their physiology and primary production are assumed to be negatively affected by increased temperatures and lower salinity. In this study, six representative benthic diatom strains were isolated from different aquatic habitats at King George Island, Antarctic Peninsula, and comprehensively identified based on molecular markers and morphological traits. Photosynthesis, respiration, and growth response patterns were investigated as functions of varying light availability, temperature, and salinity. Photosynthesis–irradiance curve measurements pointed to low light requirements, as light-saturated photosynthesis was reached at <70 µmol photons m⁻² s⁻¹. The marine isolates exhibited the highest effective quantum yield between 25 and 45 S_A (absolute salinity), but also tolerance to lower and higher salinities at 1 S_A and 55 S_A, respectively, and in a few cases even <100 S_A. In contrast, the limnic isolates showed the highest effective quantum yield at salinities ranging from 1 S_A to 20 S_A. Almost all isolates exhibited high effective quantum yields between 1.5 °C and 25 °C, pointing to a broad temperature tolerance, which was supported by measurements of the short-term temperature-dependent photosynthesis. All studied Antarctic benthic diatoms showed activity patterns over a broader environmental range than they usually experience in situ. Therefore, it is likely that their high ecophysiological plasticity represents an important trait to cope with climate change in the Antarctic Peninsula.

Monoclonal diatom strains are the basis for integrative taxonomical studies combining molecular and morphological data for better taxonomic resolution and identification. The identification of diatoms, based on morphological features of the frustule, is difficult due to the variability of characters within and amongst species. The longer the cultivation time, the more cell size is reduced during asexual reproduction. Many divisions of the cell complicate identification as morphological characters of the frustule change at different rates in different taxa and teratologies can develop. Morphological variability raises the question whether the investigated genetic markers are stable over the cultivation time span. Sixteen monoclonal pennate diatom strains, identified as 10 different species in six genera, were cultivated for up to 18 or 24 months. It was shown that molecular data for the 18S-V4, rbcL, and psbA markers were stable for the entire cultivation time of all taxa. Images of the valves of the strains were taken every six months and examined morphometrically. Depending on the time since the start of the cultures, the morphological features showed high variability, particularly in length and shape, whereas width and number of striae, costae or fibulae were more stable, depending on the taxon and strain. A limited time span and good culture conditions seem to be prerequisite to avoid the production of teratological frustules. The results of parallel cultures in a unique experimental design are compared to observations from the literature dealing with single strains or special cognitive interests.

Polar regions are among the most extreme habitats on Earth. However, diatom biodiversity in those regions is much more extensive and ecologically diverse than previously thought. The objective of this study was to add knowledge to benthic diatom biodiversity in Western Antarctic coastal zones via identification by means of morphology, DNA metabarcoding and cultured isolates. In addition, a taxonomically validated reference library for Antarctic benthic diatoms was established with comprehensive information on habitat, morphology and DNA barcodes (rbcL and 18SV4). Benthic samples from marine, brackish and freshwater habitats were taken at the Antarctic Peninsula. A total of 162 clonal cultures were established, resulting in the identification of 60 taxa. The combination of total morphological richness of 174 taxa, including the clones, with an additional 73 taxa just assigned by metabarcoding resulted in 247 infrageneric taxa. Of those taxa, 33 were retrieved by all three methods and 111 only by morphology. The barcode reference library of Antarctic species with the new references obtained through culturing allowed the assignment of 47 taxa in the metabarcoding analyses, which would have been left unassigned because no matching reference sequences were available before. Non–metric multidimensional scaling analyses of morphological as well as molecular data showed a clear separation of diatom communities according to water and substratum types. Many species, especially marine taxa, still have no record in reference databases. This highlights the need for a more comprehensive reference library to further improve routine diatom metabarcoding. Overall, a combination of morphological and molecular methods, along with culturing, provides complementary information on the biodiversity of benthic diatoms in the region.