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.

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.

In Kongsfjorden (Spitsbergen), we quantified the zonation of visually dominant macroalgal taxa and of detached macroalgae from underwater videos taken in summer 2009 at six transects between 2 and 138 m water depth. For the first time, we provide information on the occurrence of deep water red algae below the kelp forest and of detached macroalgae at water depth > 30 m. The presence and depth distribution of visually dominant red algae were especially pronounced at the outer fjord, decreased with proximity to the glacial front and they were absent at the innermost locations. Deepest crustose coralline red algae and foliose red algae were observed at 72 and 68 m, respectively. Brown algae were distributed along the entire fjord axis at 2–32 m. Green algae were only present at the middle to inner fjord and at areas influenced by physical disturbance at water depths of 2–26 m. With proximity to the inner fjord the depth distribution of all macroalgae became shallower and only extended to 18 m depth at the innermost location. Major recipients of detached macroalgae were sites at the shallower inner fjord and at the middle fjord below the photic zone at depths to 138 m. They may either fuel deep water secondary production, decompose or support carbon sequestration. Univariate and community analyses of macroalgal classes including detached macroalgae across transects and over depths reveal a considerable difference in community structure between the outermost sites, the central part and the inner fjord areas, reflecting the strong environmental gradients along glacial fjords.

The Euphrates-Tigris catchment area is one of the major drainage basins in the Middle East but climate change, overuse of waters and subsequent salinization has brought this region to the brink of ecological devastation. The aim of this study was to investigate the biodiversity of epiphytic diatoms in Iraq, for the first time using a combination of morphological and metabarcoding methods. 47 samples were collected at six sampling sites along the Rivers Tigris and Euphrates in northern Basra, Southern Iraq, during the summer of 2019 (dry season) and winter of 2020 (rainy season). The composition of the epiphytic diatoms in each sample was compared with both methods. The microscopic-morphological analysis of the environmental samples resulted in the identification of 284 infrageneric taxa in 59 genera. For the metabarcoding analysis the V4 region of the 18S marker gene was used and resulted in 1454 ASVs (Amplicon sequence variants) comprising 54 genera and 108 species with several ASVs belonging to the same taxa. The non-metric multidimensional scaling (NMDS) plots for morphology showed a clear seasonal effect in the diatom community composition, while the NMDS plots for 18SV4 analyses did not show a seasonal effect in community composition for the same samples. The incomplete taxonomic reference database for the studied sites turned out to be the major limitation of the molecular approach. Nevertheless, the combination of morphological and molecular methods increased the detection and identification of the diatom assemblages and laid the foundation for understanding their biodiversity in this region.

The diatom genus Chamaepinnularia was first published by Lange-Bertalot et Krammer in 1996 to accommodate several small species previously included within Navicula and Pinnularia. Despite its morphological similarity to those two genera, the family-level classification of Chamaepinnularia has been considered incertae sedis since its description almost three decades ago. We provide the first molecular characterisation (18S and rbcL) of the genus based on cultured polar strains and investigated its phylogenetic placement. Molecular data are complemented with observations on living cells, as well as detailed examination of oxidized material with light microscopy and scanning electron microscopy. The 12 investigated strains were morphologically identified as three taxa: two already described species (C. gerlachei, C. krookii) and one here newly described (C. australis sp. nov.). These species formed three separate, well-supported branches in the phylogeny. Our analyses placed Chamaepinnularia as a sister group to members of the Sellaphoraceae for which sequence data exist (i.e. Sellaphora, Fallacia, Rossia and Diprora). Based on the presence of hymenate areolae, a single Hshaped plastid with girdle appressed plates and the results of the phylogenetic trees, we propose the inclusion of Chamaepinnularia within the family Sellaphoraceae, which altogether form a supported monophyletic group.

Since knowledge on the biodiversity of marine benthic diatoms from the Arctic-North Atlantic region is still limited, a detailed examination of unialgal strains was undertaken using light microscopy, scanning electron microscopy and molecular markers. Three samples were collected from marine habitats along the west coast of Iceland and eight unialgal strains representing six taxa were established. The combined evidence from micromorphological and morphometric traits, together with molecular data support the description of five new species. Three of them belong to the genus Navicula Bory: Navicula jahniae sp. nov., Navicula vesturlandica sp. nov. and Navicula pseudoperminuta sp. nov. In addition, Nitzschia olafsvikensis sp. nov. and Tryblionella ytritungaensis sp. nov. are described. Also included in this study is a species with cosmopolitan distribution, Navicula gregaria Donkin . The Icelandic strain exhibits molecular differences compared to strains from other localities, suggesting the existence of geographically distinct populations. The high number of new species observed in this study, based on only eight strains, suggests that a considerable degree of unknown diversity still exists among marine benthic diatoms. Consequently, careful taxonomic work remains necessary in under-explored coastal regions such as Iceland.