Diatoms are among the most diverse and environmentally significant protists on Earth. Like many other organismal groups, a large portion of their diversity appears to lie beyond the resolution of the traditional light microscopy-based methods routinely and sometimes exclusively utilized in their investigation. Although the technological and conceptual developments in the fields of molecular biology and bioinformatics unlocked a remarkable opportunity to study diatoms in a previously unimagined depth and breadth, molecular diatomists anecdotally claim that diatoms remain genetically understudied compared to other taxa. However, this claim has never been quantified and rigorously tested. Therefore, we performed a bibliometric analysis of over 42,000 WoS-indexed diatom documents published in the past 35 years, between 1988 and 2023. The claim is confirmed: only ~15% of the analyzed diatom literature incorporated molecular data, about half compared to other groups, including other algae, cyanobacteria, plants, fungi, and animals. Interestingly, research for all groups seems to asymptotically saturate with molecular methods once they are used in about one-third of the documents annually, an observation which has important implications. In addition, past trends in the use of molecular data in diatomology were explored and some future ones were predicted.

Diatoms are among the most important primary producers in alpine and polar freshwaters. Although temperate diatoms are sensitive to freezing, polar diatoms often exhibit more resistance. This is particularly true for members of the (predominantly terrestrial) Pinnularia borealis species complex. However, it remains unclear to what extent this resistance applies to other representatives of the genus. Here, we compare the freezing-stress tolerance of 11 freshwater, benthic strains representing different species of Pinnularia (including Caloneis) from polar, alpine, and temperate habitats. As vegetative cells, strains were exposed to freezing temperatures of −4, −10, −20, −40, −80, and −196°C. Survival was assessed by light microscopy and photosynthetic measurements. We observed vegetative cells to be sensitive to low freezing temperatures; only “mild” freezing was survived by all tested strains, and most tested strains did not survive treatments ≤−10°C. However, individual strain sensitivities appeared related to their original habitats. For example, polar and alpine strains better withstood “mild” and “moderate” freezing (−4 and −10°C, respectively); although temperate strains were significantly affected by the “mild” freezing treatment, polar and alpine strains were not. The −10°C treatment was survived exclusively by polar strains, and only P. catenaborealis survived all treatments. Interestingly, this species exhibited the lowest survival in the −10°C treatment, potentially implying some metabolic activity even at freezing temperatures. Thus, despite more extensive sampling throughout the genus and finer temperature scaling compared to previous studies, the remarkable freezing-stress tolerance of the P. borealis species complex remains unique within the genus.

This archive contains datasets for our submitted bibliometric analysis. It will be made public upon acceptance of the manuscript for publication.

Diatoms are among the most species-rich and environmentally significant protists on the Earth, playing important roles in biogeochemical cycles and food webs of many aquatic and semiaquatic ecosystems. However, a large portion of their diversity appears to lie beyond the resolution of the traditional microscopy-based methods routinely and sometimes exclusively utilized in the investigations of their populations, species, and communities. One such cryptic species complex is Pinnularia acidicola which was recently shown to consist of at least three different species-level lineages with distinct geographic ranges on the islands of the Southern Hemisphere and (when looked beyond morphological characters traditionally utilized in identification of diatom species) promising species-specific morphological differences. In this study, we are (1) adding one more P. acidicola complex species represented by seven monoclonal strains from the Northern Hemisphere, (2) analyzing all available evidence including morphology, genetics, phylogenetics, biogeography and ecology, and (3) suggestig taxonomic changes to formally resolve the P. acidicola complex. The name shall be reserved for the type population found on Ille de la Possession and Marion Island, while the populations found on Amsterdam Island, Réunion, and in Sweden should be described as three new species. For now, the species appear not only unambiguously distinguishable using standard diatom barcode markers 18S rDNA and rbcL, but also semicryptic (i.e., distinguishable by distinct geographic ranges), and to some extent even pseudocryptic (i.e., three out of four species appear distinguishable even under the LM due to discontinuities in relative fascia coverage). On the other hand, the species does not appear to be differentiated ecologically with all of them preferring slightly acidic soils, lakes, or streams.

Glacier recession is creating new waterbodies in proglacial forelands worldwide, including Antarctica. Yet, it is unknown how microbial communities of recently formed 'young' waterbodies (originating decades to a few centuries ago) compare with established 'old' counterparts (millennia ago). Here, we compared benthic microbial communities of different lake types on James Ross Island, Antarctic Peninsula, using 16S rDNA metabarcoding and light microscopy to explore bacterial and diatom communities, respectively. We found that the older lakes host significantly more diverse bacterial and diatom communities compared to the young ones. To identify potential mechanisms for these differences, linear models and dbRDA analyses suggested combinations of water temperature, pH, and conductivity to be the most important factors for diversity and community structuring, while differences in geomorphological and hydrological stability, though more difficult to quantify, are likely also influential. These results, along with an indicator species analysis, suggest that physical and chemical constraints associated with individual lakes histories are likely more influential to the assembly of the benthic microbial communities than lake age alone. Collectively, these results improve our understanding of microbial community drivers in Antarctic freshwaters, and help predict how the microbial landscape may shift with future habitat creation within a changing environment.