Freezing stress tolerance of benthic freshwater diatoms from the genus Pinnularia: Comparison of strains from polar, alpine, and temperate habitats.

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.

Recently formed Antarctic lakes host less diverse benthic bacterial and diatom communities than their older counterparts.

Glacier recession is creating new water bodies 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.

On the relativity of species, or the probabilistic solution to the species problem.

For centuries, both scientists and philosophers have discussed the nature of species resulting in c. 35 species concepts proposed to date. However, in our opinion, none of them incorporated neither recent advances in evolutionary genomics nor dimensionality of species in befitting depth. Our attempt to do so resulted in the following conclusions. Due to the continuous nature of evolution (regardless of its rate and constancy), species are inevitably undefinable as natural discontinuous units (except those originating in saltatory speciation) whenever the time dimension is taken into consideration. Therefore, the very existence of species as a natural discontinuous entity is relative to its dimensionality. A direct consequence of the relativity of species is the duality of speciators (e.g., incipient species) meaning that, in a given time, they may be perceived as both being and not being a species. Finally, the most accurate way to reflect both the relativity of species and the duality of speciators in species delimitation is probabilistic. While the novelty of these ideas may be questionable, they still deserve more extensive attention from the biological community. Here, we hope to draw such attention by outlining one of the possible pathways towards a new kind of probabilistic species delimitation methods based on the probability of irreversible divergence of evolutionary lineages. We anticipate that our probabilistic view of speciation has the potential to facilitate some of the most serious and universal issues of current taxonomy and to ensure unity of the species-level taxonomy across the tree of life.

Cationic Versus Anionic Phthalocyanines for Photodynamic Therapy: What a Difference the Charge Makes.

The literature reports on cationic and anionic phthalocyanines (Pcs) for photodynamic therapy suggest systematically significant differences in activity. In this work, ten different zinc(II) Pcs with carboxylate functions or quaternary nitrogens (hydrophilic anionic, hydrophilic cationic, amphiphilic anionic, and amphiphilic cationic) were investigated, with the aim of revealing reasons for such differences. In vitro assays on HeLa, MCF-7, and HCT-116 cells confirmed higher photoactivity for cationic Pcs (EC50 ∼ 3-50 nM) than for anionic Pcs (EC50 ∼ 0.3-10 µM), the latter being additionally significantly more active in serum-free medium. The environmental pH, binding to serum proteins, interaction with biomembranes, differences in subcellular localization, and relocalization after irradiation were found to be the main factors contributing to the generally lower photoactivity of anionic Pcs than that of the cationic derivatives. This result is not limited only to the presented derivatives and should be considered in the design of novel photosensitizers.

A polyphasic approach to the delimitation of diatom species: a case study for the genus Pinnularia (Bacillariophyta).

Diatoms are one of the most abundant and arguably the most species-rich group of protists. Diatom species delimitation has often been based exclusively on the recognition of morphological discontinuities without investigation of other lines of evidence. Even though DNA sequences and reproductive experiments have revealed several examples of (pseudo)cryptic diversity, our understanding of diatom species boundaries and diversity remains limited. The cosmopolitan pennate raphid diatom genus Pinnularia represents one of the most taxon-rich diatom genera. In this study, we focused on the delimitation of species in one of the major clades of the genus, the Pinnularia subgibba group, based on 105 strains from a worldwide origin. We compared genetic distances between the sequences of seven molecular markers and selected the most variable pair, the mitochondrial cox1 and nuclear encoded LSU rDNA, to formulate a primary hypothesis on the species limits using three single-locus automated species delimitation methods. We compared the DNA-based primary hypotheses with morphology and with other available lines of evidence. The results indicate that our data set comprised 15 species of the P. subgibba group. The vast majority of these taxa have an uncertain taxonomic identity, suggesting that several may be unknown to science and/or members of (pseudo)cryptic species complexes within the P. subgibba group.