Artyom Marchenkov

Thaumatomonads are common in soil and fresh water and play an important role in microbial communities. They use silicon for the intracellular synthesis of species‐specific silica‐scales that cover the surface of their cells. Their diversity is not well studied, and the combination of electron microscopy and molecular phylogeny of marker genes allowed the discovery of a new taxon. In this work, we established the Thaumatomonas strain IW08m from the Irkutsk Reservoir Bay (Eastern Siberia, Russia) into a monoculture and, using various microscopy methods, described its cell structure, including the stage of silica‐scale formation. Based on combined data from the fine structure of the scales and phylogenetic analysis of the 18S rRNA gene, we showed congruence of morphological differences and genetic distances in the genus Thaumatomonas and described a new species, Thaumatomonas sibirica n. sp., and thus expanded our understanding of the diversity of thaumatomonads.

The study of aquatic biodiversity in the context of ecosystem sustainability is of urgent research importance, with several existing knowledge gaps. Among the under-studied groups are silica-scaled chrysophytes. Their cells are covered with silica scales and bristles/spines, the species-specific structure of which can be distinguished only by electron microscopy. In June and August 2024, samples were collected from a broad aquatic system comprising the southern part of Lake Baikal and a cascade of four reservoirs formed after the construction of hydroelectric dams on the Angara River flowing from Lake Baikal. Using electron microscopy, we identified 45 species of silica-scaled chrysophytes in phytoplankton in 2024, and the overall checklist was expanded to 57, accounting for interannual differences. Clear differences in species composition and richness were observed both between seasons and among reservoirs. Approximately a quarter of the recorded species were heterotrophs, which do not contribute to primary production, whereas 44% were phototrophs and 31% mixotrophs, both groups contributing to the Si cycle and to primary production. Continuous monitoring of reservoirs is essential for understanding the processes shaping silica-scaled chrysophytes diversity and may serve as an additional criterion for assessing the sustainability and transformation of freshwater ecosystems.

Diatoms demonstrate a high degree of differentiation, which allows them to quickly adapt to new environmental conditions. We analyzed genetic differentiation for monoclonal strains of the genus Ulnaria isolated from geographically distant water bodies. As a result of the cox1 analysis, it was found that the species Ulnaria acus, Ulnaria danica and Ulnaria ulna are characterized by population heterogeneity. It was found that the species U. acus has geographic isolation, as exemplified by the genotype from Lake Matano. For U. danica and U. ulna the geographical isolation of individual genotypes was not detected. In the course of our work, for the first time we were able to establish the fact of interspecific hybridization of diatoms by analyzing marker genes: nuclear (18S rRNA), chloroplast (rbcL) and mitochondrial (cox1, atp6).

Artificial reservoirs formed by hydroelectric dams are young ecosystems requiring water quality monitoring, as they often serve local populations. Traditionally, this is performed through hydrochemical and sanitary assessments, alongside phytoplankton composition analysis. This study aimed to assess the seasonal dynamics of microbial communities—both bacterioplankton and microeukaryotes including phytoplankton—in the Irkutsk Reservoir (IR), which is fed by the cold oligotrophic waters of Southern Baikal (SB). Using parallel metabarcoding of 16S and 18S rRNA gene fragments, we analyzed community composition during the open-water season and evaluated the ecological connectivity between these two freshwater systems. We demonstrated that seasonal changes in microeukaryotic communities were closely linked between SB and IR, with the greatest divergence observed in early summer and progressive convergence by autumn. Metabarcoding confirmed microscopy-based observations while providing higher taxonomic resolution and detecting otherwise overlooked groups. Bacterioplankton communities also exhibited seasonal variation and were shaped by environmental gradients and reservoir characteristics. Cyanobacteria peaked in SB in late summer but did not dominate communities, unlike in mesotrophic lowland reservoirs. These findings demonstrate the value of metabarcoding for freshwater monitoring and provide new insights into microbial community dynamics in river–reservoir systems influenced by oligotrophic lake inflow.

On a model natural object, the Lake Baikal–Angara River–Irkutsk Reservoir (IR), we studied changes in the qualitative and quantitative characteristics of phytoplankton communities over three seasons in 2023 depending on seasonal changes in habitat parameters. Of the 151 identified taxa, Chrysophyta (57), Chlorophyta (41) and Bacillariophyta (24) predominated in diversity. Over the entire observation period, the highest values of total biomass and total abundance were detected in the IR in June (hydrological spring) at a water temperature of 10.0–12.7 °C, and the lowest in August, despite the fact that the water warmed up to 20 °C. No mass blooms of Cyanobacteria were observed. Statistical analysis of species abundance profiles revealed that phytoplankton community structure varied across time and space. The direct effect of cold lake waters on the structure of phytoplankton in the reservoir was observed only in early June. In summer and autumn, the structures of phytoplankton in the lake and in the reservoir differed, even at the same water temperature. Low concentrations of phosphates and nitrates, high species diversity, the presence of cold-water species and species with a wide range of temperature preferences formed a dynamic spatiotemporal structure of IR phytoplankton, distinct from other temperate reservoirs, including Lake Baikal. The results obtained are important for understanding the mechanisms of formation of the flora of artificial reservoirs of temperate latitudes and for their monitoring, taking into account seasonal dynamics and the context of global climate warming.

In the context of global climate changes, it is important to assess the sustainability perspective of aquatic ecosystems based on marker organisms. In this work, we analyzed seasonal dynamics of silica-scaled chrysophytes in freshwater communicating environments which have considerable differences in water temperature between two ecosystems: the deep and cold oligotrophic Lake Baikal versus the shallower and warmer downstream Irkutsk Reservoir having mesotrophic features. During three seasons of the open water period of 2023, 38 species of silica-scaled chrysophytes were observed at 17 stations using scanning and transmission microscopy. The distribution of silica-scaled chrysophytes was shown to correlate with the water temperature. The greatest species richness was observed in the spring season in a large bay of the Irkutsk Reservoir (23 species), the smallest in the cold spring waters of Southern Baikal (up to 7 species). Widespread species living in Southern Baikal continued to grow in warmer waters of the reservoir. Using the example of silica-scaled chrysophytes, the stability of the high-latitudinal freshwater ecosystems affected by climate change is discussed. Continuous increment of the water temperature can lead to an increased abundance of widespread species and the displacement of psychrophilic species, affecting the overall biodiversity in such ecosystems.