Description and Genomic Analysis of the First Facultatively Lithoautotrophic, Thermophilic Bacteria of the Genus Thermaerobacter Isolated from Low-temperature Sediments of Lake Baikal

Members of the genus Thermaerobacter belong to the phylum Firmicutes and all isolates characterised to date are strictly aerobic and thermophilic. They were isolated from a mud sample of the Challenger Deep in the Mariana Trench, hydrothermal vents, and silt compost. A novel thermophilic, facultatively lithoautotrophic bacteria of the genus Thermaerobacter, strain PB12/4term (=VKM B-3151T), with a metabolism that is uncharacteristic of the type species, was isolated from low-temperature surface sediments near the Posolsk Bank methane seep, Lake Baikal, Russia. The new strain grows with molecular hydrogen as electron donor, elemental sulfur, and thiosulfate as electron acceptors, and CO2/[Formula: see text] as carbon source. The genome of strain PB12/4term consists of one chromosome with a total length of 2.820.915 bp and the G+C content of the genomic DNA was 72.2%. The phylogenomic reconstruction based on 120 conserved bacterial single-copy proteins revealed that strain PB12/4term belongs to the genus Thermaerobacter within in the class Thermaerobacteria, phylum Firmicutes_E. The strain PB12/4term is closely related to Thermaerobacter subterraneus DSM 13965 (ANI=95.08%, AF=0.91) and Thermaerobacter marianensis DSM 12885 (ANI=84.98%, AF=0.77). Genomic and experimental data confirm the ability of the Thermaerobacter PB12/4term pure culture to facultatively lithotrophic growth, which is provided by the presence of [NiFe]hydrogenase enzymes that are absent in T. marianensis DSM 12885 and T. subterraneus DSM 13965. The data obtained on the physiological and biochemical differences of strain PB12/4term provide a deeper insight into the species diversity and functional activity of the genus Thermaerobacter.

Toxic Effect of Anionic Surfactants on Freshwater Sponge Lubomirskia baikalensis and Its Endosymbiotic Microalgae Chlorella sp

A number of reports on sponge diseases, including from Lake Baikal, have increased dramatically all over the world in recent years. Herewith, there are various hypotheses for sponge mortality. Lubomirskia baikalensis (phylum Porifera, order Spongillida) is a unique endemic freshwater sponge of Lake Baikal that contains a complex community of eukaryotic and prokaryotic endosymbiotic microorganisms. In this work, we present the first results for the effect of anionic surfactants viz. linear alkylbenzene sulphonates (LAS) at low 10 and 20 µg L−1 concentrations on Baikal sponge species and their symbiotic community as an experimental model. A new toxicity test protocol under conditions close to natural is proposed. It uses the sponge amoebocytes called as SA1-cells, which contain eukaryotic green microalgae Chlorella sp. These SA1-cells are shown to be representative indicator in assessing the impact of anionic surfactants. The acute toxic effect resulted in 97–100% sponge cell death in less than 48 h, as well as 100% symbiotic microalgae Chlorella sp. death over 72 h was noted under LAS solution (20 μg L−1) exposure during in vivo experiments. This includes the cell membrane fatty acid changes, change in the cell sizes, cell swelling, and lysis. Long term exposure to LAS solution (10 μg L−1) reflected in cellular stress (oxidative stress) and accompanied by malondialdehyde formation (0.16–2.0 μg g−1 of dry weight) during 14-day exposure was noted. Oxidative stress and mortality of L. baikalensis are associated with their low antioxidant activity. Trolox-equivalent antioxidant capacity (TEAC) found in the range from 0.00031 to 0.00077 Trolox equivalents for these freshwater sponges.

Nonspecific response of Lake Baikal phytoplankton to anthropogenic impact

In this study, we present the first results on oxidation stress in Lake Baikal phytoplankton and its adaptation to environmental changes under anthropogenic impact. As was shown, the changing of the dominant species of phytoplankton collected from the surface water layer (~0.3 m) took place from February to June 2021. Phytoplankton were collected at a nearshore station (a littoral station at a distance of ~0.01 km from the shoreline, depth to bottom is ~5 m) and an offshore station (a pelagic station at a distance of ~1 km from the shoreline, depth to bottom is ~543 m). In February, dinoflagellates were dominant (~40 %) as well as diatoms (≤33 %) and green algae (≤12 %). Their biomass was 100 mg·m–3. In March, chrysophytes were dominant (up to 50 %) as well as cryptophytes (≤43 %) and dinoflagellates (≤30 %). Their biomass was 160–270 mg·m–3. In April, biomass increased up to 700–3100 mg·m–3 with the dominance of large cell dinoflagellates (up to 99 %), chrysophytes (up to 50 %), and cryptophytes (up to 35 %). By the end of the first decade of May, the percentage of dinoflagellates decreased and that of cryptophytes increased. In the second decade of May, the percentage of diatoms increased up to ~26–38 % but phytoplankton biomass was minimal (13–30 mg·m–3). By June, the percentage of diatoms in the samples reached 44–75 % at 60–550 mg·m–3. The oxidation stress of phytoplankton as a nonspecific adaptive response to a prolonged, intensive, or recurrent effect of a stress factor was estimated from the content of thiobarbituric acid reactive substances (TBARS). The mean content of these substances (markers of the lipid peroxidation) was determined spectrophotometrically. The oxidation stress of phytoplankton was revealed only when diatom algae dominated. It can be explained by adaptation of algae of other classes to the stress factor. The content of the lipid peroxidation markers in the coastal phytoplankton collected close to the settlement of Listvyanka known as a large touristic center was estimated from 100 to 500 μg·g–1 of dry weight of sample. During the period of diatom blooming in 2016 and 2018, oxidation stress of phytoplankton collected near large settlements was found. In phytoplankton from deep-water pelagic stations most remote from settlements, stress was not revealed. Using the method of gas chromatography, we showed a lower (up to 15 %) content of polyunsaturated fatty acids in phytoplankton characterized by stress occurrence. This confirms cell membrane damages. In Lake Baikal surface water, we found a higher content of synthetic anionic surfactants (sodium alkylbenzene sulfonates), which are components of detergents and cause oxidation stress of hydrobionts (up to 30 ± 4 μg·L–1). The presence of these substances in a water ecosystem can result in exhausting of phytoplankton cell resources, homeostasis imbalance, stress, pathological changes, and rearrangements in phytoplankton assemblage