Ancient lakes are among the most interesting models for evolution studies, because their biodiversity is the result of a complex combination of migration and speciation. Here, we investigate the origin of single celled planktonic eukaryotes from the oldest lake in the world – Lake Baikal. By using 18S rDNA metabarcoding we recovered 1,427 Operational Taxonomic Units (OTUs) belonging to protists populating surface waters (1-50 meters) and belonging to pico- and nano-plankton size fractions. The recovered communities resembled other lacustrine freshwater assemblages found elsewhere, especially the unclassified protists. However, our results suggest that a fraction of Baikal protists originated evolutionary recently from marine/brackish ancestors. Moreover, our results suggest that rapid radiation may have occurred among some protist taxa, partially mirroring what was already shown for multicellular organisms in Lake Baikal. We found potential species flocks in Stramenopiles, Alveolata, Opisthokonta, Archaeplastida, Rhizaria and Hacrobia. Putative flocks predominated in Chrysophytes, which are highly diverse in Lake Baikal. Some of the species, especially those from these flocks, may be endemic from Lake Baikal, because their 18S rDNA differed > 10% from known DNA. Overall, our study points to novel diversity of planktonic protists in Lake Baikal, some of which may have emerged in situ after evolutionary diversification.

Solar ultraviolet (UV) is among the most important ecological factors shaping the composition of biota on the planet surface, including upper layers of waterbodies. Inhabitants of dark environments recently descending from surface organisms provide natural opportunities to study the evolutionary losses of UV adaptation mechanisms and better understand how those mechanisms function at the biochemical level. Ancient Lake Baikal is the only freshwater reservoir, where deep-water fauna emerged, and its diverse endemic amphipods (Amphipoda, Crustacea) now inhabit the whole range from highly transparent littoral to dark depths of over 1600 m, which makes them a convenient model to study UV adaptation. Here we show that adults of deep-water Baikal amphipods Ommatogammarus flavus and O. albinus indeed have high sensitivity to environmentally relevant UV levels in contrast to littoral species Eulimnogammarus cyaneus and E. verrucosus. The UV intolerance was more pronounced in deeper-dwelling O. albinus and was explainable by lower levels of carotenoids and carotenoid-binding proteins. Signs of oxidative stress were not found but specifically UV-B seemingly led to accumulation of toxic compounds. Overall, the obtained results demonstrate that UV is an important factor limiting distribution of deep-water amphipods into the littoral zone of Lake Baikal.

We found out main regularities of surface waters pollution by diesters of о-phthalic acid (PAEs) at the background level of concentrations. Lake Baikal was selected as a model for our studies. The composition of PAEs in Baicalins waters is represented by dimethyl phthalate (DMP), diethyl phthalate (DEP), di-n-butyl phthalate (DnBP), and di-(2-ethylhexyl) phthalate (DEHP). Their concentrations are assessed in statistically valuable average values and ranges: 0.02 (0.01 – 0.02), 0.07 (0.06 – 0.09), 0.55 (0.47 – 0.66), 0.30 (0.26 – 0.34), µg/L, respectively. Main factors determining PAEs concentration are found out: year and season of sampling. The distribution of PAEs in the water body meets their hydrophobicity and is characterized by: i) even distribution of minor hydrophilic congeners DMP and DEP in the whole water area; ii) maximal concentrations of hydrophobic congeners DnBP and DEHP in the upper and near-bottom layers of the water column; iii) decrease of concentration of hydrophobic congeners in the near-shore zone. The contribution of near-shore sources of PAEs into the pelagic site is minimal, the main pollutants source is atmospheric transfer. A valuable contribution of DEHP from biogenic sources into the background level of PAEs is shown. Within RQ coefficients, we assessed the environmental risk for Baicalins biota.