Abstract
The paper presents the results of a study of the hypothetical evolution of unicellular hydrobionts from Verhniy Kaban Lake of Kazan by 18S rRNA marker gene with aim of identifying species saprobity based on molecular phylogenetic analysis. Verhniy Kaban Lake is an isolated freshwater reservoir. Its coastline is relatively smooth except for the northern end, which forms swampy and overgrown bay. To the West of lake an urban settlement is situated and to the East are highway and garden plots. Lake length is about 1 km, average and maximum widths are 245 m and 370 m respectively. Deepest point is 15 m; average depth is 8 m; lake area – 25 hectares. Quality of lake water is classified as polluted. One of methods used for ecological assessment of water reservoirs is the bioindication method based on ability of indicator species to survive in water of certain pollution degree. At present, freshwater hydrobiont species from V.Sladechek’s list (1973) are used as indicators. Indicator species lists expand very slowly due to the long process of experimental observations. At the same time, price drop of the next-generation sequencing technologies allows to identify more organisms and run molecular phylogenetic analysis with aim of identifying species saprobity based on study of hypothetic evolution of hydrobionts by marker genes including 18S rRNA. The advantage of using 18S rRNA/16S rRNA ribosomal genes is that they are present in all organisms. Ribosomal genes are ones of the most conservative genes. Therefore, the systematic position of the organism and the time of divergence with closely related species can be determined via analysis of similarities and differences in rRNA sequences. Molecular phylogenetic trees were constructed by the neighbour joining (NJ) and maximal parsimony (MP) methods for Verhniy Kaban Lake hydrobionts identified with next-generation sequencing method (Illumina) by 18S rRNA marker gene (SRR7510986, SRR7465374, SRR7516513). Phylogenetic analysis of hydrobionts of Verhniy Kaban Lake by 18S rRNA marker gene revealed 12 clusters with a high bootstrap value (68–100 %), containing 12 existing indicator species of hydrobionts and 13 unicellular algae and ciliates – indicator species of different saprobity that did not previously have the status of bioindicators – algae: Diplosphaera mucosa (o-b-saprobity), Mucidosphaerium pulchellum (p-a-saprobity), Dinobryon pediforme (b-mesosaprobity), Gomphoneis minuta (b-mesosaprobity), Nitzschia amphibia (o-b-saprobity), Rhabdonema adriaticum (o-b-saprobity), Synura mammillosa (b-o-saprobity), Cryptomonas ozolini (b-mesosaprobity); ciliates: Astylozoon enriquesi (b-mesosaprobity), Blepharocorys curvigula (p-saprobity), Enchelys gasterosteus (p-saprobity), Monocoronella carnea (a-b-mesosaprobity), Plagiopogon loricatus (b-a-mesosaprobity). Most identified indicator species group near b-mesosaprobity. This indicates ecological state of Verhniy Kaban Lake as polluted by ecological assessments.
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