Diatom assemblages of the brackish Bolshaya Samoroda River (Russia) studied via light microscopy and DNA metabarcoding

Diatoms are highly diverse and widely spread aquatic photosynthetic protists. Studies of regional patterns of diatom diversity are substantial for understanding taxonomy and biogeography of diatoms, as well as for ecological perspectives and applied purposes. DNA barcoding is a modern approach, which can resolve many problems of diatoms identification and can provide valuable information about their diversity in different ecosystems. However, only few studies focused on diatom assemblages of brackish rivers and none of them applied the genetic tools. Herein, we analyzed taxonomic composition and abundance of diatom assemblages in the brackish mixohaline Bolshaya Samoroda River flowing into the Elton Lake (Volgograd region, Russia) using light microscopy and high-throughput sequencing of the V4 region of the 18S rDNA gene amplicons. In total, light microscopy of the samples taken in 2011–2014 and 2018 allowed to distinguish 39 diatom genera, represented by 76 species and infraspecies taxa. Twenty three species of diatoms were recorded in the river for the first time. Next-generation sequencing revealed a larger number of diatom taxa (26 genera and 47 OTUs in two samples vs. 20 genera and 37 species estimated by light microscopy). As a result, sequences of Haslea, Fistulifera, Gedaniella were recorded in the river for the first time. Significant differences in the data obtained with molecular and light microscopy approaches are discussed. Some V4 18S rDNA sequences were characterized by a low similarity with homologues from the reference database. We revealed high spatial-temporal heterogeneity of the diatom assemblages, occurrence of freshwater species together with brackish and marine ones, and predominance of benthic and plankto-benthic species. Thus, investigations of diatoms in brackish rivers based on both morphological and molecular approaches provide a good chance of improving an understanding of diversity, ecology and biogeography of Bacillariophyta.