Saline and hypersaline waters are one of the most peculiar ecosystems of our planet, characterized by extreme life conditions. Despite their worldwide distribution, the diversity and abundance of protist communities in these ecosystems remain poorly studied. Here, we analyze planktonic communities of protists sampled across 38 saline and hypersaline water environments (2–390‰) from arid climatic zones of the South Urals and Crimea in light of environmental data using high-throughput 18S rDNA amplicon sequencing. A total of 9 eukaryotic supergroups, 34 phyla, 104 classes, 184 orders, 315 families and 548 genera have been identified. We revealed significant differences in the taxonomic structure of protist communities depending on salinity, geographic location and pH. The protist communities demonstrated linear regression of richness and diversity and growth of the percentage of unclassified Eukaryota (up to 43%) with the increase in salinity. Centrohelids demonstrated the ability to inhabit a broad range of salinities, up to 320‰, which is four times higher than previously reported. Centrohelid species Pinjata ruminata and Yogsothoth sp. are assumed to be specifically adapted to salinity of 3–210‰. The obtained results provide insight into the taxonomy and diversity of protists in saline and hypersaline environments and highlight the great potential for the discovery of new taxa due to the large number of unclassified 18S rDNA sequences.

Introduction. Acinetobacter baumannii is a nosocomial pathogen that causes hospital-acquired infection in immunocompromised individuals and has multiple drug resistance. A. baumannii is the priority microorganism that requires new therapeutic alternatives to replace the use of antibiotics. Corynebacterium spp. may represent a promising therapeutic alternative, given the important role of individual representatives in microbiome-mediated defense against pathogens. The aim of the work is to investigate the antimicrobial activity of cell-free supernatants (CFSs) of Corynebacterium amycolatum against clinical isolates of A. baumannii. Materials and methods. The effect of CFSs of C. amycolatum on the growth of planktonic culture and biofilm formation of clinical isolates of A. baumannii was studied in 96-well polystyrene plates. The production of exopolysaccharides by A. baumannii after co-cultivation with CFSs of C. amycolatum was studied using the phenol-sulfuric acid method. The morphology of biofilms and the viability of test strains after preliminary treatment with CFSs of C. amycolatum were studied using scanning electron and fluorescence microscopy. The composition of the CFS of extract was studied using the GC-MS method. Results. An inhibitory effect of CFSs of C. amycolatum on the growth of planktonic culture of all A. baumannii test strains was established. With regard to biofilm formation, the effect of CFSs was multidirectional. SEM results showed that after co-cultivation with CFSs of C. amycolatum, A. baumannii cells were scattered over the glass surface, their cell membrane was not damaged. Using fluorescence microscopy, the absence of metabolic activity of most cells, characterizing their death, was detected. GC-MS analysis revealed the presence of 4 volatile compounds including 1,4-diaza-2,5-dioxo-3 isobutylbicyclo[4.3.0]nonane, 1,2-Benzenedicarboxylic acid, dibutyl ester, ergotaman-3',6',18-trione, 12'-hydroxy-2'-methyl-5'-(phenylmethyl)-, (5'alpha)- and pyrrolo[1,2-a]pyrazine-1,4-dione, hexahydro-3-(phenylmethyl)-. Conclusions. The obtained data on the antimicrobial and antibiofilm activity of C. amycolatum CFSs against A. baumannii are another argument indicating the probiotic properties of C. amycolatum. The identified chemical compounds in CFS open up the prospect for further research aimed at developing new strategies to combat infections caused by A. baumannii.