In the marine environment, bacteria and viruses play a significant role in carbon fluxes, remineralization processes, and the infection of various organisms. We performed a survey in the northeastern Barents Sea, a region adjacent to the Arctic Ocean, to investigate spatial patterns of microbial plankton, after the main productive period, in October 2020. Two main water masses occurred in the study region—colder Arctic Water and warmer Barents Sea Water, representing transformed Atlantic Water. Multivariate analyses detected patchiness in the horizontal distribution of bacteria and viruses, and their abundances showed no clear association with the water masses. There was an obvious vertical pattern in microbial concentration, with the highest estimates in the upper layers. Surface viral and bacterial abundance varied in a wide range (2.20 × 105–10.7 × 105 cells mL−1 and 0.86 × 106–14.98 × 106 particles mL−1, respectively) and were correlated with each other. Bacterioplankton was dominated by small-sized cells (<2 μm, 0.04–0.06 µm3), and the average volume of bacterial cells tended to increase toward the seafloor. The ratio of viral to bacterial abundance (VBR) was 11 ± 1 and did not differ between water masses and depth layers. VBR were higher, compared to summer values, suggesting a strong impact of viruses on bacterioplankton, after the main productive season. Redundancy and correlation analyses showed that inorganic nutrients (nitrate and phosphate) and organic carbon from zooplankton were most responsible for the total variability in the microbial parameters. Water temperature and salinity, also, had a measurable impact, but their influence was lower. Bacterial abundance was lower than in other seasons and regions of the Barents Sea, while viral abundance was comparable, suggesting a stronger viral impact on Arctic marine bacteria in the autumn season.

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We studied the spatial patterns of the planktonic ecosystems at two Arctic sites strongly affected by Atlantic Inflow (FS, the Fram Strait; and BS, the Barents Sea). A high degree of similarity in the bacterial abundance (mean: 3.1 × 105 cells mL−1 in FS vs. 3.5 × 105 cells mL−1 in BS) was found, while other plankton characteristics were different. Bacterial biomass reached a maximum in BS (3.2–7.9 mgC m−3), while viral abundances tended to be higher in FS (2.0–5.7 × 106 particles mL−1). Larger bacterial cells were found in BS, suggesting the presence of different bacterial populations at both locations. The virus-to-bacteria ratio was significantly higher in FS than in BS (13.5 vs. 4.7). Chlorophyll a concentration was extremely low (<0.25 mg m−3). The highest zooplankton abundance was in the surface layer (919 individuals m−3 in FS vs. 602 ind. m−3 in BS). Zooplankton biomass strongly varied (1–39 mgC m−3), with the maximum in BS. High proportions of boreal taxa in the total zooplankton abundance indicate the Atlantification of pelagic ecosystems in the Arctic. Plankton indicators are correlated with temperature, salinity, and sampling depth. Strong intercorrelations were found between major plankton groups, suggesting tight links in the studied plankton ecosystems.

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The purpose of our research was to study the effects of probiotic feed supplement “Bio-Mos” on the structure changes of digestive apparatus of two-year-old channel catfish and carp. Materials and methods of research. Studies on the influence of a probiotic supplement on the organism of two-year-old channel catfish were conducted on the basis of the Prydniprovsky industrial warm-water fish farm under the conditions of the State Enterprise "Nyvka" of the Institute of Fisheries of the National Academy of Agrarian Sciences of Ukraine. Carp and channel catfish intestines were investigated during the growing season (May-September). Fish received a probiotic additive “Bio-mos” with feed. The preparation was fed for carp in a concentration of 2%, and for channel catfish – 5% of the feed weight. For histological studies, the samples of biological material were carried out using an express method of modification of the pouring of fish tissues into paraffin mixtures. Histometry of objects (determination of linear dimensions of histological objects) was carried out according to Avtandilov G. G. For microscopic histocyte studies, a manual "Atlas of histology and embryology of industrial fish of Ukraine", "Atlas of microscopic structure of fish liver", "Atlas of histology and histochemistry of freshwater fish" and "Fish Histology and Histopathology" were used. Results of the research. As histologic studies revealed the complex use of the preparation "Bio-mos" in the composition of feedstuffs had a positive effect on the development of certain sections of the fish intestinal tube. Probably this may be explained by the synergy of the individual components of the diet. Concerning the two-year-old species of the channel catfish, we can say that the histostructure of the wall of the esophagus and the thick intestine in the control and experimental individuals were almost identical and did not reveal any changes in the entire length of the sites. Separate modifications at the microlevel were recorded in the stomach wall. Our studies showed that usage of the drug "Bio-mos" in the composition of feed in the area of large curvature of stomach of investigated individuals increased secretory activity of the glandular structures. It was also found in fish that received feed supplement, enlightenment of most gastric glands, somewhat expanded. It should be noted that exorcinocytes, due to the influence of individual components of the drug "Bio-mos", gain increased secretory activity. It was noted that the increase of secretory activity of the gastric wall of experimental fish had a focal character, which shows an increase in the intensity of the body as a whole. Investigation of the structure of the thin intestine of carp and channel catfish at the microlevel revealed a number of common features that were observed within the studied species: The cell walls of a single-layered high prismatic epithelium of the mucous membrane were almost obscured indifferently, in connection with which the entire cellular layer was observed in the form of a homogeneous, optically homogeneous mass. Cytoplasm of epithelial cells was characterized by increased oxyfilm. At the apical end of the cells there was a low rim, formed by a huge amount of cytoplasmic processes that had its own type of dark strip. The nuclei of the epitheliocytes were oval or highly elongated, oriented closer to the basal end; in them there were clearly visible lumps of chromatin and sharply oxyphilic nucleols. Occasionally there were mitotic cells. Among the prismatic cells were often glandulotsity. Under the epithelium was a plate of mucous membrane, represented by a layer of loose connective tissue or the same, it consisted of collagen membranes that form a three-dimensional barely noticeable network. The muscle layer of the mucous membrane was represented only by separate elongated very thin smooth muscle cells with rod-shaped cores lying under the epithelium. The muscle of the intestinal tube was two-layer. The inner layer of the circular muscle on the histopreparation was cut along. The nucleus of the muscle cells was round, lying in the center and surrounded by a light rim of the cytoplasm (where the incision did not take the nucleus, nuclear-free sites were visible). Between the circular and longitudinal layers of smooth muscle lied the intermucosal layer of fine puffy connective tissue, in it – the capillaries, which were determined by the nuclei of the endothelium. On the histological preparations of the small intestine of the carp, it was noted that the circular muscle layers, the own plate of the mucosa with accompanying intramural ganglia, vascular elements and connective tissue, as well as the components of the intestinal folds within the studied groups, were morphologically identical. In fish of the control group, the vast majority of intestinal folds were in a free state, at the same time, in the majority of experimental individuals, peculiar anastomoses of the folds were observed in the mucous membrane of the small intestine. This phenomenon had a local character, but it clearly illustrated the increase in surface area of absorption. The experimental group was found to change the intestinal fold and had a relatively developed capillary network, indicating optimization of metabolic processes in the intestinal tube. The own plate of the fold of the mucous membrane had a pronounced hyperchromatosis. Probably, their concentration within this histological structure was stable in nature – this fact clearly reflected an increase in the immune status of fish, and, consequently, an improvement in the health indicators of individuals. It should be noted that in the investigated fish, modification changes in the form of individual centers are considered also in the caudal direction of the intestinal tube, but were not expressed in such contrast. Conclusions. Investigation the influence of the probiotic drug "Bio-mos" on the intestines of the channel catfish were established changes in its structure, and in particular noted that exorcinocytes, as a result of the influence of individual components of the drug become elevated secretory activity. This phenomenon is focal in nature, and shows an increase in the intensity of the body as a whole. In carps, that received the drug Bio-mos in the composition of feed, the length of the intestinal folds exceeds the length of the individuals in the control group (an average of 1.6 times). Accordingly, the change in length increases the surface area of absorption. Complex use of the preparation "Bio-mos" in the composition of feed for fish, has a modifying effect on the mucous membrane of the intestinal tube in the form of stimulation of secretory activity of the glands of the stomach, increasing the area of the absorbent surface of the small intestine, accelerating blood flow and modulating the immune status of fish organism in whole.