Among the Arctic seas, the largest volume of river runoff (~45% of the total river-water inflow into the Arctic Ocean) enters the Siberian Kara Sea. The viral communities of the Kara Sea are important for the functioning of the marine ecosystem. Studies of virus–prokaryote interactions on the Kara Sea shelf have been conducted only in spring and autumn. Here, we investigated the abundance of free viruses, viruses attached to prokaryotes, and pico-sized detrital particles; the morphology (shape and size) of the viruses, viral infection and virus-mediated mortality of prokaryotes in early summer, i.e., during a seasonal ice melting period and maximum inflow of river-water volumes with high concentrations of dissolved and suspended organic carbon. Seawater samples for microbial analyses were collected across the Kara Sea shelf zone on board the Norilskiy Nickel as a research platform from June 29 to July 15, 2018. Abundances of prokaryotes (range (0.6–25.3) × 105 cells mL−1) and free viruses (range (10–117) × 105 viruses mL−1) were correlated (r = 0.63, p = 0.005) with an average virus: prokaryote ratio of 23.9 ± 5.3. The abundance of free viruses and viral-mediated mortality of prokaryotes were significantly higher in early summer than in early spring and autumn. Free viruses with a capsid diameter of 16–304 nm were recorded in the examined water samples. Waters in the Kara Sea shelf contained high concentrations of suspended organic particles 0.25–4.0 µm in size (range (0.6–25.3) × 105 particles mL−1). The proportions of free viruses, viruses attached to prokaryotes, and viruses attached to pico-sized detrital particles were 89.8 ± 6.0%, 2.2 ± 0.6% and 8.0 ± 1.3%, respectively, of the total virioplankton abundance (on average (61.5 ± 6.2) × 105 viruses mL−1). Viruses smaller than 60 nm clearly dominated at all studied sites. The majority of free viruses were not tailed. We estimated that an average of 1.4% (range 0.4–3.5%) of the prokaryote community was visibly infected by viruses, suggesting that a significant proportion of prokaryotic secondary production, 11.4% on average (range 4.0–34.0%), was lost due to viral lysis. There was a negative correlation between the abundance of pico-sized detrital particles and the frequency of visibly infected prokaryotic cells: r = −0.67, p = 0.0008.

We present a study of the distribution of galaxies along the radius of 157 groups and clusters of galaxies (200 km s −1 < 𝜎 < 1100 km s −1 ) of the local Universe (0.01 < 𝑧 < 0.1). We introduced a new boundary of galaxy systems and identified it with the splashback radius 𝑅 𝑠 𝑝 . We also identified the central region of galaxy systems with a radius of 𝑅 𝑐 . These radii are defined by the observed integral distribution of the total number of galaxies depending on the squared distance from the center of the groups/clusters coinciding, as a rule, with the brightest galaxy. We show that the radius 𝑅 𝑠 𝑝 is proportional to the 𝑅 200𝑐 (radius of the virialized region of a galaxy cluster) and to the radius of the central region 𝑅 𝑐 with a slope close to 1. Among the obtained dependences of the radii on X-ray luminosity, the log 𝑅 𝑠 𝑝 -log 𝐿 𝑋 relation has the lowest scatter. We measured < 𝑅 𝑠 𝑝 > = 1.67 ± 0.05 Mpc for the total sample, < 𝑅 𝑠 𝑝 > = 1.14 ± 0.14 Mpc for galaxy groups with 𝜎 ≤ 400 km s −1 , < 𝑅 𝑠 𝑝 > = 2.00 ± 0.20 Mpc for galaxy clusters with 𝜎 > 400 km s −1 . We found the average ratio of the radii 𝑅 𝑠 𝑝 /𝑅 200𝑐 = 1.40 ± 0.02 or 𝑅 𝑠 𝑝 /𝑅 200𝑚 = 0.88 ± 0.02.