The article analyses biotic interactions (competition, predation and commensalism) between the main components of the microbial “loop” during summer and winter in the upper mixed layer of the Black Sea pelagic zone. In summer, the interaction between bacteria and phytoplankton in a phosphate-deficient environment resulted in a high contribution of bacteria and phytoplankton to the total phosphate uptake by microplankton, which was 47 and 53%, respectively, indicating a strong competition between bacteria and phytoplankton for uptake of this nutrient. The high total rate of phosphate uptake by bacteria and phytoplankton (119·10–4 µM·h–1) and its low concentrations in the upper mixed layer ensured a short turnover time of 1.9 hours, which contributed to accelerated phosphate recycling in this layer. In the interaction (predator–prey) between protozoa and picofraction (bacteria+ picophytoplankton), high values of the specific rate of phosphate excretion by protozoa (zooflagellates, ciliates) were noted. This suggests their active participation in the consumption of bacteria, picophytoplankton and heterotrophic nanoplankton, thus indicating the high functional activity of the microbial “loop”. In winter, in the upper mixed layer above the pycnocline, at a temperature of 5.5–10°C, the production and daily coefficient Р/B of bacteria, picophytoplankton, zooflagellates, and ciliates decreased significantly compared to the summer period, with the exception of high levels of phytoplankton biomass and production. The contribution of the picofraction (bacteria+ pico-phytoplankton) to the total phosphate uptake decreased to 8% compared to summer. The average total rate of phosphate uptake by microplankton decreased to 15·10–4 μM·h–1 relative to the summer period, with an interval of 9·10 –4–24·10 –4 μM·h –1. At the same time, the phosphate turnover time in the upper mixed layer increased to 38 hours, indicating a slowdown in its recycling and a low level of functional activity in the microbial “loop”. A data analysis suggests that the uptake of nutrients by bacteria and phytoplankton and their excretion by protozoa are subject to seasonal fluctuations, which should be taken into account when calculating the annual balance of matter and energy in the Black Sea pelagic zone.