Antarctic sea ice is exposed to a wide range of environmental conditions during its annual existence, however, there is very little information describing the change in sea iceassociated microbial communities SIMCO during the changing seasons. It is well known that, during the solar seasons, SIMCO plays an important role in the polar carbon cycle, by increasing the total photosynthetic primary production of the South Ocean and participating in the remineralization of phosphates and nitrogen. What remains poorly understood is the dynamic of SIMCO populations and their ecological contribution to carbon and nutrient cycling throughout the entire annual life of Antarctic sea ice, especially in winter. Sea ice at this time of the year is an extreme environment, characterized by complete darkness which stops photosynthesis, extremely low temperatures in its upper horizons down to 45C, and high salinity up to 150_250 PSU in its brine inclusions, where SIMCO thrive. Without a permanent station, wintering expeditions in Antarctica are technically difficult therefore, in this study, the process of autumn freezing was modeled under laboratory conditions, and the resulting young ice was further incubated in cold and darkness for one month. The ice formation experiment was primarily designed to reproduce two critical conditions: i total darkness causing the photosynthesis to cease, and ii the presence of a large amount of algae-derived organic matter. As expected, in the absence of photosynthesis, the activity of aerobic heterotrophs quickly created microoxic conditions, which caused the emergence of new players, namely facultative anaerobic and anaerobic microorganisms. Following this finding, we can state that the Antarctic pack ice and its surrounding ambient under_ice seawater and platelet ice are likely to be very dynamic and can quickly respond to environmental changes, caused by the seasonal fluctuations. Given the size of the Antarctic pack ice, even in complete darkness and cessation of photosynthesis, its ecosystem appears to remain active, continuing to participate in the global carbon and sulfur cycling under harsh conditions.