The objective of this study was to assess the O2 budget in the water under sea ice combining observations and modelling. Modelling was used to discriminate between physical processes, gas-specific transport (i.e., ice-atmosphere gas fluxes and gas bubble buoyancy) and bacterial respiration (BR) and to constrain bacterial growth efficiency (BGE). A module describing the changes of the under-ice water properties, due to brine rejection and temperature-dependent BR, was implemented in the one-dimensional halo-thermodynamic sea ice model LIM1D. Our results show that BR was the dominant biogeochemical driver of O2 concentration in the water under ice (in a system without primary producers), followed by gas specific transport. The model suggests that the actual contribution of BR and gas specific transport to the change in seawater O2 concentration was 37% during ice growth and 48% during melt. BGE in the water under sea ice, as retrieved from the simulated O2 budget, was found to be between 0.4 and 0.5, which is in line with published BGE values for cold marine waters. Given the importance of BR to seawater O2 in the present study, it can be assumed that bacteria contribute substantially to organic matter consumption and gas fluxes in ice-covered polar oceans. In addition, we propose a parameterization of polar marine bacterial respiration, based on the strong temperature dependence of bacterial respiration and the high growth efficiency observed here, for further biogeochemical ocean modelling applications, such as regional or large-scale Earth System models.