ABSTRACT: Antarctic sea ice provides a stable environment for the colonisation of diverse and highly specialised microbes that play a central role in the assimilation and regulation of energy through the Antarctic food web. However, little is known about how the bacterial community composition changes within the sea ice, the functional role bacteria play in the sea-ice microbial loop, and the effect of variations in the environment on these patterns and processes. Using terminal restriction fragment length polymorphism (T-RFLP) analysis of the total (16S rRNA gene) and the active (16S rRNA) community, we characterised spatial patterns in Antarctic sea-ice bacterial communities (SIBCO). In addition, bacterial enzymatic activities were identified using synthetic fluorogenic substrates. Both bacterial community structure and enzymatic activity were compared to changing environmental conditions vertically down through the sea ice. The structure of the DNA- and RNA-derived bacterial communities exhibited strong vertical zonation through the ice. There was no direct relationship between changes in chlorophyll a (chl a) and bacterial numbers; however, there were direct relationships between chl a, bacterial community structure and metabolic function, thus providing evidence for a coupling of the microbial loop. The SIBCO were involved in phosphate recycling and selectively assimilated proteins over carbohydrates in response to the N‑ and P-poor environment. We propose that Antarctic sea ice has an active microbial loop in which the spatial dynamics of bacterial communities are influenced by algal-derived substrates and nutrient availability.