Microbial polymers have diverse chemical structures, which determine their functional properties. Many microbial sources produce either intracellular or extracellular biopolymers. For example, Azotobacter vinelandii is a non-pathogenic soil bacterium that produces alginate, an extracellular polysaccharide. Alginates are used mainly in the food and pharmaceutical industries as stabilizing, thickening, gel- or film-forming agents. Due to its characteristics of biocompatibility, biodegradability and non-antigenicity, new applications for alginate are being discovered, such as biomaterial in the biomedical field and tissue engineering. It is well established that alginate is first synthesized as a polymannuronate from its cytosolic precursor. However, the mechanisms involved in the polymerization, modification (acetylation, epimerization and depolymerization) and translocation of alginate have been poorly elucidated. Two of the most important parameters in alginate production by A. vinelandii are dissolved oxygen tension (DOT) and the oxygen supply conditions, as these impact both the polymer concentration and its composition, particularly its molecular weight (MW). Several studies have revealed that increased alginate molecular weight occurs in oxygen limited conditions, specifically at oxygen concentrations near zero. This article reviews recent studies examining the influence of oxygen, under limitation (microaerophilic) and non-limitation conditions (measured as DOT and oxygen transfer rate, OTR), on the polymerization and degradation of alginate produced by A. vinelandii. This review also provides evidence for understanding these processes at the cellular level and the effect of oxygen on alginate biosynthesis. (c) 2014 Society of Chemical Industry