AimsThe sigma E (AlgU) in Azotobacter vinelandii has been shown to control the expression of cydR gene, a repressor of genes of the alternative respiratory chain, and alginate has been considered a barrier for oxygen diffusion. Therefore, the aim of the present study was to compare the respiratory activity of an alginate nonproducing strain, lacking the sigma factor E (algU-), and polymer-producing strains (algU+) of A. vinelandii under diazotrophic conditions at different aeration conditions. Methods and ResultsOur results reveal that under diazotrophic and high aeration conditions, A. vinelandii strain OP (algU-) had a specific oxygen consumption rate higher (30 and 54%) than those observed in the OP algU+-complemented strain, named OPAlgU+, and the ATCC 9046 respectively. However, the specific growth rate and biomass yields (based on oxygen and sucrose) were lower for OP cultivations as compared to the algU+ strains. These differences were partially explained by an increase in 15-fold of cydA relative expression in the OP strain, as compared to that obtained in the isogenic OPAlgU+ strain. ConclusionsOverall, our results confirm the important role of algU gene on the regulation of respiratory metabolism under diazotrophic growth when A. vinelandii is exposed to high aeration. Significance and Impact of the StudyThis study highlights the role of AlgU to control respiration of A. vinelandii when exposed to diazotrophy.