Oxygen transfer rate during the production of alginate by Azotobacter vinelandii under oxygen-limited and non oxygen-limited conditions

BACKGROUND: The oxygen transfer rate (OTR) and dissolved oxygen tension (DOT) play an important role in determining alginate production and its composition; however, no systematic study has been reported about the independent influence of the OTR and DOT. In this paper, we report a study about alginate production and the evolution of the molecular mass of the polymer produced by a wild-type A. vinelandii strain ATCC 9046, in terms of the maximum oxygen transfer rate (OTR(max)) in cultures where the dissolved oxygen tension (DOT) was kept constant. RESULTS: The results revealed that in the two dissolved oxygen conditions evaluated, strictly controlled by gas blending at 0.5 and 5% DOT, an increase in the agitation rate (from 300 to 700 rpm) caused a significant increase in the OTR(max )(from 17 to 100 mmol L(-1 )h(-1 )for DOT of 5% and from 6 to 70 mmol L(-1 )h(-1 )for DOT of 0.5%). This increase in the OTR(max )improved alginate production, as well as the specific alginate production rate (SAPR), reaching a maximal alginate concentration of 3.1 g L(-1 )and a SAPR of 0.031 g (alg )g (biom)(-1 )h(-1 )in the cultures at OTR(max )of 100 mmol L(-1 )h(-1). In contrast, the mean molecular mass (MMM) of the alginate isolated from cultures developed under non-oxygen limited conditions increased by decreasing the OTR(max), reaching a maximal of 550 kDa at an OTR(max )of 17 mmol L(-1 )h(-1 ). However, in the cultures developed under oxygen limitation (0.5% DOT), the MMM of the polymer was practically the same (around 200 kDa) at 300 and 700 rpm, and this remained constant throughout the cultivation. CONCLUSIONS: Overall, our results showed that under oxygen-limited and non oxygen-limited conditions, alginate production and its molecular mass are linked to the OTR(max), independently of the DOT of the culture.