Two-stage fermentation process for alginate production by Azotobacter vinelandii mutant altered in poly-β-hydroxybutyrate (PHB) synthesis

Abstract

A two-stage fermentation strategy, based on batch cultures conducted first under non-oxygen-limited conditions, and later under oxygen-limited conditions, was used to improve alginate production by Azotobacter vinelandii (AT6), a strain impaired in poly-beta-hydroxybutyrate (PHB) production. The use of sucrose as carbon source, as well as a high oxygen concentration (10%), allowed to obtain a maximum biomass concentration of 7.5 g l(-1) in the first stage of cultivation. In the second stage, the cultures were limited by oxygen (oxygen close to 0%) and fed with a sucrose solution at high concentration. Under those conditions, the growth rate decreased considerably and the cells used the carbon source mainly for alginate biosynthesis, obtaining a maximum concentration of 9.5 g l(-1), after 50 h of cultivation. Alginate concentration obtained from the AT6 strain was two times higher than that obtained using the wild-type strain (ATCC 9046) and was the highest reported in the literature. However, the mean molecular mass of the alginate produced in the second stage of the process by the mutant AT6 was lower (400 kDa) than the polymer molecular mass obtained from the cultures developed with the parental strain (950 kDa). The use of a mutant of A. vinelandii impaired in the PHB production in combination with a two-stage fermentation process could be a feasible strategy for the production of alginate at industrial level.