Biosynthesis of multiple biopolymers by Sinorhizobium meliloti CFR 14 in high cell density cultures through fed batch fermentation

Rhizobium spp. produce a variety of commercially important polymers during free living growth, which includes polyhydroxyalkanoate (PHA), exopolysaccharides (EPS), β-(1,2)-glucan, etc. Shake flask cultivation of Sinorhizobium meliloti CFR 14 for 72 h with ammonium sulfate (0.318 g/L of N) and sucrose (12.64 g/L of C) resulted in 3.5 g/L of biomass with 1.7 g/L of PHA and 1.5 g/L of EPS. Corresponding yields enhanced to 5.0, 2.5 and 2 g/L, in a 48 h batch fermentation process at 30 °C, pH 7.0 and 40% air saturation level. Fed batch fermentation process with ammonium sulfate and sucrose as feed nutrients supported the production of 34.5 g/L of biomass with 17.2 g/L of PHA and 3.5 g/L of EPS. Concentration of glycogen, cellular and excreted β-(1,2)-glucan were about 0.3 g/L each under this condition. Fed batch process operated using ammonium hydroxide for dual purpose as a nitrogen source and for pH adjustment, lead to decreased biomass (29.6 g/L) and PHA yields (9.4 g/L). However, higher synthesis of EPS, glycogen, cellular and excreted β-(1,2)-glucan amounting to 9.0, 1.1, 1.3 and 2.0 g/L, respectively, were obtained indicating that the fermentation conditions like pH, dissolved oxygen level, amount of carbon and nitrogen play a significant role in the synthesis of intra- and extra-cellular polymers by the free living cells of S. meliloti.