Enhanced curdlan production with nitrogen feeding during polysaccharide synthesis by Rhizobium radiobacter

• Rhizobium radiobacter capable of producing curdlan was identified.
• The cells displayed a diauxie growth profile upon (NH4)2HPO4 addition during curdlan producing phase.
• An enhanced curdlan production of 65.27 g/L was attained by continuous (NH4)2HPO4 feeding.

Curdlan is a secondary metabolite synthesized by Agrobacterium sp. and some other bacteria. A newly isolated exopolysaccharide-producing strain was identified to be Rhizobium radiobacter CGMCC 12099. The polysaccharide product was confirmed to be curdlan with a molecule weight of 1.4 × 105 Da, and its molecular structure was determined by HPLC and infrared spectrum. Although nitrogen source is necessary for cell reproduction, curdlan production is largely dependent on nitrogen limitation, as well as cell vitality. Here, a nitrogen feeding strategy was investigated to elevate the curdlan production by R. radiobacter. The optimal concentration and addition time of (NH4)2HPO4 were investigated. The results showed that the enhanced cell density was correlated to the amount of (NH4)2HPO4 added. Also, nitrogen addition in earlier fermentation stage was beneficial to the cell growth and curdlan production. Furthermore, continuously feeding strategy was employed by feeding (NH4)2HPO4 at a constant rate of 1.24 g/h at 35th h of fermentation for 9 h, achieving a final curdlan production of 65.27 g/L, productivity of 0.544 g/L/h and glucose conversion rate of 38.89%. The curdlan production was improved by 2.1 times compared with that without nitrogen addition. This study provides a feasible and cheap nitrogen feeding strategy to enhance curdlan production.