Effect of limiting substrate concentration, growth rate and aeration on alginate composition and production by Azotobacter vinelandii in continuous culture

Abstract

The extracellular production of the acetylated alginate of Azotobacter vinelandii strain E was studied in PO 4 2− limited continuous cultures. The uronate composition and fine structure of the isolated alginates was determined using 1H-n.m.r. Cultures maintained at low phosphate concentrations (0.24 mM) produced less biomass and polysaccharide than those maintained at higher PO 4 2− concentrations (2.0 mM). The ratio of polysaccharide produced:dry cell wt was not influenced by the degree of PO 4 2− limitation. Alginate production rates were not influenced greatly by changes in culture dilution rates but at very low (<0.08/h) and very high (>0.25/h) dilution rates, reduced production was observed. At low (100 ml/min) and high (800 ml/min) aeration rates alginate production was less than at intermediate aeration rates (300–500 ml/min). Alginates with a great diversity of uronate composition and fine structure were isolated from the cultures although no relationship between composition and growth rate or aeration rate was observed. From cultures with 2.0 mM PO 4 2−, alginates with high mannuronate levels (60–70%, with an essentially polymannuronate and heteropolymeric structure) were isolated whilst those from cultures with 0.24 mM PO 4 2− had high guluronate levels (70%, with essentially polyguluronate and heteropolymeric structure). Increasing the dilution rate of cultures (0.05–0.3/h) resulted in the isolation of alginates with increasing degrees of acetylation.