Response of an exopolysaccharide-producing heterocystous cyanobacterium to changes in metabolic carbon flux

Abstract

The response of the exopolysaccharide-producing heterocystous cyanobacteriumCyanospira capsulata to changes in metabolic carbon flux was investigated to estimate the potential for improvement of the exopolysaccharide (EPS) yield. Carbon flux was altered by transferring the organism either to an argon atmosphere or to medium containing the nitrogen assimilation inhibitors L-methionine-D,L-sulfoximine (MSX), O-diazoacetyl-L-serine (AZAS) or D,L-7-azatryptophan (AZAT), or by adding glyoxylate, known to stimulate carbon metabolism. When carbon flux was modified by interfering with nitrogen metabolism, the concentration of total carbohydrates exceeded that of the control culture only in Ar- or AZAS-treated cell suspensions, but this difference was mainly due to enhancement of the quantity of bound carbohydrates. On the other hand, when carbon flux was modified by a single addition of glyoxylate (30 mM) or by daily additions of 10 mM glyoxylate without interfering with nitrogen metabolism, carbohydrate release into the medium was stimulated markedly; after 5 days of growth in the presence of the organic compound, the concentration of EPS was 43 % higher than in the control culture. The results demonstrate that, with enhanced carbon flux, the excess carbon is preferentially channeled byC. capsulata cells into the synthesis of an overflow product like EPS, whereas, with mere diversion of carbon flux from the process of nitrogen assimilation, the synthesis of carbon reserves is more heavily favoured.