Extracellular polysaccharides produced by a tropical cryptophyte as a carbon source for natural bacterial populations

Abstract

The extracellular polysaccharides (EPS) released by a freshwater cryptophyte, Cryptomonas tetrapyrenoidosa were characterized and their degradation by heterotrophic microbial populations from the same habitat, a tropical eutrophic reservoir, was evaluated. The EPS were purified by anion exchange column chromatography, the monosaccharide composition was determined by gas-chromatography, and the linkages of the monosaccharides by GC-MS. The EPS were separated by anion exchange into two different acidic fractions with completely different compositions. Fraction 1 was a complex branched heteropolysaccharide dominated by fucose, N-acetyl glucosamine, mannose and galactose. Fraction 2 showed a less complex pattern, which was composed mainly of 1,3-linked glucuronic acid and 1,3-linked galactose. We monitored the concentrations of the monosaccharides in the EPS during microbial degradation using Sepharose CL6B gel filtration and PAD-HPLC. After seven days, the main EPS were degraded into galactose-rich lower molecular weight carbohydrates, evidencing degradation in two steps. The decay patterns of the monosaccharides varied and, in the higher molecular weight (Fraction A), fucose and galactose were degraded more rapidly than rhamnose, glucose and man/xyl. In Fraction B, although the decay coefficients were lower and more homogenous, fucose and rhamnose were degraded more rapidly than galactose, glucose and man/xyl. In spite of the complexity of the EPS, and the specialized degradation pattern, the natural microbial community from the Barra Bonita reservoir was able to degrade the entire polysaccharide mixture in about 21 days.