ABSTRACT
Cyanobacterial EPSs are mainly composed of highmolecular-mass heteropolysaccharides, with variable compositions and roles according to the microorganism and the environmental conditions (Pereira et al. 2009). Cyanobacteria possess a unique cell wall that combines the presence of an outer membrane and lipopolysaccharides, as in Gram-negative bacteria, with a thick and highly cross-linked peptidoglycan layer similar to Gram-positive bacteria (Hoiczyck and Hansel 2000, Stewart et al. 2006). The EPS associated with the cell surface can be referred to as sheaths, capsules, and slimes, according to their thickness, consistency, and appearance (De Philippis and Vincenzini 1998, 2003). The sheath is dened as a thin, dense layer loosely surrounding cells or cell groups usually visible in light microscopy without staining. The sheath of the cyanobacterium Lyngbya aestuarii has been shown to have a sulfated proteoglycan (Robbins et al. 1998). The polypeptide comprises 12.9% of the sheath dry weight and sulfate esters account for 2.0%. Aspartic acid and alanine represent 32.5% of the polypeptide component. The dominant monosaccharide is glucose, averaging 18.0% of the dry weight. At least 13 different monosaccharide linkages have been identied. This morphologically rigid sheath is a single sulfated proteoglycan (Robbins et al. 1998). Both the sulfate groups and the uronic acids contribute to the anionic nature of the EPS, conferring a negative charge and a “sticky” behavior to the overall macromolecule (Decho 1990, Leppard et al. 1996, Arias et al. 2003, De Philippis and Vincenzini 2003, Mancuso Nichols et al. 2005). To date, up to 12 different monosaccharides have been identied in cyanobacterial EPS: the hexoses (glucose, galactose, mannose, and fructose); the pentoses (ribose, xylose, and arabinose); the deoxyhexoses (fucose, rhamnose, and methyl rhamnose); and the acidic hexoses (glucuronic and galacturonic acid) (De Philippis and Vincenzini 1998, 2003, De Philippis et al. 2001). In a few cases, the presence of additional types of monosaccharides such as N-acetyl glucosamine, 2, 3-O-methyl rhamnose, 3-O-methyl rhamnose, 4-O-methyl rhamnose, and 3-O-methyl glucose has been reported (Hu et al. 2003). The monosaccharide most frequently found at the highest concentration in cyanobacterial EPS is glucose, although there are polymers where other sugars, such as xylose, arabinose, galactose, or fucose, are present at higher concentrations than glucose (Tease et al. 1991, Bender et al. 1994, Gloaguen et al. 1995, Fischer et al. 1997, De Philippis and Vincenzini 1998, 2003, Parikh and Madamwar 2006). It has been reported that cyanobacterial EPSs are composed not only composed of carbohydrates but also of
other macromolecules such as polypeptides (Kawaguchi and Decho 2000). Polypeptides enriched with glycine, alanine, valine, leucine, isoleucine, and phenylalanine have been reported in the EPS of Cyanospira capsulata and Nucula calcicola (Flaibani et al. 1989, Marra et al. 1990), and in Schizothrix sp., small proteins specically enriched with aspartic and glutamic acid have been observed (Kawaguchi and Decho 2002). In general, the chemical composition, the type, and the amount of the exopolysaccharides produced by a given cyanobacterial strain are stable features, mostly depending on the species and the cultivation conditions (Nicolaus et al. 1999). However, the sugar composition of the EPS produced by a certain strain may, qualitatively and quantitatively, vary slightly, especially with the age of the culture (Gloaguen et al. 1995, De Philippis and Vincenzini 1998).
