Cyanobacterial extracellular polymeric substances (EPS) are mainly composed of high-molecular-mass heteropolysaccharides, with variable composition and roles according to the microorganism and the environmental conditions. The number of constituents - both saccharidic and nonsaccharidic - and the complexity of structures give rise to speculations on how intricate their biosynthetic pathways could be, and how many genes may be involved in their production. However, little is known regarding the cyanobacterial EPS biosynthetic pathways and regulating factors. This review organizes available information on cyanobacterial EPS, including their composition, function and factors affecting their synthesis, and from the in silico analysis of available cyanobacterial genome sequences, proposes a putative mechanism for their biosynthesis.

Cyanobacteria are photoautotrophic prokaryotes which include a large variety of species of widespread occurrence and with diverse morphological, physiological and biochemical properties. Many cyanobacteria are known to be able to synthesise outermost slimy investments and to release polysaccharidic material into the culture medium during cell growth. These released polysaccharides (RPSs), being easily recoverable from the culture medium, are attracting much interest in view of their possible uses in several industrial applications. In this paper, an overview of the current knowledge on both RPS-producing cyanobacterial strains (including the possible roles of the exopolysaccharides) and chemical characteristics of the cyanobacterial RPSs is given, with particular emphasis on RPS properties and possible industrial applications. On the whole, cyanobacterial RPSs are characterised by a great variety in both number (from two to 10) and type of constitutive monosaccharides (various arrangements of acidic and neutral sugars). Most polymers show an anionic nature due to the presence of uronic acids and/or other charged groups such as pyruvyl or sulfate. Polypeptide moieties as well as acetyl substituents have also sometimes been found, causing additional structural complexity. All the cyanobacterial RPSs so far tested showed a pseudoplastic behaviour, but with marked differences in both viscosity values and shear thinning. In terms of RPS production, the responses of cyanobacteria to changes of culture conditions appear strain-dependent. RPS productivities shown by some cyanobacteria are well comparable with those reported for other photosynthetic microorganisms proposed for polysaccharide production, but very low in comparison with those of heterotrophic microorganisms. Nevertheless, cyanobacteria may be regarded as a very abundant source of structurally diverse polysaccharides, some of which may possess unique properties for special applications, not fulfilled by the polymers currently available. However, much work has still to be done to bridge the wide gap existing between data on the biology of the RPS-producer strains and information concerning technological and other useful properties of the cyanobacterial RPS. z

Cyanobacteria are photoautotrophic prokaryotes which include a large variety of species of widespread occurrence and with diverse morphological, physiological and biochemical properties. Many cyanobacteria are known to be able to synthesise outermost slimy investments and to release polysaccharidic material into the culture medium during cell growth. These released polysaccharides (RPSs), being easily recoverable from the culture medium, are attracting much interest in view of their possible uses in several industrial applications. In this paper, an overview of the current knowledge on both RPS‐producing cyanobacterial strains (including the possible roles of the exopolysaccharides) and chemical characteristics of the cyanobacterial RPSs is given, with particular emphasis on RPS properties and possible industrial applications. On the whole, cyanobacterial RPSs are characterised by a great variety in both number (from two to 10) and type of constitutive monosaccharides (various arrangements of acidic and neutral sugars). Most polymers show an anionic nature due to the presence of uronic acids and/or other charged groups such as pyruvyl or sulfate. Polypeptide moieties as well as acetyl substituents have also sometimes been found, causing additional structural complexity. All the cyanobacterial RPSs so far tested showed a pseudoplastic behaviour, but with marked differences in both viscosity values and shear thinning. In terms of RPS production, the responses of cyanobacteria to changes of culture conditions appear strain‐dependent. RPS productivities shown by some cyanobacteria are well comparable with those reported for other photosynthetic microorganisms proposed for polysaccharide production, but very low in comparison with those of heterotrophic microorganisms. Nevertheless, cyanobacteria may be regarded as a very abundant source of structurally diverse polysaccharides, some of which may possess unique properties for special applications, not fulfilled by the polymers currently available. However, much work has still to be done to bridge the wide gap existing between data on the biology of the RPS‐producer strains and information concerning technological and other useful properties of the cyanobacterial RPS.

Exopolysaccharides (EPSs) are an important class of biopolymers with great ecological importance. In natural environments, they are a common feature of microbial biofilms, where they play key protective and structural roles. As the primary colonizers of constrained environments, such as desert soils and lithic and exposed substrates, cyanobacteria are the first contributors to the synthesis of the EPSs constituting the extracellular polymeric matrix that favors the formation of microbial associations with varying levels of complexity called biofilms. Cyanobacterial colonization represents the first step for the formation of biofilms with different levels of complexity. In all of the possible systems in which cyanobacteria are involved, the synthesis of EPSs contributes a structurally-stable and hydrated microenvironment, as well as chemical/physical protection against biotic and abiotic stress factors. Notwithstanding the important roles of cyanobacterial EPSs, many aspects related to their roles and the relative elicited biotic and abiotic factors have still to be clarified. The aim of this survey is to outline the state-of-the-art of the importance of the cyanobacterial EPS excretion, both for the producing cells and for the microbial associations in which cyanobacteria are a key component.

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Microorganisms can remove metals from the surrounding environment with various mechanisms, either as metabolically mediated processes or as a passive adsorption of metals on the charged macromolecules of the cell envelope. Owing to the presence of a large number of negative charges on the external cell layers, exopolysaccharides (EPS)-producing cyanobacteria have been considered very promising as chelating agents for the removal of positively charged heavy metal ions from water solutions, and an increasing number of studies on their use in metal biosorption have been published in recent years. In this review, the attention was mainly focused on the studies aimed at defining the molecular mechanisms of the metal binding to the polysaccharidic exocellular layers. Moreover, the few attempts done in the use of EPS-producing cyanobacteria for metal biosorption at pilot scale and with real wastewaters are here reviewed, discussing the main positive issues and the drawbacks so far emerging from these experiments.

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