Ernesto Micheletti

Complexity of cyanobacterial exopolysaccharides: composition, structures, inducing factors and putative genes involved in their biosynthesis and assembly

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.

Exopolysaccharide-producing cyanobacteria in heavy metal removal from water: molecular basis and practical applicability of the biosorption process

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.

Using extracellular polymeric substances (EPS)- producing cyanobacteria for the bioremediation of heavy metals: do cations compete for the EPS functional groups and also accumulate inside the cell?

Many cyanobacteria produce extracellular polymeric substances (EPS) mainly of polysaccharidic nature. These EPS can remain associated to the cell surface as sheaths, capsules and/or slimes, or be liberated into the surrounding environment as released polysaccharides (RPS). The ability of EPS-producing cyanobacteria to remove heavy metals from aqueous solutions has been widely reported in the literature, focusing mainly on the biotechnological potential. However, the knowledge of the effects of the metals in the cells survival/growth is still scarce, particularly when they are simultaneously exposed to more than one metal. This work evaluated the effects of different concentrations of Cu(2+) and/or Pb(2+) in the growth/survival of Gloeothece sp. PCC 6909 and its sheathless mutant Gloeothece sp. CCY 9612. The results obtained clearly showed that both phenotypes are more severely affected by Cu(2+) than Pb(2+), and that the mutant is more sensitive to the former metal than the wild-type. Evident ultrastructural changes were also observed in the wild-type and mutant cells exposed to high levels (10 mg l(-1)) of Cu(2+). Moreover, in bi-metal systems, Pb(2+) was preferentially removed compared with Cu(2+), being the RPS of the mutant that is the most efficient polysaccharide fraction in metal removal. In these systems, the simultaneous presence of Cu(2+) and Pb(2+) caused a mutual inhibition in the adsorption of each metal.

Selectivity in the heavy metal removal by exopolysaccharide‐producing cyanobacteria

Aims:  The aim of this study was to assess the selective removal of Cu(II), Cr(III) and Ni(II) by strains of exopolysaccharide (EPS)‐producing cyanobacteria, and to investigate the interaction of sorption in solutions with multiple‐metals.

Characteristics and role of the exocellular polysaccharides produced by five cyanobacteria isolated from phototrophic biofilms growing on stone monuments.

Three coccoid and two filamentous cyanobacterial strains were isolated from phototrophic biofilms exposed to intense solar radiation on lithic surfaces of the Parasurameswar Temple and Khandagiri caves, located in Orissa State, India. Based on to their morphological features, the three coccoid strains were assigned to the genera Gloeocapsosis and Gloeocapsa, while the two filamentous strains were assigned to the genera Leptolyngbya and Plectonema. Eleven to 12 neutral and acidic sugars were detected in the slime secreted by the five strains. The secretions showed a high affinity for bivalent metal cations, suggesting their ability to actively contribute to weakening the mineral substrata. The secretion of protective pigments in the polysaccharide layers, namely mycosporine amino acid-like substances (MAAs) and scytonemins, under exposure to UV radiation showed how the acclimation response contributes to the persistence of cyanobacteria on exposed lithoid surfaces in tropical areas.

Sheathless Mutant of Cyanobacterium Gloeothece sp. Strain PCC 6909 with Increased Capacity To Remove Copper Ions from Aqueous Solutions.

Cyanobacteria are a large and widespread group of photoautotrophic microorganisms that combine the ability to perform an oxygenic plant-like photosynthesis with typical prokaryotic features (40). Outside their atypical gram-negative cell walls (15), many strains possess outermost structures, mainly of a polysaccharidic nature, that differ in thickness and consistency. These structures can be referred to as sheaths, capsules, and slimes (4). The term sheath is applied to the usually thin and electron-dense layer surrounding the cells or the cell groups; the capsule refers to the thick and gelatinous layer intimately associated with the cell surface and presenting sharp outlines, whereas the term slime is used to designate the mucilaginous material that is dispersed around the organism but does not reflect the shape of the cells. During cell growth, aliquots of these polysaccharides can be released into the surrounding medium (referred to as released exocellular polysaccharides [RPS]), causing a progressive increase in its viscosity (4). The cyanobacterial exopolysaccharides (EPSs; a general term which includes both the polysaccharidic structures that remain intimately associated with the cell, i.e., the sheath and capsule, and those released into the culture medium, i.e., the RPS) exhibit some typical characteristics that distinguish them from the polymers synthesized by other bacteria: they frequently contain two different uronic acids, they build a polymer particularly rich in negatively charged groups, they possess a larger number of different monosaccharides, which increases the number of possible conformations of the polymer, and they contain sulfate groups, a unique feature among prokaryotes and one that is shared by the EPSs produced by Archaea (5, 31). At present, heavy metals are one of the most widespread causes of pollution, and their continuous accumulation in water bodies and soils constitutes a serious hazard to both the environment and human health (11, 20). The use of EPS-producing microorganisms (or isolated EPSs) is a valid alternative to conventional chemical and physicochemical methods to remove metallic cations from polluted waters (35). This new technology presents advantages such as the use of natural and renewable sources, reduced costs, rapid kinetics of metal removal, the ability to remove metallic ions present at low concentrations, the possibility to treat contaminated waters simultaneously with several different metal ions, and the possibility of recovering valuable metals from the biosorbent (6, 18). In this context, EPS-producing cyanobacteria appear to be promising candidates due to the unique characteristics of their polysaccharidic envelopes (see above). The efficiency of cyanobacterial EPS in the removal of metal ions has been discussed previously, with an emphasis on the monosaccharidic composition of the polymer, the isolation of EPS, and the subsequent utilization with removal assays (5, 8, 21). Although the chemical composition of the sheaths of several cyanobacterial strains has been determined (14, 15, 32, 33, 38, 39) and the importance of the capsules and RPS in the metal-removal process has been recognized (6), information about the exact contribution of each type of EPSs and/or functional group to the biosorption of the metal is still very limited. This work aimed to understand the role of the various outermost polysaccharidic investments in the process of copper removal by the unicellular N2-fixing cyanobacterium Gloeothece sp. strain PCC 6909. This strain is characterized by a well-defined laminated sheath that encloses cells and cell groups, maintaining a firm colonial structure. In this study, both the wild-type and a sheathless mutant, previously obtained by chemical mutagenesis, were used to elucidate the contribution of each type of EPS to the metal-removal process. Moreover, chemical and physical analyses were performed to identify the major sites responsible for the metal binding.

Optimization of copper sorbing–desorbing cycles with confined cultures of the exopolysaccharide‐producing cyanobacterium Cyanospira capsulata

Aim:  The aim of this study was to compare the copper removal capability of the exopolysaccharide‐producing cyanobacterium Cyanospira capsulata confined into various filtering devices and to assess its reuse for several metal sorbing‐desorbing cycles.