Roberto De Philippis

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 and their possible exploitation: A review

Since the early 1950s, more than one hundred cyanobacterial strains,belonging to twenty different genera, have been investigated with regard tothe production and the released exocellular polysaccharides (RPS) into theculture medium. The chemical and rheological properties show that suchpolysaccharides are complex anionic heteropolymers, in about 80% casescontaining six to ten different monosaccharides and in about 90% casescontaining one or more uronic acids; almost all have non-saccharidiccomponents, such as peptidic moieties, acetyl, pyruvyl and/or sulphategroups. Based on such ingredients, cyanobacterial RPSs show promise asthickening or suspending agents, emulsifying or cation-chelating compoundsand the residual capsulated cyanobacterial biomass, following RPSextraction, could be an effective cation-chelating material. Indeed, wheneleven unicellular and filamentous RPS-producing cyanobacteria, selectedon the basis of the anion density of their RPSs and on the abundance oftheir outermost investments, were screened for their ability to removeCu2+ from aqueous solutions, a quick and most effective heavy metaladsorption was observed for the unicellular Cyanothece CE 4 and thefilamentous Cyanospira capsulata. These results suggest the possibilityto accomplish, through the exploitation of RPS-producing cyanobacteria,a multiproduct strategy to procure a wide range of biopolymers suited tovarious industrial applications, in addition to the residual biomass effectivein the recovery of heavy metals from polluted waters.

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.

Exopolysaccharide production by a unicellular cyanobacterium isolated from a hypersaline habitat

The unicellular cyanobacterial strain 16Som2, isolated from a Somaliland saltpan and identified asCyanothece sp., is characterized by cells surrounded by a thick polysaccharidic capsule, the external part of which dissolves into the medium during growth, causing a progressive increase in culture viscosity. In spite of this, the thickness of the capsule remained almost constant under all the culture conditions tested, demonstrating that the processes of its synthesis and solubilization occurred at a similar rate. The synthesis of carbohydrates was neither enhanced by increasing salinity (sea-water enriched with NaCl in the range 0 to 2.0 M) nor by Mg2+, K+ or Ca2+ deficiencies. In contrast, N-limitation and, to a lesser extent, P-limitation induced a significant enhancement of carbohydrate synthesis; in particular, N-deficiency stimulated the synthesis of all the carbohydrate fractions (intracellular, capsular and soluble). The soluble polysaccharide, separated from the culture medium and hydrolyzed with 2N trifluoroacetic acid, showed a sugar composition consisting of glucuronic acid: galacturonic acid: galactose: glucose: mannose: xylose: fucose in a molar ratio of 1: 2: 2.4: 6.8: 4.8: 2.9: 1.6.Cyanothece sp. culture subjected to nitrogen starvation synthesized polysaccharide with a mean productivity of 115 mg (EPS) l−1d−1, for the polymer solubilized into the medium, and of 15 mg (CPS) l−1d−1 for the capsular polysaccharide.

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.

Assessment of the metal removal capability of two capsulated cyanobacteria, Cyanospira capsulata and Nostoc PCC7936

Two capsulated, exopolysaccharide-producing cyanobacteria, Cyanospira capsulata and Nostoc PCC7936, were tested with regard to their metal removal capability by using copper as model metal. The experiments, carried out with the sole cyanobacterial biomass suspended in distilled water and confined into small dialysis tubings, showed that C. capsulata biomass is characterized by the best efficiency in metal removal, with a qmax (maximum amount of copper removed per biomass unit) of 96 ± 2 mg Cu(II) removed per g of protein in comparison with the value of 79 ± 3 of Nostoc PCC7936 biomass. The experimental data obtained with both cyanobacterial biomass best fit the Langmuir sorption isotherm. The sorption of copper started from the first minutes of contact with the metal and attained the equilibrium state, when no more copper removal was evident, after 5 and 6 hours, for C. capsulata and Nostoc PCC7936, respectively. The best efficiency in Cu(II) removal was obtained at pH 6.1–6.2, while the presence of Mg2+ or Ca2+ reduced copper removal capability of both species to 60–70% of their qmax. The results showed that the biomass of C. capsulata and Nostoc PCC7936 possesses a high affinity and a high specific uptake for copper, comparable with the best performances shown by other microbial biomass, and suggest the possibility to use the capsulated trichomes of the two cyanobacteria for the bioremoval of heavy metals from polluted water bodies.

STUDIES ON EXOPOLYSACCHARIDE RELEASE BY DIAZOTROPHIC BATCH CULTURES OF CYANOSPIRA CAPSULATA

SummaryDiazotrophic batch cultures of Cyanospira capsulata producing large amounts of a soluble exopolysaccharide (EPS) were studied over a period of about 30 days under continuous illumination. The thickness of the capsule surrounding the trichomes remained almost the same throughout the growth phases and the EPS was continuously released into the medium at a rate which was roughly constant throughout the culture period. A mean EPS productivity of about 6 g m−2 day−1 was attained. Purified EPS samples exhibited a saccharidic composition consisting of four neutral sugars (glucose, mannose, fucose and arabinose) and galacturonic acid in a molar ratio of 1:1:1:1:2, respectively. The EPS was also characterized by the presence of pyruvic residues and by a protein content of about 2%. O-Acetyl groups and sulphate residues were not detected. The massive release of this polysaccharidic material into the liquid medium made the cultures progressively more viscous.

Role of Cyanobacterial Exopolysaccharides in Phototrophic Biofilms and in Complex Microbial Mats

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.

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.

H and poly-β-hydroxybutyrate, two alternative chemicals from purple non sulfur bacteria

The feasibility of a process for the photoproduction of both H 2 and poly-b-hydroxybutyrate (PHB)-containing biomass has been tested utilizing semi-continuous cultures of Rhodopseudomonas palustris growing in a tubular system with limiting amounts of fixed nitrogen. A two-stage batch process, consisting in a first period of nitrogen-limited cell growth followed by a second period of cell cultivation under conditions of phosphorus shortage, showed the possibility to separate the H production phase from the PHB accumulation phase, making possible to carry out processes that otherwise would be in competition.