Heavy metal bioremoval with EPS-producing cyanobacteria

Many cyanobacterial strains possess, outside their outer cell membrane, additional surface structures, mainly of polysaccharidic nature, that may be referred to three distinct types: sheaths, capsules and slimes. In addition, many polysaccharide-producing strains release, into the culture medium, aliquots of their sheath, capsule or slime as water-soluble polymeric material. Most of these polymers are characterized by an anionic nature, owing to the presence of uronic acids and/or of other charged groups. The use of microorganisms for the removal of toxic heavy metals from polluted waters has been intensively investigated in recent years as an alternative to the conventional methods, that are considered as less effective or too expensive if heavy metals are dissolved in huge volumes of water at relatively low concentrations. The good efficiency in metal chelation showed by many of the microorganisms studied has been related to the presence of a high number of negatively charged groups on the cell envelope. In this respect, the utilization of exopolysaccharide (EPS)-producing cyanobacteria seems to be quite promising, owing to their anionic nature. Indeed, a large number of researches demonstrated the very good efficiency of some of the EPS-producing cyanobacteria in the removal of positively charged metal ions. In this lecture, the attention will be mainly focused on the studies aimed at defining the molecular mechanisms of the metal binding to the polysaccharidic exocellular layers. From the results so far published, it is possible to infer that the mechanisms of interaction between the EPS-producing cyanobacteria and metals are very complex, being involved in this process a large number of factors. In the lecture will also be referred the few attempts done in the use of EPS-producing cyanobacteria for metal biosorption at pilot scale and with real wastewaters, discussing the main positive issues and the drawbacks so far emerging from these experiments