Exploitation of microbial photosynthesis for environmentally sustainable biotechnological processes

Microbial photosynthesis is a metabolic process that dramatically changed the environmental conditions of Earth some billions of years ago and human beings owe to the photosynthesis large part of the primary productivity on the planet. Furthermore, microbial photosynthesis can be exploited for developing several sustainable and environmentally friendly processes. In this lecture some of the processes deriving from the exploitation of oxygenic and anoxygenic microbial photosynthesis will be described. In particular, the exploitation of exopolysaccharide (EPS)-producing cyanobacteria for the removal of heavy metals from industrial waste waters or for the stabilization of desert soils and the utilization of purple non sulfur bacteria (PNSB) for the production of hydrogen will be reviewed. 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. Recently, 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. It was shown 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. A number of attempts done at pilot scale and with real wastewaters also demonstrated the potential of the use of EPS-producing cyanobacteria for metal biosorption. Among the roles proposed for the outermost polysaccharidic investments of cyanobacteria, the protection against desiccation of the cells embedded into the polymeric matrix has been reported. Indeed, the capability of polysaccharide-producing cyanobacteria to overcome the stress due to desiccation or to low water activity in desert or saline environments has been described, attributing to the secreted glycan the role of a repository for water. The exploitation of some EPS-producing cyanobacteria for the stabilization of desert soils by their artificial inoculation on sand dunes will be presented, describing the role of the EPSs in the hydrological behaviour of the crusts. The exploitation of PNSB for the production of hydrogen has been frequently claimed as very promising. Indeed, PNSB are generally considered among the most promising microbial systems for the biological production of hydrogen owing to (i) their high theoretical substrate-to-hydrogen conversion yields, (ii) their lack of O2-evolving activity, that avoids the inhibition of the nitrogenase-mediated H2 production due to the presence of oxygen, (iii) their capability to use a wide spectrum of light and (iv) to metabolize organic substrates derivable from industrial wastes or from the degradation of plant and animal residues, suggesting their use in association with wastewater treatments. In this lecture, the results obtained at the University of Florence (Italy) in the production of hydrogen with PNSB growing on substrates derived from the fermentation of vegetable residues or growing on synthetic media in outdoor 50L photobioreactors will be presented.