Seasonal succession of phototrophic biofilms in an Italian wastewater treatment plant: Biovolume, spatial structure and exopolysaccharides

A multiphasic approach was applied to investigate the structural features of phototrophic biofilms that grow in a wastewater treatment plant (WWTP) at Fiumicino Airport (Rome, Italy). Seasonal variations in species composition, biomass and exopolysaccharides produced were analyzed by light (LM) and electron microscopy (SEM), high performance liquid chromatography (HPLC) and circular dichroism (CD). Phototroph contribution to the 3-dimensional structure of the biofilm and its development was assessed by confocal laser scanning microscopy. Analysis of biofilms grown on polypropylene slides showed a stable species composition; seasonal changes in biomass were mostly due to changes of major cyanobacterial and algal taxonomic groups. Extensive growth was evident on the range of artificial substrata that were implanted in the treatment plant. CD spectra and HPLC analyses of 2 operationally defined exopolysaccharide fractions extracted from samples scraped off the tank walls revealed that negatively charged heteropolysaccharides comprised most of the matrix and capsular components of the biofilms. Cytochemical staining distinguished between acidic and sulphated residues in the samples observed by LM. The data provide a new insight into the structural integrity and development of phototrophic biofilms in this hyper-eutrophic environment, indicating a potential use of autochthonous consortia in an environmentally sound tertiary water treatment alternative to conventional chemico-physical technologies.