Several wastewater stabilization ponds (WSP) in Tunisia suffer periodically from the ‘red-water’ phenomenon due to blooming of purple sulfur bacteria, indicating that sulfur cycle is one of the main element cycles in these ponds. In this study, we investigated the microbial diversity of the El Menzeh WSP and focused in particular on the different functional groups of sulfur bacteria. For this purpose, we used denaturing gradient gel electrophoresis of PCR-amplified fragments of the 16S rRNA gene and of different functional genes involved in microbial sulfur metabolism (dsrB, aprA, and pufM). Analyses of the 16S rRNA revealed a relatively high microbial diversity where Proteobacteria, Chlorobi, Bacteroidetes, and Cyanobacteria constitute the major bacterial groups. The dsrB and aprA gene analysis revealed the presence of deltaproteobacterial sulfate-reducing bacteria (i.e., Desulfobacter and Desulfobulbus), while the analysis of 16S rRNA, aprA, and pufM genes assigned the sulfur-oxidizing bacteria community to the photosynthetic representatives belonging to the Chlorobi (green sulfur bacteria) and the Proteobacteria (purple sulfur and non sulfur bacteria) phyla. These results point on the diversity of the metabolic processes within this wastewater plant and/or the availability of sulfate and diverse electron donors.Electronic supplementary materialThe online version of this article (doi:10.1007/s00253-012-3931-5) contains supplementary material, which is available to authorized users.

The molecular diversity of the purple photosynthetic bacteria was assessed during temporal pigmentation changes in four interconnected wastewater stabilization ponds treating domestic wastewater by denaturant gel gradient electrophoresis method applying pufM gene. Results revealed high phylogenetic diversity of the purple phototrophic anoxygenic bacteria community characterized by the presence of the purple non-sulfur, purple sulfur, and purple aerobic photosynthetic anoxygenic bacteria. This phototrophic bacterial assemblage was dominated by the purple non-sulfur bacteria group (59.3 %) with six different genera followed by the purple sulfur community (27.8 %) with four genera and finally 12.9 % of the pufM gene sequences were assigned throughout the aerobic anoxygenic phototrophic bacterial group. The purple phototrophic bacterial community was characterized by the presence of salt-dependant bacterial species belonging to the genus Marichromatium, Thiorhodococcus, Erythrobacter, and Roseobacter. The wastewater treatment plant performances were unsatisfactory, and the biological and chemical parameters suggested that the purple photosynthetic bloom was correlated with the eutrophic state.