In the present work, we describe for the first time the utilization of a complex microbial biofilm for the treatment of sulfide-containing effluents. A non-aerated packed-column reactor was inoculated with anoxic lake sediment and exposed to light. A biofilm developed in the column and showed a stable oxidation performance for several weeks. Microbial species composition was analyzed by microscopy, pigment analysis and a bacterial 16S rRNA gene clone library. Colorless sulfur bacteria, green algae and purple sulfur bacteria were observed microscopically. Pigment composition confirmed the presence of algae and purple sulfur bacteria. The clone library was dominated by alpha-Proteobacteria (mostly Rhodobacter group), followed by gamma-Proteobacteria (Chromatiaceae-like and Thiothrix-like aerobic sulfur oxidizers) and the Cytophaga-Flavobacterium-Bacteroides group. Plastid signatures from algae were also present and a few clones belonged to both the beta- (Rhodoferax sp., Thiobacillus sp.) and delta-Proteobacteria (Desulfocapsa sp.) and to the low G+C Gram-positive bacteria (Firmicutes group). The coexistence of aerobic, anaerobic, phototrophic and chemotrophic microorganisms in the biofilm, the species richness found within these metabolic groups (42 operational taxonomic units) and the microdiversity observed within some species could be very important for the long-term functioning and versatility of the reactor. © Springer-Verlag 2004.