The structure (biomass, species or Operational Taxonomic Unit [OUT] composition) and function (photosynthetic performance) of autotrophic and heterotrophic biofilm communities were investigated along a chemical gradient created downstream of a small Fe spring (Llémena watershed, Girona, northeastern Spain). Diatom species composition, diatom abundance, and algal biomass (measured as basal fluorescence [Fo]) followed the chemical gradient. Site ordination scores based on the photosynthetic performance of biofilms (measured as effective quantum yield [Yeff] and photosynthesis–irradiance curve parameters) and on bacterial densities and denaturing gradient gel electrophoresis (DGGE) data (OTUs for Fungi and Bacteria) were not linearly related to the chemical gradient. The extreme chemical environment selected for low-productivity, stress-tolerant community characterized by low algal biomass, absence of cyanobacteria, and dominance of the diatom taxon Navicula cincta. The unimodal pattern of response of heterotrophs to the chemical gradient and the observed coincidence with the pattern of photosynthetic performance indicate that interspecies competition within a trophic level may be obscured by biotic interactions between primary producers and decomposers. We hypothesize that these biotic interactions may be important under extreme chemical conditions because of differences in sensitivity between autotrophic and heterotrophic components of the biofilm to chemical stress or limiting resources.