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We investigated the effects of copper on the structure and physiology of freshwater biofilm microbial communities. For this purpose, biofilms that were grown during 4 weeks in a shallow, slightly polluted ditch were exposed, in aquaria in our laboratory, to a range of copper concentrations (0, 1, 3, and 10 microM). Denaturing gradient gel electrophoresis (DGGE) revealed changes in the bacterial community in all aquaria. The extent of change was related to the concentration of copper applied, indicating that copper directly or indirectly caused the effects. Concomitantly with these changes in structure, changes in the metabolic potential of the heterotrophic bacterial community were apparent from changes in substrate use profiles as assessed on Biolog plates. The structure of the phototrophic community also changed during the experiment, as observed by microscopic analysis in combination with DGGE analysis of eukaryotic microorganisms and cyanobacteria. However, the extent of community change, as observed by DGGE, was not significantly greater in the copper treatments than in the control. Yet microscopic analysis showed a development toward a greater proportion of cyanobacteria in the treatments with the highest copper concentrations. Furthermore, copper did affect the physiology of the phototrophic community, as evidenced by the fact that a decrease in photosynthetic capacity was detected in the treatment with the highest copper concentration. Therefore, we conclude that copper affected the physiology of the biofilm and had an effect on the structure of the communities composing this biofilm.

The effects of copper (Cu) on photosynthetic riverine biofilms were studied in artificial stream channels. Direct effects on the composition and functioning of the biofilms were investigated using plant pigments, community-level physiological profiles (CLPP), and pulse-amplitude-modulated (PAM) fluorescence. Copper caused a significant reduction of microalgal biomass and induced a shift in the population from diatoms to cyanobacteria. However, a decrease in biomass indicated that the replacement of species was not totally effective to counteract the toxic effects of Cu. A direct effect of Cu could also be shown in the bacterial community, and, furthermore, changes in the CLPP could be related to the Cu treatment. Copper-exposed biofilms lost the capacity to use between 11 and 15% of the substrates, but many of the remaining capacities became more robust, indicating an increased Cu tolerance due to the exposure. The change in the biofilm microbial composition points to the indirect effects of Cu on biofilms due to the close interdependence between biofilm autotrophic and heterotrophic compartments. Grazing by snails, which appeared to be an important factor structuring biofilms without any Cu addition, had a very minor effect on Cu-exposed biofilms. Although grazing changed the bacterial composition, its effects were not detected either on the algal community or on the biofilm community tolerance to Cu.

Photosynthetic biofilms are capable of immobilizing important concentrations of metals, therefore reducing bioavailability to organisms. But also metal pollution is believed to produce changes in the microalgal species composition of biofilms. We investigated the changes undergone by natural photosynthetic biofilms from the River Meuse, The Netherlands, under chronic copper (Cu) exposure. The suspended particles in the river water had only a minor effect on reduction of sorption and toxicity of Cu to algae. Biofilms accumulated Cu proportionally to the added concentration, also at the highest concentration used (9 microM Cu). The physiognomy of the biofilms was affected through the growth of the chain-forming diatom Melosira varians, changing from long filaments to short tufts, although species composition was not affected by the Cu exposure. The Cu decreased phosphate uptake and algal biomass measured as chl a, which degraded exponentially in time. Photosynthetic activity was always less sensitive than algal biomass; the photon yield decreased linearly in time. The protective and insulating role of the biofilm, supported by ongoing autotrophic activity, was indicated as essential in resisting metal toxicity. We discuss the hypothesis that the toxic effects of Cu progress almost independently of the species composition, counteracting ongoing growth, and conclude that autotrophic biofilms act as vertical heterogeneous units. Effective feedback mechanisms and density dependence explain several discrepancies observed earlier.