Divergent composition of algal-bacterial biofilms developing under various external factors

The influence of external factors other than nutrients on biofilm development and composition was studied with a combination of optical (Confocal Laser Scanning Microscopy, PAM fluorometry) and chemical methods (EPS extraction, HPLC, TOC determination). The development of algal-bacterial biofilms was followed from bare surfaces to mature biofilms in two water reservoirs on Dutch filtration dunes. Biofilms developed under the influence of grazing, light limitation or undisturbed by either of these two factors. Biofilms appeared similar at the beginning of the colonization period at the three sites and laser microscopy observations revealed the predominance of bacteria and capsular EPS (extracellular polymeric substances) in young biofilms. After 3 weeks development, the biofilms had a higher number of taxa; undisturbed biofilms presented the highest biomass, the longest developmental period and showed a significant correlation between organic carbon, chlorophyll and EPS production, indicating a close coupling between autotrophic carbon production and EPS. In light-limited biofilms, this coupling was weaker (although the organic carbon content was similar to that of the undisturbed biofilms) and a lower algal biomass was reached. Light-limited biofilms were mostly composed of diatoms, which are more efficient in low irradiances than green microalgae. Biofilms grazed by the snail Potamopyrgus antipodarum presented the lowest biomass level, but the highest proportion of EPS. Grazing seemed to favour the predominance of EPS-rich algae, as well as firmly attached diatoms. Although filamentous cyanobacteria were found in mature biofilms at the three locations, they were more abundant in the grazed biofilms. The differences in carbon uptake with respect to its allocation indicated that external factors influencing biofilm development affect the cycling and transport of carbon in biofilms and hence influence the effect of biofilm metabolism on the overlying water quality.