Pioneering bacterial and algal communities and potential extracellular enzyme activities of stream biofilms

Microbial biofilms are important for the turnover of organic matter in small streams. A rapid colonization of the epilithic surface will become more important given the predicted increase of flood events. Here, we elucidated the pioneering community structure and activity of stream biofilms. Colonization of glass slides exposed in a small stream for 1, 4, 8, 12, and 24 h was compared with those exposed for 7 days or 5 months. Forty thousand microbial cells and 10 algae cm−2 attached to the glass slides within 1 h of exposure. Catalyzed reporter deposition-FISH demonstrated that the pioneer community that settled within 12 h was dominated by Cytophago–Flavobacteria. Later stages were characterized by an enrichment of Gammaproteobacteria and Betaproteobacteria. However, a major fraction of the detected bacterial cells could not be identified beyond the domain level. Green algae dominated the pioneering algal groups, but were outnumbered by filamentous algae after the attachment period. Potential activity of alkaline phosphatase was already detected after 4 h, β-glucosidase after 8 h, and β-xylosidase only after 7 days of biofilm formation. Thus, biofilm formation occurred rapidly and the functionality of the assemblages was evident within a few hours. However, the ratios of β-xylosidase : β-glucosidase suggested that the initial biofilms relied more on autochthonous than on allochthonous carbon sources in contrast to mature biofil