The ecology and biogeochemistry of stream biofilms

Key Points Biofilms dominate microbial life in stream ecosystems. These matrix-enclosed and surface-attached microbial communities are ubiquitous, prolific and highly active at the interfaces of the streambed. The biofilm mode of life is advantageous in streams with a fast flow of water and continuous export of nutrients and organic matter. Biofilms in streams can be considered a 'microbial skin', regulating the processing and export of nutrients and organic matter from catchments and influencing the dispersal of microorganisms and their biodiversity dynamics at the scale of entire stream networks. Interactions between the growth of biofilms, streamwater flow and substratum chemistry produce emergent environmental complexity in the streambed. Proteobacteria and Bacteroidetes often dominate the communities of stream biofilms. Flavobacteriia and Sphingobacteriia seem to be especially important members of these communities. Archaea are found within niche microenvironments established by the metabolic activity of other microorganisms. High biodiversity in stream biofilms is supported by continuous input of microorganisms from upstream catchments, environmental sorting induced by habitat heterogeneity (ranging from the scale of the biofilm to large stream networks) and episodic disturbance from streamwater flow. New interdisciplinary approaches are needed to link structure and function of biofilms to their environment and, ultimately, to ecosystem processes and biogeochemical fluxes in streams. This is crucial to understand and predict implications of global ecosystem change and climate change on the microbial ecology and functioning of stream ecosystems. Biofilms dominate microbial life in streams and rivers. In this Review, Battin and colleagues describe the interactions between the microbiome of stream biofilms and ecosystem processes, and they consider the effects of global ecosystem change and climate change on these biofilms. Streams and rivers form dense networks, shape the Earth's surface and, in their sediments, provide an immensely large surface area for microbial growth. Biofilms dominate microbial life in streams and rivers, drive crucial ecosystem processes and contribute substantially to global biogeochemical fluxes. In turn, water flow and related deliveries of nutrients and organic matter to biofilms constitute major constraints on microbial life. In this Review, we describe the ecology and biogeochemistry of stream biofilms and highlight the in