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Biofilms in the stream food web

Microbial biofilms are composed of microscopic heterotrophic, photosynthetic and chemotrophic bacteria, fungi, diatoms and algae in a complex polymer linked assemblage. Organisms within the biofilm recycle organic detritus in the stream, use dissolved organic matter, fix nitrogen or recycle organic nitrogen, and fix energy and carbon by photosynthesis and chemosynthesis. So they play a major role in the bottom up supply of energy and organic matter to the food web.

Within stream food webs, biofilms are intensively grazed by protozoa, macrobenthic invertebrates and some fish. A recent hypothesis (Sheldon et al 1997) is that the specific microbial composition of the biofilm will modify the nutritional quality of the material for grazing species. For example, a dominance of heterotrophic bacteria and nitrogen fixing cyanobacteria will provide higher levels of protein and carbohydrates, both as cell mass and in extracellular polymers (low C:N ratio) than will a biofilm dominated by photosynthetic algae where polycaccharide (including cellulose) levels are high but protein relatively low (high C:N ratio). In this situation extracellular polymers are made up of polysaccharides, protein and nucleic acids, and this material is secreted by bacteria to enhance adherence to surfaces and provide a protection from the surrounding environment. The mass of extracellular polymer in a biofilm may be 10 times that of the bacterial cell mass (Hall and Meyer 1998). According to ecological principles stream biofilm composition will be modified by the physicochemical characteristics of the environment, predation by grazers and presence or absence of specific biofilm species. Biofilm composition then, along with habitat factors, will influence the population level and composition of grazing organisms and so on through the food chain.

Sheldon F. & Walker K.F. (1997). Changes in biofilms induced by flow regulation could explain extinctions of aquatic snails in the lower River Murray, Australia. Hydrobiologia 347: 97-108. 1997

Hall, R. O. J. & Meyer, J. L. (1998). The trophic significance of bacteria in a detritus-based stream food web. Ecology 79, 1995-2012

 

 

 

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Stream Biofilm Research Group
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The University of Auckland
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