Streams biofilm bacteria

Because of the high area of solid surfaces covered with biofilms, these biofilms dominate the heterotrophic metabolism in many aquatic ecosystems. In streams, rivers, and shallow lakes, bacterial activity in epilithic and epiphytic biofilms may be several times higher on an areal basis than the activity of free living bacteria. By the differential use of specific DOM fractions, biofilm bacteria influence the biogeochemical composition of DOM in these ecosystems. Biofilms thus can control biogeochemical fluxes of DOM and are important sinks of organic matter. 

Aside from adding defined compounds, experimental additions of natural DOM mixtures suspected to vary in lability have helped test ideas about the contribution of various DOM sources to aquatic ecosystems. In a nice example using manipulation of natural DOM sources, Battin et al. (1999) used flowthrough microcosms to measure the relative uptake rates of allochthonous and autochthonous DOM by stream sediments. They documented greater than fivefold differences or more in uptake and respiration, depending on whether the DOM was extracted from soil or periphyton. Moreover, they were able to show, via transplant experiments, several cases where prior exposure to a particular source of DOM increased the ability of that community to metabolize the DOM supplied. There appears to be some preadaptation of microbial catabolic capacity when these stream biofilms were re-exposed to a familiar type of DOM. Similarly, the response of heterotrophic bacteria to carbon or nutrient addition was greatest when the source community was particularly active (Foreman et al., 1998). Kaplan et al. (1996) showed that fixed film bioreactors, colonized on one water source, were unable to rapidly metabolize DOC in water from another source. 

Microbial cells transported with the stream of fluid above the surface interact with conditioning films. Immediately after attachment, microorganisms initiate production of slimy adhesive substances, predominantly exopolysaccharides (EPS) that assist the formation of microcolonies and microbial films. EPS create bridges for microbial cells to the substratum and permit negatively charged bacteria to adhere to both negatively and positively charged surfaces. EPS may also control interfacial chemistry at the mineral/biofilm interface. 

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