Ecosystem Consequences

Biofilms enhance bacteria-DOM interactions by several means. Their spatial and chemical heterogeneity provides additional sorption sites for DOM compared with clean surfaces. Their loose architecture with interstitial voids and channels increases diffusivity and to some extent allows convective flow within biofilm structures. Because bacteria metabolize organic matter sorbed to the biofilm, a diffusion flux from the free water to the biofilm is maintained. Large proportions of organic matter sorbed to the biofilm are not instantly turned over but remain in the biofilm as a reservoir, which buffers direct effects of DOM depletion in the water column. 

For bacteria, there may be some adverse effects of living in a biofilm, especially in regions close to the substratum there, bacteria are to some extent cut off from the flux of DOM from the surrounding water. Additionally, bacterial enzymes can be inactivated if they are sorbed to surfaces. On the other hand, fixed positions of bacteria in biofilms support interactions between bacteria either within bacterial clones or between different bacterial species. They take advantage of growing in clones by saving energy spent for the production of extracellular enzymes, and they mutually interact between species in several ways. 

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. 

I thank Hans-Curt Flemming and Thomas Griebe for helpful comments on an earlier draft of this contribution. 

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