Streams bacterial activity

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. 

It is instructive to calculate ASO - H20(%o), which is the difference between the 5 0 values of sol and H2O in mine water (Fig. 12.22). In the absence of isotope fractionation and with Fe as the oxidant, a zero difference would be expected. Taylor et al. (1984b) propose that Fe dominates pyrite oxidation in submersed environments. For such conditions ASO - H20(%o) values exceed zero due to isotope fractionation, ranging from about 4 to 6 in sterile systems to about 11 in the presence of T. ferrooxidans. Fractionation is caused by the greater reactivity of light 0 than heavy 0, with thus favored in product sulfate. Bacterial activity (in this case T. ferrooxidans) causes more fractionation than takes place in the abiotic oxidation reaction. ASO - H20(% ) values range up to about 18 when O2 is the chief oxidant in the presence of bacteria. S uch conditions are expected in aerated streams and in shallow unsaturated materials (Taylor et al. 1984a, 1984b). 

The predilection of bacteria to associate with surfaces is the primary organizing principle of community structure. ZoBell (1943) presented a conceptual model of this organization surfaces concentrate nutrients bacterial attachment to surfaces maximizes concentration gradients and reduces diffusion distances packing cells on surfaces improves retention of extracellular enzymes. Over the next 40 years, the study of attached microbial communities became an integral part of several fields (see Chapter 12). The importance of periphyton, or aufwuchs, in stream metabolism was established (Hynes, 1970 Geesey et al., 1978 Stevenson et al., 1996). Comparative studies of community organization and activity in relation to substratum produced a taxonomy for attached communities epilithon, epiphyton,... 

Humic substances are abundant in DOM as well as POM, but the role of phenol oxidases in DOM metabolism has been little studied (Munster and De Haan, 1998). Foreman (1999) found that phenol oxidase activity in water from five contrasting Michigan streams was correlated with phenol concentration and that bacterial production in systems with significant humic DOC input was stimulated by the addition of phenols, suggesting that phenols may be an important growth substrate for some bacterial guilds. 

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