Uptake of DOC

DOM is removed from streamwater by both abiotic and biotic processes. The principal biotic processes are uptake by microorganisms, assimilation of the organic C into microbial biomass, consumption of this heterotrophic production, and its eventual remineralization to CO2 by community respiration. DOC is also removed from the water column by abiotic sorption and transformed into other compounds by photodegradation. Dahm (1981) estimated that adsorption onto clays and chemical complexing with oxides of aluminum and iron accounted for up to one third of the initial removal of DOC from the water column. Over a period of several days, however, microbial uptake was responsible for the majority of DOC disappearance from the water column into the sediment layer. Photochemical degradation results in the transformation of DOC into other inorganic and organic compounds. Although it is not clear whether these organic products are more or less available to bacteria than the initial DOC, most studies conducted in freshwater systems using humic compounds or DOC from vascular plants have found that photochemical degradation enhances biological availability (Moran and Covert 2003).

The incorporation of DOC into microbial biomass is of interest because of its potential as an energy input into stream food webs. It is a central tenet of this chapter that detrital energy pathways can be as or more important than primary production, and DOC can be a major C source for heterotrophic microorganisms. Bacteria likely play an even greater role in this regard than do fungi, but it is also apparent that different microorganisms are intimately intertwined with various OM sources as well as with algae in complex energy-processing sites known as biofilms (Figure 7.13). Experimental study of the response of bacterial abundance and biomass to different C sources and nutrient levels is the basis for most current knowledge, but rapid advances in microbial ecology including the ability to assay for key enzymes and to survey microorganisms for the functional genes that encode particular enzymes promise new insights

FIGURE 7.13 A structural and functional model of the organic microlayer-microbial community found as a surface biofilm on stones and other submerged objects in streams. The matrix of polysaccharide fibrils produced by the microbial community binds together bacteria, algae, and fungi, and is inhabited by protozoans and micrometazoans that graze on this material. Detrital inputs include dissolved, colloidal, and fine particulate organic matter, while light energy is trapped by algal photosynthesis. Within the matrix, extracellular release and cell death result in enzymes and other molecular products that are retained due to low diffusion rates and thus available for utilization by other microorganisms. (Modified from Lock 1981.)

FIGURE 7.13 A structural and functional model of the organic microlayer-microbial community found as a surface biofilm on stones and other submerged objects in streams. The matrix of polysaccharide fibrils produced by the microbial community binds together bacteria, algae, and fungi, and is inhabited by protozoans and micrometazoans that graze on this material. Detrital inputs include dissolved, colloidal, and fine particulate organic matter, while light energy is trapped by algal photosynthesis. Within the matrix, extracellular release and cell death result in enzymes and other molecular products that are retained due to low diffusion rates and thus available for utilization by other microorganisms. (Modified from Lock 1981.)

into such processes as leaf breakdown and biofilm activity (Findlay and Sinsabaugh 2006, Zak et al. 2006).

In addition to surface biofilms, bacteria in the water column of rivers and attached to particles can process available DOC and potentially be important in food webs. Bacteria are responsible for a large proportion of ecosystem respiration in large rivers (Richey et al. 1990; Benner et al. 1995), and the quantity of C lost by respiration is nearly 13 times greater than total organic C transported by the Amazon River to the ocean (Richey et al. 2002). Carbon is generally considered to be an important limiting resource for bacterial production, and so variation in the quantity and quality of DOC and POC sources is of considerable importance. In addition, nutrients, temperature, oxygen, and many other environmental factors will influence the incorporation of these C sources into microbial biomass. The following sections summarize current knowledge of microbial utilization of DOM, primarily by bacteria, on and within the streambed and in the water column. Because microorganisms commonly are associated with particulate matter, utilization of DOM does not separate easily from utilization of fine POM.

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