The Role of Microbial Extracellular Enzymes in the Transformation of Dissolved Organic Matter in Humic Waters

Abstract

Non-living natural organic matter (NOM) comprises in most aquatic environments the largest organic matter (OM) fraction, and it harbours also the largest organic carbon pool (Allen 1976; Wetzel 1983; Steinberg and Munster 1985; Thurman 1985; Hedges 1992; Munster 1993). Furthermore, NOM has been recognized as the most important carbon and energy resource in the detritus food chain processes (Williams 1981; Hobbie 1992; Wetzel 1992). Non-predatory processing and transfer of organic carbon in parallel with the classical grazing food chain are significant fractions of NOM passing and channelled through the aquatic food chain (Azam and Cho 1987; Cho and Azam 1988; Williams 1990; Ducklow 1991). The NOM pools can be subdivided into dissolved (DOM) and particulate organic matter (POM). Both of these summarize the major part of the organic detritus pools (Wetzel 1983; Thurman 1985). Besides NOM utilization by phagotrophic microbes such as flagellates (Sherr 1988; Sanders 1991; Tranvik et al. 1993), osmotrophic microorganisms such as bacteria and fungi are the major sinks for NOM, and by utilizing primarily DOM resources (Benner et al. 1986, 1989; Azam and Cho 1987; Cho and Azam 1988) they are closing the link between the classical grazing food chain and the microbial loop (Pomeroy 1974; Sorokin 1977; Williams 1981; Azam et al. 1983; Wetzel 1992). The ratio of living to nonliving OM in many aquatic environments has been estimated in the range of 100 to 10:1 and it empasizes the importance of NOM sources in aquatic ecosystem processes (Allen 1976; Wetzel 1984, 1990; Farrington 1992; Hobbie 1992; Munster and Albrecht 1994). The flow of OM through the aquatic food chain is mostly dominated at first by the photosynthetic production of OM, the utilization and transformation of NOM by microheterotrophs via the microbial food web, and finally its transference into the higher food chain, partially respired and/or its sedimentation to the bottom (Fig. 9.1).