Extracellular Microbial Activity

All soil metabolic proce.sses are driven by enzymes. The main sources of enzymes in soil are roots, animals, and microorganisms the last are considered to be the most important (49). Once enzymes are produced and excreted from microbial cells or from root cells, they face harsh conditions most may be rapidly decomposed by organisms (50), part may be adsorbed onto soil organomineral colloids and possibly protected against microbial degradation (51), and a minor portion may stand active in soil solution (52). The fraction of extracellular enzyme activity of soil, which is not denaturated and/or inactivated through interactions with soil fabric (51), is called naturally stabilized or immobilized. Moreover, it has been hypothesized that immobilized enzymes have a peculiar behavior, for they might not require cofactors for their catalysis. 

Evidence has been accumulating that extracellular metabolic activity of microorganisms, mainly bacteria, occurs within skin glands and on the skin surface (Albone, 1997). Sterile human apocrine secretions do not develop the characteristic axillary odour (Shelley et al., 1953). In the salivary secretions of the boar, transformations of the pheromonal 16-androstene steroids 2. were attributed to the microbial flora (Booth, 1987). 

Measurement of exoenzymatic activities is potentially useful in detecting the effects of toxicants on heterotrophic biofilm communities. Sensitivity and direct relationship with organic matter use and, therefore, microbial growth make extracellular enzyme activities a relevant tool to assess the toxicity of specific compounds. Use of novel approaches that combine enzymatic and microscopic tools (e.g. ELF-phosphatase) may be extremely useful to detect anomalies at the sub-cellular scale. 

The diversity of functions within a microbial population is important for the multiple functions of a soil. The functional diversity of microbial communities has been found to be very sensitive to environmental changes (Zak et al. 1994 Kandeler et al. 1996,1999). However, the methods used mainly indicate the potential in vitro functionality. Functional diversity of microbial populations in soil may be determined by either expression of different enzymes (carbon utilization patterns, extracellular enzyme patterns) or diversity of nucleic acids (mRNA, rRNA) within cells, the latter also reflecting the specific enzymatic processes operating in the cells. Indicators of functional diversity are also indicators of microbial activity and thereby integrate diversity and function. 

Attempts to identify extracellular esterase activity in PVA-contaminated sites with proven microbial degradation activity showed no substantial results. The breakdown of the polymer proceeds without concomitantly high extracellular esterase activities [36]. These findings suggest that intracellular esterases are the... 

Component enzymes of the cellulase system have been purified from several microbial species (1-13), among which mutants of the imperfect fungus Trichoderma provide the highest levels of extracellular enzyme activity (14). From this organism have been purified / -glucosi-dases (EC 3.2.1.21), endo-l,4-/ -D-glucanases (EC 3.2.1.4) and 1,4-/ -d-... 

Chappell, K. R., and R. Goulder. 1995. A between-river comparison of extracellular-enzyme activity. Microbial Ecology 29 1-17. 

Hoppe, H.-G. 1991. Microbial extracellular enzyme activity A new key parameter in aquatic ecology. In Microbial Enzymes in Aquatic Environments. Springer-Verlag, Berlin. 

Karner, M., and F. Rassoulzadegan. 1995. Extracellular enzyme activity Indications for high short-term variability in a coastal marine ecosystem. Microbial Ecology 30 143-156. 

Keith, S. C., and C. Arnosti. 2001. Extracellular enzyme activity in a river-bay-shelf transect Variations in polysaccharide hydrolysis rates with substrate and size class. Aquatic Microbial Ecology 24 243—253... 

Meyer-Reil, L.-A. 1987. Seasonal and spatial distribution of extracellular enzymatic activities and microbial incorporation of dissolved organic substrates in marine sediments. Applied Environmental Microbiology 53 1748-1755. 

Munster, U. 1991. Extracellular enzyme activity in eutrophic and polyhumic lakes. In "Microbial Enzymes in Aquatic Ecosystems (R. J. Chrost Ed.), pp. 96-122 Springer-Verlag, New York. 

Hoppe, H.-G. 1993. Use of fluorogenic model substrates for extracellular enzyme activity (EEA) measurement of bacteria. In Handbook of Methods in Aquatic Microbial Ecology (P. F. Kemp, B. F. Sherr, E. B. Sherr and J. J. Cole, Eds.), pp. 423-432. Lewis Publishers, Ann Arbor, MI. 

Hoppe H., S. Kim, and K. Gocke. 1988. Microbial decomposition in aquatic environments Combined process of extracellular enzyme activity and substrate uptake. Applied and Environmental Microbiology 54 784—790. 

Mineralization of organic macromolecules is initiated by extracellular enzymes because bacteria are unable to hydrolyze substrates that are much larger than about 600 Da (Weiss et al., 1991). Not all bacteria are capable of synthesizing these enzymes, as is often the case with those responsible for terminal decomposition and some intermediary metabolisms. As a result, these terminal organisms depend heavily on the activities of other bacteria for substrates. It is clear that polymer hydrolysis occurs since these compounds are required to support microbial activities in sediments, but some studies have failed to detect polymer hydrolysis potentials sufficient to support in situ rates of metabolism (Arnosti, 1998). Such studies underscore the difficulties of examining hydrolytic processes. 

Mn oxidation and scavenging in plumes appear to be dominated by microbial activity (see review in Winn etal., 1995). Plume bacteria are often characterised by capsules of abundant extracellular polymer matrices that are believed to scavenge Mn from solution (e.g. Cowen etal., 1986, 1999). Radiotracer experiments (54Mn uptake) and elevated microbial biomass at plume depths suggest that microbial metabolic activity enhances Mn scavenging (Cowen etal., 1986). However, it is not clear if this activity includes active Mn2+ oxidation or is simply due to non-enzymatic processes. 

Whitton, B.A., Grainger, S.L.J., Hawley, G.R.W. and Simon, j.W. (1991) Cell-bound and extracellular phosphatase activities of cyanobacterial isolates. Microbial Ecology 21,85-98. 

Because extracellular enzyme activity is the rate-limiting step in microbially mediated decomposition of plant detritus in wetlands and aquatic systems, many researchers have studied enzyme... 

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