Microbial activity populations

In situations where conserved internal markers caimot be used, such as in spills of essentially pure compounds, the evidence for enhanced biodegradation may have to be more indirect. Oxygen consumption, increases in microbial activity or population, and carbon dioxide evolution have all been used with success. 

Several studies have indicated that the species diversity of indigenous soil communities will influence the species composition of ectorhizosphere populations (176). On mature roots, seasonal successions may be observed as the soil microbial activity varies with temperature, water content, nutrition, and root exudation. Acero et al. (177) found that the composition of alder (Almis) rhizosphere populations alternated between one dominated by Bacillus spp. in autumn and winter and one dominated by Pseudomonas spp. in spring and summer. 

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. 

Indicators of microbial activity in soil represent measurements at the ecosystem level (e.g., processes regulating decomposition of organic residues and nutrient cycling, especially nitrogen, sulfur and phosphorus). Measurements at the community level include bacterial DNA and protein synthesis. Frequency of bacteriophages is a measurement at the population level. 

Wetland remediation involves a combination of interactions including microbial adsorption of metals, metal bioaccumulation, bacterial oxidation of metals, and sulfate reduction (Fennessy Mitsch, 1989 Kleinmann Hedin, 1989). Sulfate reduction produces sulfides which in turn precipitate metals and reduce aqueous metal concentrations. The high organic matter content in wetland sediments provides the ideal environment for sulfate-reducing populations and for the precipitation of metal complexes. Some metal precipitation may also occur in response to the formation of carbonate minerals (Kleinmann Hedin, 1989). In addition to the aforementioned microbial activities, plants, including cattails, grasses, and mosses, serve as biofilters for metals (Brierley, Brierley Davidson, 1989). 

Jordahl et al. (1997) reported that hybrid poplar root exudates stimulated soil microbial activity involved in chemical degradation. A significant increase occurred in populations involved in the degradation of several chemicals, but only a minor increase occurred in populations that degrade atrazine. Rhizosphere effects on atrazine degradation are unclear and remain under investigation. 

A study was conducted to determine the effects of combinations of organic amendments and benzaldehyde on plant-parasitic and non-parasitic nematode populations, soil microbial activity, and plant growth (Chavarria-Carvajal et al., 2001). Pine bark, velvetbean and kudzu were applied to soil at rates of 30 g/kg and paper waste at 40 g/kg alone and in combination with benzaldehyde (300 mul/kg), for control of plant-parasitic nematodes. Pre-plant and post-harvest soil and soybean root samples were analyzed, and the number of parasitic and non-parasitic nematodes associated with soil and roots were determined. Soil samples were taken at 0, 2, and 10 weeks after treatment to determine population densities of bacteria and fungi. Treatment... 

In this chapter I describe how microbial activity may be estimated and what data are presently available on a) rates of microbial transformation and utilization, b) phenolic acid effects on soil and rhizosphere microbial populations, and c) the influences of soil and rhizosphere microbial populations on phenolic acid phytotoxicity. The resulting insight is then used to suggest a possible approach by which this hypothesis may be tested experimentally. 

Although all of these approaches to estimate microbial activity (i.e., enzyme activity, utilization of substrates, formation of products, respiration, or changes in microbial populations) could be determined, only changes in microbial populations that can utilize phenolic acids as a sole carbon source have been related to phenolic acid depletion from soil solutions.3... 

Given the bacterial populations that utilized p-coumaric acid as a sole carbon source and the physicochemical (e.g., constant temperature, adequate nutrition and moisture) and biotic conditions of these two laboratory systems, utilization of p-coumaric acid ranged from 0.6 to 5.0 pg/g soil/h for the open systems and 8.6 pg/g soil/h for the closed system. The pg values for the open system represent steady-state rates as modified by nutrition, while the pg values for the closed system represent maximum rates. Whether such rates ever occur in field soils is not known, since the physicochemical and biotic environments of field soils are so different from those of laboratory systems. Laboratory soil systems provide potential rates of utilizations, but until field rates are determined the importance of microbial activity in phenolic acid depletion from soil solutions will not be known. 

Stanley, P. M., Gage, M. A., and Schmidt, E. (1979). Enumeration of specific populations by immunofluorescence. In Methodology of biomass determinations and microbial activities in sediments, . Vol ASTP STP 673 American Society for Testing and Materials, West Consho-hocken, PA. pp. 46—55. 

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