Growth substrate transformation

To describe the observed enhancement in the rate of cometabolism in the presence of growth substrate, a modification of equation 8 was recently proposed [6]. This modification introduces a second transformation capacity term, the growth substrate transformation capacity Tc ... 

Tc = growth substrate transformation capacity, mg nongrowth substrate/mg growth substrate consumed during growth. 

For those pesticides that are cometabolized, ie, not utilized as a growth substrate, the assumption of first-order kinetics is appropriate. The more accurate kinetic expression is actually pseudo-first-order kinetics, where the rate is dependent on both the pesticide concentration and the numbers of pesticide-degrading microorganisms. However, because of the difficulties in enumerating pesticide-transforming microorganisms, first-order rate constants, or half-hves, are typically reported. Based on kinetic constants, it is possible to rank the relative persistence of pesticides. Pesticides with half-hves of <10 days are considered to be relatively nonpersistent pesticides with half-hves of >100 days are considered to be relatively persistent. 

TCE is the other major contaminant at the site and is a common groundwater contaminant in aquifers throughout the United States [425]. Since TCE is a suspected carcinogen, the fate and transport of TCE in the environment and its microbial degradation have been extensively studied [25,63, 95,268,426,427]. Reductive dechlorination under anaerobic conditions and aerobic co-metabolic processes are the predominant pathways for TCE transformation. In aerobic co-metabolic processes, oxidation of TCE is catalyzed by the enzymes induced and expressed for the initial oxidation of the growth substrates [25, 63, 268, 426]. Several growth substrates such as methane, propane, butane, phenol, and toluene have been shown to induce oxygenase enzymes which co-metabolize TCE [428]. 

In fermentation reactors, cell growth is promoted or maintained to produce metabolite, biomass, transformed substrate, or purified solvent. Systems based on macro-organism cultures are usually referred as tissue cultures. Those based on dispersed non-tissue forming cultures of micro-organisms are loosely referred as microbial reactors. In enzyme reactors, substrate transformation is promoted without the life-support system of whole cells. Frequently, these reactors employ immobilized enzymes, where an enzyme is supported on inert solids so that it can be reused in the process. Virtually all bioreactors of technological importance deal with a heterogeneous system involving more than two phases. 

The substrate is added to the fermentation medium at the time of inoculation or during a later phase of microbial growth1. The optimum time of substrate addition must be determined, whereafter incubation is continued until maximum yield of transformation has been reached. The level of the enzyme responsible for the desired hydroxylation may be enhanced by induction if the substrate is added during active growth of the microorganism. On the other hand, if the substrate inhibits cell growth, substrate addition must be delayed until maximum cell mass is obtained. 

Microorganisms including yeasts and bacteria are able to produce D-amino acid oxidases, and interest has centered particularly on the use of /7-alkane as growth substrates (Kawamoto et al. 1977). The enzyme has achieved importance for its role in carrying out the first step in the transformation of cephalosporin C to 7-aminocephalosporanic acid that is an intermediate for the synthesis of semisynthetic cephalosporins. The nucleotide sequence of the enzyme from Rhodotorula gracilis ATCC 26217 has been established and the gene could be overexpressed in Escherichia coli (Alonso et al. 1998). 

Transformations of growth substrate by bacteria can generally be described using saturation kinetics [42] ... 

When a growth substrate inhibits its own transformation at high concentrations, Haldane kinetics are often used ... 

DCM can serve as a growth substrate under anaerobic conditions [93, 99]. The principal transformation pathways are oxidation to CO and fermentation to acetate. COj-reducing methanogens use some of the... 

The morphology of the product derived from the vapor phase (Table III) depends on concentration and diffusion factors [27], the particular phase transformation and in turn on the melting point of the metal catalyst being used, and on the liquidus temperature of the growth substrate. 

The pH is therefore very important and comes into play at several levels in the selection of the best adapted strains in the growth rate and yield in the malolactic activity and even in the nature of the substrates transformed. 

Funk et al. (13) reported that the biotransformation of TNT occurred over an experimental time frame of 4 days, and RDX over 24 days. This long period required for the observed biotic transformation may be due to the limited amount of growth substrate supplied to the consortium in these experiments. The comparatively short time required for... 

The reactant is referred to as a substrate. Alternatively it may be a nutrient for the growth of cells or its main function may require being transformed into some desirable chemical. The cells select reactants that will be combined and molecules that may be decomposed by using enzymes. These are produced only by living organisms, and commercial enzymes are produced by bacteria. Enzymes operate under mild conditions of temperature and pH. A database of the various types of enzymes and functions can be assessed from the following web site http //www.expasy.ch/enzyme/. This site also provides information about enzymatic reactions. 

Like enzymes, whole cells are sometime immobilized by attachment to a surface or by entrapment within a carrier material. One motivation for this is similar to the motivation for using biomass recycle in a continuous process. The cells are grown under optimal conditions for cell growth but are used at conditions optimized for transformation of substrate. A great variety of reactor types have been proposed including packed beds, fluidized and spouted beds, and air-lift reactors. A semicommercial process for beer used an air-lift reactor to achieve reaction times of 1 day compared with 5-7 days for the normal batch process. Unfortunately, the beer suffered from a mismatched flavour profile that was attributed to mass transfer limitations. 

---