Microbial Catalytic Activities

The anodic microbial catalysts are expected to play a key role for the power density and efficiency of an MFC. The highest power generation of MFCs seems to be produced by those operating with a mixed culture, or a microbial community, rather than those operating with a pure culture. The structure and activity of the bacterial community are sensitive to various environmental conditions, such as solution pH, electrode potential, ionic strength, and temperature. Since these environmental parameters can also affect other processes (e.g. the proton transfer efficiency, and cathode performance), it is now more difficult to conclude a quantified relationship between the microbial community and these parameters. Moreover, the configuration and operating mode of MFCs also appear important for the composition and activity of anodic biofilms. 

The inoculum source is another factor affecting the anodic microbial community and activity. Kim et al. [67] suggested that start-up of an MFC is most successful with biofilm harvested from the anode of an existing MFC. In another study, enrichment of the bacteria on the anode of an MFC resulted in increased power output and a change in the bacterial community [17]. 

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Chemical pollution is the diversion of chemical elements from the natural biogeo-chemical cycles. The carbon, nitrogen, and phosphate in municipal wastes released to streams and lakes are removed from the soil-plant cycle, which is the source of the nitrogen and much of the phosphate. If those substances were instead put back directly into the soils from whence they came, much less pollution would result. Air and water only slowly convert their wastes back into their natural sites in plants and soils. Soil, on the other hand, has enormous surface area and microbial catalytic activity plus oxygen and water with which to deactivate pollutants. Soil degrades most... 

Up to date, performances of laboratory MFCs are still much lower than the ideal performance. Electricity generation in an MFC is a combined effect of (i) microbial catabolism, (ii) electron transfer from microbes to the anode (anode performance), (iii) reduction of electron acceptors at the cathode (cathode performance), and (iv) proton transfer from the anode to cathode. Factors affecting each of the above processes may have a great influence on the overall MFC performance, thus appropriate experimental conditions is very important. Many modifications have been carried out to improve each of these processes, leading to higher total power output of MFCs. As listed in Table 2.5, major factors affecting the power generation of MFCs include microbial catalytic activity, anode and cathode performances, proton transfer efficiency and solution chemistry. 

Microbial catalytic activity Use mixed cultures Inoculated with pre-acchmated bacteria Afjply appropriate anode potential Add surfactants into the anode biofilm... 

We can also use microbial cells (fermentation) containing the desired catalytic activity without isolating the enzymes responsible. 

Vazquez-Duhalt, R. Semple, K. M. Westlake, D. W. S., and Fedorak, P. M., Effect of Water-Miscible Organic-Solvents on the Catalytic Activity of Cytochrome-C. Enzyme and Microbial Technology, 1993. 15(11) pp. 936-943. 

Therefore the catalytic activity of these enzymes varies enormously and it is in theory possible that at least in some cases the quantity or activity of the hydrogen evolving enzyme could limit the overall process. However, there is little evidence for this being the limiting factor in any system. Indeed, in many microbial systems, potential catalytic activity far surpasses the amount of hydrogen produced, suggesting that other metabolic factors are limiting. 

Microbial-enhanced oil recovery, 78 630 Microbial enzymes, 70 262, 263 catalytic activity of, 76 413 producing, 76 403 Microbial genes, sources of, 72 474 Microbial genomics, 72 472 Microbial growth, in the papermaking process, 78 127... 

However, if one accepts the hypothesis that only microbially promoted processes are occurring, it becomes necessary to explain the approximately linear dependence of the oxidation rate on 02 partial pressure as a conveniently exact effect on metabolic reaction rates. Ross and Bartlett (76) suggested that oxidation of Mn2+ is initiated by bacteria but that subsequent reaction is dominated by abiotic autocatalytic processes, once catalytically active colloids and particles are formed in sufficient numbers. 

The sol-gel procedure enables encapsulation of enzymes in optically transparent, porous silicate matrices, under mild room-temperature conditions. The small pores prevent microbial degradation and, due to the biomolecule size, they will not diffuse out of the polymer network. The physical encapsulation avoids self-aggregation effects as well as protein unfolding and denaturalization. At the same time, the catalytic activity is maintained as the enzymes are able to react with small substrates that can transfer across or within the support, assuring continuous transformations [75]. 

Microbial cells contain or produce at least 2500 different enzymes which can catalyze biochemical reactions leading to growth, respiration and product formation. Most of these enzymes can readily be separated from cells and can display their catalytic activities independently of the cells. Although microbial enzyme activities have been observed for many centuries, only recently have microbial enzymes been commercially utilized. 

Microbes can control the local geochemical environment of actinides and alfect their solubility and transport. Francis et al. (1991) report that oxidation is the predominant mechanism of dissolution of UO2 from uranium ores. The dominant oxidant is not molecular oxygen but Fe(III) produced by oxidation of Fe(II) in pyrite in the ore by the bacteria Thiobacillus ferroxidans. The Fe(III) oxidizes the UO2 to UOl. The rate of bacterial catalysis is a function of a number of environmental parameters including temperature, pH, TDS, fo2, and other factors important to microbial ecology. The oxidation rate of pyrite may be increased by five to six orders of magnitude due to the catalytic activity of microbes such as Thiobacillus ferroxidans (Abdelouas et al., 1999). 

When proteinoids were heated in buffer at pH 6.2 or 6.8, loss of catalytic activity was observed. The extent of loss ranged from 95 to 11% (Table II). Those proteinoids that initially showed higher levels of activity relative to histidine were the most affected by the heat treatment. After heating, the level of activity was comparable to that of the equivalent amount of histidine, or to that of mineral acid hydrolysates of the polymer. Under similar conditions, a-chymotrypsin was 97% inactivated. The fact that the control tests on L-histidine or A -carbo-benzoxy-L-histidine showed no effect is consistent with the inference that inactivation is due to disruption of a macromolecular conformation. Copolymers prepared from only aspartic acid and histidine were also active on NPA and were inactivated by the heat treatment. The percentages of inactivation ranged from 62 to 19. Polymers prepared and processed under aseptic conditions were both catalytically active and subject to inactivation by heat. These experiments were performed as routine verification that the respective phenomena do not result from the presence, and subsequent denaturation, of contaminating microbial enzymes. 

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