Carbon flows

Some chemicals such as iadigo, tryptophan, and phenylalanine are overproduced ia bacteria by pathway engineering (36—38). In this method, the enzymes iavolved ia the entire pathway are overproduced. In addition, the host bacterium is also altered such that the carbon flow is directed toward the engiaeered pathway (38). E. colih.2LS been modified to overproduce iadigo and tryptophan and phenylalanine. CoTjnebacteriumglutamicum has been engiaeered to overproduce tryptophan from 28 to 43 g/L. Similarly, attempts are underway to overproduce several vitamins by pathway engineering (34,38). 

Three commercial processes that use these various hot carbonate flow arrangements are the promoted Benfield process, the Catacarb process, and the Giammarco-Vetrocoke process (26—29). Each uses an additive described as a promoter, activator, or catalyst, which increases the rates of absorption and desorption, improves removal efficiency, and reduces the energy requirement. The processes also use corrosion inhibitors, which aHow use of carbon—steel equipment. The Benfield and Catacarb processes do not specify additives. Vetrocoke uses boric acid, glycine, or arsenic trioxide, which is the most effective. 

To find out where the carbon flow goes to we can simply investigate Figure 85. It is obvious that... 

DeRito CM, GM Pnmphrey, EL Madsen (2005) Use of field-based stable isotope probing to identify adapted popnlations and track carbon flow throngh a phenol-degrading soil microbial conunnnity. Appl Environ Microbiol 71 7858-7865. 

S. J. Grayston, D. Vaughan, D. Jones. Rhizosphere carbon flow in trees, in comparison with annual plants the importance of root exudation and its impact on microbial activity and nutrient availability. Appl. Soil Ecol. 5 29 (1996). 

A. A. Meharg, A critical review of labelling techniques used to quantify rhizosphere carbon flow. Plant and Soil 166 55 (1994). 

A. A. Meharg and K. Killham, The effect of. soil pH on rhizosphere carbon flow of Lolium perenne. Plant and Soil 123 1 (1990). 

Recurrent is the lack of adequate techniques to assess carbon flows through the plants and microbes into soil organic matter (151). Most important is the development of techniques and protocols to separate rhizosphere from nonrhizosphere soil as well as possibly to facilitate analyses of soil carbon dynamics. The use of carbon isotopes, and, where possible, application of double labeling with C and C, seems inevitable in order to separate the contribution of different substrates to the formation of the soil organic matter pool and to get to an understanding of the ecological advantage of exudates and rhizodeposits. 

W. Cheng, D. C. Coleman, C. R. Carroll, and C. A. Hoffman, Investigating shortterm carbon flows in the rhizosphere of different plant species using isotopic trapping. Ai>ron. J. 86 782 (1994). 

E. Paterson, J. M. Hall, E, A. S. Rattray, B. S. Griffiths, K. Ritz, and K. Killham, Effect of elevated COj on rhizosphere carbon flow and soil microbial processes. Glob. Change Biol. 5 363 (1997). 

Methodological Approaches to the Study of Rhizosphere Carbon Flow and Microbial Population Dynamics... 

Such differences in the amount and type of rhizodeposition that occur on the root with time result in concomitant variations in microbial populations in the rhizosphere, both within the root (endorhizosphere), on the surface of the root (rhizoplane), and in the soil adjacent to the root (ectorhizosphere). The general microbial population changes and specific interaction of individual compounds from specific plants or groups of plants with individual microbial species are covered in detail elsewhere (Chap. 4). Consequently, this chapter is restricted to consideration of methodologies used to study carbon flow and microbial population dynamics in the rhizosphere, drawing on specific plant-microbe examples only when required. 

Plants grown for longer periods in solid supports such as sand or soil repre-.sent the next level of complexity and, although other techniques are available, carbon flow is most frequently estimated using C labeling experiments. In the laboratory, COt can be supplied to shoots either as a short pulse or continuously, and the carbon flow can be monitored. In the field, due to technical limitations, only COi pulse labeling procedures are possible. A final approach, termed crop studies, involves the measurement of components of crop growth from which... 

Rather than measuring rhizodeposition in nutrient. solutions chemically, another approach has been to expose shoots to COi for a short period of time and to follow the spread of the through the plant, into the roots, and then into the nutrient solution. Kinetics of carbon flow and quantification of rhizodeposition can then be obtained (e.g.. Ref. 24). Advantages and limitations of this approach are discussed more fully in Sec. II.C. 

Two different approaches to the use of C02 labeling have been applied to measure carbon flow within plant-soil systems—pulse-chase and continuous labeling. 

Table 2 Examples of CO Labeling Studies to Examine the Effect of Treatment Influencing Carbon Flow in Soil... 

One practical aspect of the procedure for monitoring carbon flow following C labeling is the need to separate roots from the soil for analysis. Incomplete removal of roots can lead to an overestimation of rhizodeposition, but overzealous washing of soil may lead to leaching of " C or loss of fine roots. This problem has been examined in detail for wheat and barley, and procedures to correct for these errors have been developed (69). 

Table 3 Examples of Experiments Employing Crop Study Techniques to Examine the Effect of Treatments Influencing Carbon Flow to Roots or Soil...

A final, highly specialized procedure for monitoring carbon flow in plants involves the use of "CO . The use of this positron gamma-emitting isotope of C, with a half-life of 20.3 min, allowed several physiological parameters of my-corrhizal and non-mycorrhizal plants of Panicum coloratum to be measured simultaneously in real time (82). However, the technical problems associated with using "COi, particularly the ability to produce i.sotopes of carbon, and with handling this short half-life are likely to limit this approach to specialised facilities. 

III. SIGNIFICANCE OF CARBON FLOW IN MICROBIAL POPULATION DYNAMICS... 

It has taken time for methods to be developed that meet the desire of microbial ecologists to learn more about carbon flow and microbial communities in natural environments. In recent years techniques have developed at considerable speed that offer the potential to answer some of the commonly asked questions. Use of C techniques for measuring carbon flow is one example. While questions still remain about the interpretation and relevance of some modern methods regarding the real makeup and interactions in mixed populations, there is no doubt that the use of biochemical or molecular signatures provides the best opportunity to advance our knowledge in this area. 

J. M. Norton, J. L. Smith, and M. K. Firestone, Carbon flow in the rhizosphere of ponderosa pine seedlings. Soil Biol. Biochem. 22 449 (1990). 

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