Alternative Carbon Sources

The dsz genes in ECRD-1 are expressed at a baseline level constitutively in the presence of sulfate, although their levels are presumably higher in the absence of sulfate. Studies were conducted with and without sulfate. The Rhodococcus cells are known to use alkanes as substrates, thus an alternate carbon source was used to minimize hydrocarbon degradation. The hydrocarbon profile of the oils was demonstrated to be the same before and after the desulfurization. The experiments were aimed at identifying the extent of desulfurization of these oils and so were conducted at oil to water ratio of 1 1000. The results showed 90% or higher removal of CO to C4 DBTs in fully induced cells (without sulfate). In the presence of sulfate, the extent removal was lower. 

Alternative carbon sources (glucose/glycerol/succinate) are available, but nitrogen source is completely absent. 

Catabolite activator protein, CAP (also called cAMP receptor protein, CRP) is an activator required for high level transcription of the lac operon. The active molecule is a CRP dimer that binds 3 5 cyclic AMP to form a CRP-cAMP complex. CRP-cAMP binds to the lac promoter and increases the binding of RNA polymerase, stimulating transcription of the lac operon. CRP dimer without cAMP cannot bind to this DNA. The action of CRP depends upon the carbon source available to the bacterium. When glucose is present, the intracellular level of cAMP falls, CRP cannot bind to the lac promoter and the lac operon is only weakly transcribed. When glucose is absent, the level of intracellular cAMP rises, the CRP-cAMP complex stimulates transcription of the lac operon and allows lactose to be used as an alternative carbon source. 

Very small quantities of methanol are now consumed in the denitrification of waste water. Methanol is used as a carbon source by bacteria which convert nitrates and nitrites to nitrogen. Methanol is an efficient carbon source and decreases the production of byproducts. The potential requirement for a carbon source for water denitrification is large but the use of methanol is threatened by cheaper feedstocks. One large denitrification plant in the U.S., for example, uses brewery waste as a carbon source. In the future, water treatment plants designed for water denitrification may utilize a less expensive alternate carbon source than methanol. 

Biodegradation estimated t,/2 > 20 h in river, t,/2 < 20 h in eutrophic pond, t,/2 = 20 h in eutrophic lake and very long half-life in oligotrophic lake, based on the biodegradation rate in the presence of alternative carbon sources will be one-half the biodegradation rate of quinoline when quinoline is the only carbon source by the one compartment model (Smith et al. 1978). 

Methanol synthesis from waste C02 streams has the potential to contribute to the limitation of worldwide C02 emissions and to serve as an alternative carbon source to fossil fuels if a renewable source of hydrogen is available (see Section 5.3.1). The main obstacle to methanol synthesis from C02-rich streams is thermodynamics. The equilibrium yield of methanol from 25% C0/C02 75% H2 mixtures of varying C0/C02 ratio is shown in Figure 5.3.5. For pure CO, a one-pass methanol yield of nearly 55% can be obtained at 525 K, while pure C02 would only yield 18%. Besides the addition of CO, this equilibrium limitation can be overcome by operating at lower temperatures (an option that requires more active catalysts), implementing higher recycle ratios, or product extraction (an option that requires higher capital investment) [8]. 

Seyis, I. and Aksoz, N. (2005). Xylanase production from Trichoderma harzianum 1073 D3 with alternative carbon source and nitrogen sources. Food Technol. Biotechnol. 43,37-40. 

Terashima, M., et al., Utilization of an alternative carbon source for efficient production of human alpha(l)-antitrypsin by genetically engineered rice cell culture. Biotechnol Prog, 2001 17(3) 403-406. 

---