The Influence of Nutrients

Suppression of secondary product formation by excess nutrients, especially by glucose and other easily degradable carbon sources, but also by nitrogen-containing compounds and phosphate, is a general phenomenon in microbial cultures. 

Suppression by excess nutrients has been found in the biosynthesis of polyketides (D 3.3), of gibberellins (D 6.3), of certain antibiotics, e.g., streptomycin (D 1.3), neomycin C (D 1.3), actinomycins (D 8.4.1), chloramphenicol (D 8.2), bacitracin A (D 23), enniatin B (D 23), cephalosporins (D 23.3), and penicillins (D 23.3), of alkaloids, e.g., benzodiazepines (D 8.4.2), and ergolines (D 21.2) etc. Usually the suppression of secondary product formation is accompanied by the suppression of other characteristics of cell specialization (such as conidiospore formation in Peni-cillium cyclopium), indicating a general influence of nutrient supply on cell specialization. 

In most cases the mechanism by which nutrients control secondary metabolism is unknown. However, glucose and other rapidly used carbon sources repress the formation of iV-acetylkanamycin amidohydrolase, thought to be the final enzyme of kanamycin A biosynthesis (D 1.3) and phenoxazinone synthase, an enzyme required in actinomycin biosynthesis (D 8.4.1). 

The report that cAMP relieves glucose repression of iV-acetylkanamycin amidohydrolase in Streptomyces kanamyceticus, a prokaryote ( ), indicates that the repression mechanism resembles that of different catabolic enzymes in bacteria, which proceed via the inhibition of adenylate cyclase, the enzyme that converts ATP to cAMP (D 10.4). As a consequence the concentration of cAMP decreases and the transcription by RNA polymerase of operons subjected to cAMP control is inhibited (catabolite repression). In eucaryotes, however, catabolite repression could not be demonstrated. In Penicillium cyclopium for instance, glucose suppression of benzodiazepine alkaloid biosynthesis cannot be overcome by administration of cAMP or cAMP derivatives. 

It is of interest that glucose suppression of alkaloid metabolism in P. cyclopium (D 8.4.2) can be at least partially blocked by the morphological organization of the mycelium. In mycelial mats growing at the surface of solid or liquid media, i.e., under natural growth conditions, cell specialization, as indicated by alkaloid production and conidiospore formation, is suppressed by administration of high glucose concentrations. This is because only one side of the mat has contact with the medium which slows the permeation of nutrients to the other cells. 

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The above description of eutrophication has illustrated the complex nature of the problem, particularly in relation to the influence of nutrients, the multiplicity of sources of phosphorus and the spectrum of its bio-availability. Clearly, the most effective long-term solution to many of our eutrophication problems will be to reduce the nutrient load to affected waters. However, it has also been shown that, because the concentrations of available phosphorus required to impose a control on primary production is very low (e.g. 5-10/rgU total dissolved phosphorus), the reduction of nutrients from any one source alone is unlikely to be effective. 

The ability to change and control the composition of the nutrient solution and the relatively small size of the microcosms used enables manipulation of environmental variables and time-course studies of rhizodeposition to be made relatively easily. The influence of nutrient availability, mechanical impedance, pH, water availability, temperature, anoxia, light intensity, CO2 concentration, and microorganisms have all been examined within a range of plant species (9). A few examples to illustrate the continued interest in examining the effect of such variables on rhizodeposition in nutrient culture are given in Table 1. 

Plant response in the ASTROCULTURE flight experiment. Soc. Automotive Eng. Tech. Paper 951624. Muro, J., Diaz, V., Goni, J. L., Lamsfus, C. (1997). Comparison of hydroponic culture and culture in a peat/sand mixture and the influence of nutrient solution and plant density on seed potato yields. Potato Res., 40,431 38. Musgrave, M. E. (2002). Seeds in space. SeedSci. Res., 12, 1-16. 

Torn, M. S., Vitousek, P. M., and Trumbore, S. E. (2005). The influence of nutrient availability on soil organic matter turnover estimated by incubations and radiocarbon modeling. Ecosystems 8(4), 352-372. 

Finally, in Equation 64, Zeng (1996a) modified his own original model (presented in Equations 28 to 30), by eliminating parameter S that assumes low values for hybridomas culture. The influence of nutrients is dependent, not on nutrient concentration, but on available mass of nutrient per cell unit S/Xv. 

Suberkropp, K. (1995). The influence of nutrients on fungal growth, productivity, and... 

R. van der Heijden, R. Wijnsma, R. Verpoorte, P.A.A. Harkes and A. Baerheim Svendsen, The influence of nutrient media, auxins and cytokinins on the initiation of alkaloid producing callus on leaf explants of eight species of Tabernaemontana, Fitoterapia, 57 (1986) 415-421. 

Under the influence of nutrients, they can often multiply at a great rate. 

In conclusion, the lower MY induced by nutrient restriction in lactating dairy cows was partly due to a lower number of mammary cells. This was a consequence of a higher level of apoptosis but also to a mammary remodelling by the extracellular matrix. The future objective will be the study of the influence of nutrient restriction on mammary cell activity. 

Indergaard, M Skijak-Break, G Jensen, A. Studies on the influence of nutrients on the composition and structure of alginate in Laminaria saccharina (L.) Lamour. (Laminariales, Phaeophyceae). Botanica Marina, 1990, 33, 277-288. 

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