Carbon cycle nutrient supply

On a localized basis, human activity can affect the carbon cycle indirectly by increasing the supply of mineral nutrients to aquatic environments. A large nutrient supply supports high levels of primary productivity by phytoplankton (algae and cyanobacteria), a process described as eutrophication. Under natural conditions, eutrophication is usually seasonal outside the tropics, being related to the stratification of the water column and the succession of phytoplankton (see Section 3.2.4). When nutrients are exhausted in the upper... 

To be successful in the evolutionary sense, all organisms must grow and reproduce successfully, and to accomplish these functions they have particular requirements for environmental resources. Plants, for example, need access to an appropriate supply of sunlight, water, and inorganic nutrients such as carbon dioxide, nitrate, phosphate, calcium, and about 20 other chemicals. Similarly, animals require a suitable habitat, replete with the appropriate foods to eat and places for shelter to complete their life cycle. The requirements of organisms for resources must be satisfied within an appropriate ecological context, for example, in terms of the temperature regime, or the types of diseases, parasites, or predators that are present. 

The concept of sparing of one nutrient by another was introduced earlier, where it was demonstrated that dietary carbohydrate can spare protein. Similarly, cysteine can spare methionine and tyrosine can spare phenylalanine. A certain proportion of dietary methionine is converted to cysteine. Mediionine normally supplies part of the body s needs for cysteine. With cysteine-free diets, methionine can supply all of the body s needs for cysteine. The methionine catabolic pathway that leads to cysteine production is shown in Figure 8.27. Only the sulfur atom of methionine appears in the molecule of cysteine serine supplies the carbon skeleton of cysteine. a-Ketobutyrate is a byproduct of the pathway. a-Ketobutyrate is further degraded to propionyl-CoA by BCKA dehydrogenase or pyruvate dehydrogenase. Propionyl-CoA is then converted to succinyl-CoA, an intermediate of the Krebs cycle. 

The various essential chemical nutrients are often determined in predicted proportions in living organisms. It has been found, for instance, that the C N ratio in microbial plasma is between 4 to 10, in soil humus 6 to 35, and in forest biomass about 160. At the global level, we can consequently calculate how much nitrogen should be involved in biogeochemical cycling to provide the annual net primary productivity (NPP) of terrestrial ecosystems at this ratio for as much as 60 x 10 tons of carbon per year. Simple calculations show that at least 0.4 x 10 tons of nitrogen should be supplied... 

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