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"Assuming about equal prevalence of all the amino acids. ^Assuming 50% G + C content (equal prevalence of all four bases). cA common fatty acid.

"Assuming about equal prevalence of all the amino acids. ^Assuming 50% G + C content (equal prevalence of all four bases). cA common fatty acid.

For microorganisms, N and P are typically considered macronutrients (for plants, K would be added). These are needed in a mass ratio of about 5:1. The required C/N/P ratio, as a rule of thumb, is commonly said to be 100:5: 1. However, in this case, much of the carbon is used as an energy source rather than to make cell constituents. As can be seen in Table 11.1, the C/N ratio of a typical cell itself is around 3.6 (or 4.3 in the formula C5H7O2N) rather than 20.

The term micronutrients usually includes S and Fe, and probably K, Ca, and Mg. Trace nutrients would include the many other elements, such as cobalt (Co), nickel (Ni), copper (Cu), and zinc (Zn), needed in only very small amounts, usually for specific enzymes. Roles that a number of elements play in cell metabolism are summarized in Table 11.3.

Some organisms may need fairly high concentrations of another element for a special purpose. Diatoms, for example, need substantial amounts of silicon (Si) to construct their silica shells, and many testate amoeba need calcium for theirs. Other organisms may

TABLE 11.3 Roles of Various Elements within Microorganisms

Element

Symbol

Important Cellular Roles

Carbon, hydrogen, oxygen

C, H, O

Major constituents of organic matter

Nitrogen

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