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There are 92 naturally occurring elements on the Earth. Some are very well studied, some not at all: for example astatine (Bryson 2003). The problem is that some elements are very rare. For example, only 24.5 grams of francium occur at any time in the whole of the Earth’s crust. Only about 30 of the naturally occurring elements are widespread on Earth, and very few are important for life (Figure 1). In the solar system, stars in general, and probably the universe as a whole, the most abundant elements are the lighter elements: over 75% hydrogen (H), 25% helium (He), about 1% everything else. In the ‘everything else’ category even numbered elements are more abundant than odd numbered elements. Abundance tends to fall rapidly with increasing atomic number. However, carbon (C), oxygen (O), magnesium (Mg), silicon (Si), and iron (Fe) are anomalously high relative to these general trends, while lithium (Li), beryllium (Be), and boron (B) are anomalously low. In the Earth's crust the order of abundance is O (< 50%), Si (> 20%), Al, Fe, Mg, Ca, Na, and K. These are all the sorts of elements that rocks are mostly made out of. In the Earth as a whole, because of the core and the mantle, Fe, Ni, and Mg, become more common, while O, Si, Al remain major overall constituents (Table 1). Concerning life, elements have different functions (Table 2). We have evolved to utilize or tolerate the elements, but we live within narrow ranges of acceptance. As a rule, our tolerance for elements is directly proportionate to their abundance in the Earth’s crust (Bryson 2003).
Figure 1: The distribution of naturally occurring elements known or believed to be essential for life in the periodic table. The understanding of the ecological importance of C, N, and P is much more advanced than it is for the other elements (redrawn after Da Silva & Williams 2001)
Table 1: The occurrence of elements in % dry weight of the Earth crust, green algae and animals (data from different sources) in comparison with lettuce grown in a hydroponic system, and fish feed (Schmautz, unpublished data). Note that the frequency (and with that the availability) of elements in the Earth crust does not match the frequency in living beings
Chemical elements have different roles and are mainly involved in different functions in an organism (Table 2). Organisms do not require all these elements in the same quantities. Some elements are required in large quantities, while others are required in minute quantities. This is illustrated nicely by the tentative stoichiometric formula for a living human being (Sterner & Elser 2002):
This means that for every cobalt (Co) atom in our body, there are 132 million oxygen (O) atoms. The major nutritional requirements of plants and animals, without which they are unable to complete a normal life cycle, are outlined in Figure 2. Macronutrients are required in larger quantities. Micronutrients are required in minute quantities.
Table 2: Primary functions and the chemical elements (or associated ions) involved in performing them for organisms (modified from Sterner & Elser 2002). Elements with a relatively minor role are indicated in parentheses
Function Elements Chemical form Examples Structural (biological polymers and support materials) H, O, C, N, P, S, Si, B, F, Ca, (Mg), (Zn) Involved in chemical compounds or sparingly soluble inorganic compounds biological molecules (proteins, DNA, fats, carbohyxdrates) tissues (muscle, lung, leaves…) skeletons; shells; teeth plant support tissues (lignine, cellulose) Electrochemical H, Na, K, Cl, HPO 2-, 4 (Mg), (Ca) Free ions message transmission in nerves cellular signalling energy metabolism Mechanical 2- Ca, HPO4 , (Mg) Free ions exchanging with bound ions Muscle contraction Catalytic (acid-base) Zn, (Ni), (Fe), (Mn) Complexed with enzymes Digestion (Zn). Zinc oxidises alcohol. Hydrolysis of urea (Ni) PO4 removal in acid media (Fe, Mn) Catalytic (redox) Fe, Cu, Mn, Mo, Se, (Co), (Ni), (V) Reactions with O2 (Fe, Cu) nitrogen fixation (Mo) reduction of nucleotides (Co) Co is necessary for the creation of Vitamin B12
Figure 2: Nutritional requirements of plants and animals. Note that water (needed by all living beings) is not included in the chart. Animals obtain their nutrients from food and drink. Plants, with the exception of parasitic and carnivorous ones, absorb the essential nutrient elements from their environment (air, soil solution, nutrient solution)
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