Species biologically available

Because of its common occurrence and biological importance, it is an essential micronutrient for most organisms. Thus, a number of analytical procedures for the analysis of iron species have been developed and typically concentrate on biologically available species [9],... 

Given that the Fe and P bound to the DOM in humic lakes are probably not directly available to organisms, the DOM-Fe-P complex would have to undergo some form of transformation to yield bioavailable Fe and P species. In terms of biological uptake, in general it can be said that Fe is preferred in the dissolved free ionic form and P as orthophosphate (P04). Several known processes result in the transformation of Fe and P from the DOM-Fe-P complex into biologically available species. Two of the mechanisms are physical-chemical processes (1) UV-mediated photoreduction and (2) DOM-mediated chemical reduction or dark reduction. Other mechanisms are... 

A current area of interest is the use of AB cements as devices for the controlled release of biologically active species (Allen et al, 1984). AB cements can be formulated to be degradable and to release bioactive elements when placed in appropriate environments. These elements can be incorporated into the cement matrix as either the cation or the anion cement former. Special copper/cobalt phosphates/selenates have been prepared which, when placed as boluses in the rumens of cattle and sheep, have the ability to decompose and release the essential trace elements copper, cobalt and selenium in a sustained fashion over many months (Chapter 6). Although practical examples are confined to phosphate cements, others are known which are based on a variety of anions polyacrylate (Chapter 5), oxychlorides and oxysulphates (Chapter 7) and a variety of organic chelating anions (Chapter 9). The number of cements available for this purpose is very great. 

The species of components present will also be affected by oxidation-reduction, and pH. For example, iron is primarily in the Fe3+ (oxidized) or the Fe2+ (reduced) state depending on the oxidation-reduction potential of the soil. Speciation, which depends, in part, on the oxygen status of soil, is of environmental concern because some species are more soluble, such as Fe2+, and are thus more biologically available than others. The occurrence of a specific species is related to the chemistry occurring in a soil, which is related to its features. Thus, large features must be taken into consideration when studying soil chemistry and when developing analytical and instrumental methods. 

One of the questions that arises in soil analysis is whether a determination of the total amount of a component in soil is desired or if just the biologically available amount is more relevant. Related to this is the question of which species of the component is present. In some cases, speciation is of utmost importance. For instance, chromium can be present as Cr(III) or Cr(VI). Cr(VI) is more toxic and thus of greater concern [15], This concern is also related to the biological availability of a specific species. In this case, while knowing the total chromium content (i.e., the sum of Cr(III) and Cr(IV)... 

It is often assumed that if something is in the soil, it will be in plants. This is incorrect. Plants do not take up all of the elements or molecules present in their immediate environment. However, there are some plants, called hyperaccumulators, that accumulate higher than normal levels of some toxic elements. These plants still do not take up all the elements in their environment and they are often small, so the total amount of toxic elements removed from soil is limited. In addition, not all species of an element are toxic and some are not biologically available and thus do not enter biological systems. For example, chromium as Cr(VI) is more toxic and more biologically available than is Cr(IH) [1]. 

In some cases, resins have been used to try to determine only the plant or more generally the biological availability of an ionic species. Resins placed in soil have also been used to study ion speciation, soil microbiology, various phosphorus measurements, soil nutrient supply rate, nutrient transformations and movement, and micronutrient and metal toxicity [22-25],... 

The simple definition of biological availability as the fraction of the total trace metal available for uptake by the biota implicitly assumes that all relevant organisms will have similar uptake characteristics. In addition the use of biological availability as a chemical parameter involves the assumption that it can be identified with particular chemical species or groups of species. Perhaps surprisingly many of the studies carried out to date have lent support to these assumptions, though the detailed mechanisms of the uptake processes remain unclear. Some of the inorganic chemical species identified as... 

A further group of biologically available chemical species comprises organic compounds of the trace elements, which may be divided into two groups organic complexes of cations, and metal and non-metal alkyls (Table 3). These species are mainly neutral molecules, in contrast to the ionic species identified in Table 2, and are thought to be taken up as a consequence of their lipid-solubility (Florence et al., 1983). 

It is clear from this brief survey that biologically available chemical species can include free cations, anionic and cationic complexes, and neutral molecules, depending on the element concerned. Such a disparate collection implies a range of different uptake mechanisms probably a minimum of three for positive, neutral, and negative charge types. Furthermore, the estimation of biological availability in terms of chemistry becomes more difficult, since not only will different techniques be required for different elements, but some elements are already included in more than one species type in Tables 2 and 3 (e.g. Cd, Hg, Cu). 

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