ECOLOGICAL BIOCHEMISTRY - Study Guide - V. M. Isaenko 2005

Chapter 10. BIOTRANSFORMATION OF XENOBIOTICS

10.2. Behavior of Xenobiotics in Ecosystems

The Main Pathways of xenobiotic behavior in ecosystems are determined by degradation, redox and conjugation reactions, phase transitions between different environmental media, and adsorption onto biotic or abiotic Materials.

These processes unfold in ecosystems through the interaction of biological and abiotic factors, which determine the rate and extent of xenobiotic transformation. The rate of xenobiotic biodegradation by organisms depends on their ability to penetrate Cells and undergo the action of intracellular Enzymes, among other factors. Xenobiotic degradation can also occur through physicochemical pathways (such as Hydrolysis in Water or photolysis under the action of light).

Certain xenobiotics are characterized by high persistence in the biosphere. Moreover, the degradation products of some xenobiotics are even more persistent or toxic. For instance, the pesticide propanil is hydrolyzed in agroecosystems to yield 3,4-dichloroaniline, which remains in the soil for a very long time.

The issue of the persistence of xenobiotics and their transformation products receives extraordinary attention when assessing their potential impact on biogeocenoses. Among xenobiotics, there are those that degrade extremely slowly or hardly at all. Some are entirely resistant to decomposition or breakdown. These primarily include heavy metals; although their atoms may become incorporated into various compounds, they ultimately remain within the environment.

Pollutants that undergo extremely slow clearance in the biosphere include radioactive isotopes with long half-lives (spanning decades, centuries, and millennia).

Redox reactions, much like degradation processes, can proceed either enzymatically or under non-enzymatic conditions. The latter may occur, for example, through the action of atmospheric oxygen or oxygen dissolved in water, free radical-mediated oxidation, or hydrogen peroxide, among others.

One of the key mechanisms for xenobiotic detoxification is their conjugation with various Organic compounds. In animal organisms, conjugation typically leads to the Formation of derivatives that are much more readily eliminated from the body (via urine or feces). However, these conjugates eventually enter the soil or water, continuing to circulate within the biogeocenosis.

During transformation, xenobiotics can undergo significant changes in their water and lipid solubility. This alters their ability to cross cellular membranes and accumulate within cells, which in turn affects their accessibility to intracellular enzymes. Such availability may decrease As a result of adsorption onto particles of biotic or abiotic origin. Adsorption processes play a critical role in determining the persistence of xenobiotics in ecosystems.

The accumulation of xenobiotics in living organisms poses a substantial hazard to human health and the biosphere, potentially amplifying their toxic effects. At the same time, this phenomenon can be harnessed for biomonitoring purposes.

Crucially, depending on the stage and mechanism of biotransformation, xenobiotics and their transformation products can reversibly transfer between different compartments of the biogeocenosis—from water to soil, from water to air, from organisms to soil or water, and so forth. Such large-scale shifts are of great significance. For instance, the volatility of certain pesticides and their ability to vaporize from soil or water into the atmosphere can lead to their long-range transport by air masses.

An examination of xenobiotic METABOLISM in living organisms and the ecosystems they form reveals a fundamental commonality in their biotransformation pathways. Despite the remarkable capacity of the biochemical mechanisms through which living organisms defend themselves or mitigate the effects of xenobiotics, their capabilities are ultimately limited.