NOM as Natural Xenobiotics

Abstract

In all ecosystems, natural organic matter (NOM) comprises the major reservoir of organic carbon and contains a high proportion of humic substances (HSs). HSs are natural xenobiotics that exert indirect and direct chemical challenges to exposed organisms. Xenobiotics are chemicals that are found in, but are not produced by, an organism. Indirect xenobiotic interactions comprise the release of reactive oxygen species (ROS) from illuminated HSs in aquatic systems that in turn can place oxidative stress on exposed organisms. This oxidative stress can select robust over sensitive species and thereby structure the community, as shown by freshwater bacteria exposed to singlet oxygen. Furthermore, this oxidative stress can reduce the activity of viruses and pathogens and can oxidize various toxins released by cyanobacteria, and thereby reduce the chemical challenge of exposed organisms. Within freshwater phytoplankton, cyanobacteria appear to be more sensitive to such chemical stress than eukaryotic phototrophs. Due to the low persistence of polyphenols in eutrophic alkaline waters, it is questionable whether these xenobiotic NOM compounds are really effective. Not all stress-responses of exposed organisms are adverse. For instance, due to the coevolution of plants and soil organic matter, HSs interact with higher plants by activating genes and complex regulation, resulting in increased stress resistance. One major phenotypic result of this interaction is the remodeling of root morphology, leading to an increased absorptive surface of the root, which mycorrhizal fungi can attack and colonize. Most plants possess mycorrhiza that enable them to efficiently take up nutrients. Exposed animals also respond to NOM on the gene, protein, and metabolic level. Low-molecular weight NOM can be internalized and can provoke oxidative stress, because animals attempt to rid themselves of natural xenobiotics. However, this oxidative stress can lead to an increased lifespan, body size, and offspring number, hence, stabilizing exposed populations. Another NOM-mediated population-stabilizing mechanism is the acquisition of multiple stress resistences where one stressor prepares the organism for another to cope with adverse effects. Evidence is accumulating that this applies to a variety of natural and anthropogenic environmental stressors, such as variations in osmotic conditions, acidity, or netting and predation. Stress-tolerance can be passed to succeeding generations by epigenetic mechanisms. Other apparently population-stabilizing mechanisms are weak feminization by NOM to reduce the maintenance costs for two separate parental bodies in ecosystems running close to their carrying capacity. Several Quantitative-Structure-Activity-Relationships (QSAR) have shown that particularly phenolic, quinoid, and stable organic radicals account for various effects in organisms exposed to NOM, irrespective of its origin. The QSAR could be confirmed in tests with the nematode Caenorhabditis elegans by enriching a given HS-preparation artificially with hydroxybenzenes. Overall, it appears that NOM is a major driving force of ecosystem functions, and we are only beginning to appreciate its ecological significance.