Bioavailability of contaminants
Due to their various uses, metals and organic molecules released by chemical synthesis are part of our daily lives. Low concentrations of these molecules have permeated throughout the environment, presenting an inherent toxic danger of varying strength.
One of the obstacles to the correct evaluation of ecotoxicological risk for aquatic organisms is our ability to predict the contaminating fraction of these micropollutants capable of having a toxic impact on the organism.
In the environment, pollutants are present in various forms that do not have the same bioavailability. These differences in their ability to be in contact and interact with the organisms affects their contaminating ability. In addition to concentration levels, measured by traditional analysis methods, it is important to know the bioavailability of these molecules in order to adequately evaluate the ecotoxicology risk they present.
Bioavailability and bioaccumulation are affected by the pollutant, its environmental future and the physiology of the exposed organism (its ability to accumulate, excrete or transform pollutants).
These phenomena affect aquatic environments as well as terrestrial and atmospheric environments. For example, plants are exposed to pollutants in the soil, through water in the roots and in the air by absorbing contaminants through the leaves.
The bioavailability of metals (copper, zinc) is generally linked to a free or ionised fraction (as opposed to complex forms: carbonates, sulfates, oxides, etc.).
The bioavailability of organic pollutants (e.g., PCBs) is strongly influenced by the amount of organic matter in the water with which they could form complexes too large to cross through biological membranes.
The following methods can be used to estimate bioavailability:
- the first is purely chemical and involves describing the various chemical forms in which the contaminant is found in the environment (the term speciation is used for metals) and then making a bioavailability hypothesis for each of these forms;
- the second is both chemical and biological and involves measuring the concentration of accumulated pollutant in an exposed organism;
- the third is only biological and involves measuring a response such as a biological response to exposure at cell (e.g., enzyme response) or organism (physiological response such as mortality) level.
One of the possible consequences of pollutant bioavailability is their accumulation in living organisms. This is known as bioaccumulation. It provides information on bioavailable concentrations available in the environment without presuming toxicity.
Bioaccumulation is influenced by the physical-chemical characteristics of the environment (temperature, hardness, organic matter, etc.), the absorbed substance (e.g., hydrophobic nature) and also by the physiology of the organisms involved (ability to excrete or metabolise the substance).
Bioaccumulation could potentially be used to evaluate the environment. In fact, using traditional analysis methods to measure very low concentrations is limited by the sensitivity of these methods (quantification limits) and does not calculate the bioavailability of the pollutants for the organisms.
Today, models are used to predict the bioaccumulation of contaminants in the environment, both dissolved and as particles for organisms (fish and invertebrates), and to estimate the bioavailable fraction in the environment using organism concentrations. This is the case for biodynamic models. These multi-compartment models take into account adsorption of a metal or organic component by water and food and physiological processes that regulate bioaccumulation in each organism.
Challenging bioavailability with passive sensors
While measuring the level of contaminant found in organisms within an environment is the best way of evaluating the bioavailability of contaminants in an environment for species that live there, techniques are also available to sample contaminants in the environment and attempt to link them to an ecotoxic risk.
Since the 1980s, several different systems of passive sampling have been developed to measure chemical contaminants in the water in order to improve the relevance of chemical measurement in determining toxic impacts. Immersed for several days or weeks, these systems extract and concentrate available contaminants in situ. These techniques make it possible to measure weak levels of contamination while improving the inclusion of time in measurement, particularly in relation to chronic exposure of aquatic organisms.
They are particularly used to identify traces of heavy metals:
- DGT (Diffusive Gradient in Thin Films) for PAH metals, PCBs or hydrophobic pesticides
- SMPD (Semi-permeable Membrane Device) for organic hydrophobic molecules and more hydrophilic emerging substances (drug residues, hormones, etc.).
- POCIS (Polar Organic Compounds Integrative Samplers) for hydrophilic organic micropollutants
The role of microorganisms in bioavailability
Microorganisms (bacteria, fungus) are likely to regulate pollutants using interactions at a cellular or community level.
- At a cellular level, several mechanisms may intervene in bioavailability, including bisorption, biosequestration and chemical transformation (oxidation, creation of insoluble precipitate, etc.).
- At a community level, biofilms are capable of secreting polymer substances that interact with metal or organic substances in the environment, and interact with them by trapping or damaging them and adapting quickly to environmental changes.
To each their bioavailability?
If our understanding of contaminant bioavailability is an essential component in evaluating environmental hazards, the idea only imperfectly covers the complexity of interactions that may occur between substances, regardless of whether they are useful (e.g., nutrients) or toxic, and species present in the environment. Today, bioavailability is primarily described using chemical models. However, these models do not represent ecosystems particularly well and the issue of how the characteristics of living organisms, both individual and communities, change the bioavailability of substances to which they are exposed over time remains to be explored.
Part of the “Ecotoxicology” series in the Earth - Environment Systems Collection coordinated by Jeanne Garric IRSTE Editions
In terrestrial and aquatic environments, living species are organized into communities that interact with their environment. This volume offers several examples of interactions between toxic and community interactions (microorganisms, invertebrates, etc.) in their environment.
For more information
- Consult the web pages of the RiverLy research unit and the Irstea Lyon Villeurbanne Center
- Find the book Biodisponibles - Une histoire entre le vivant et son exposome - Volume 3 (Bioavailability - A Story between the living and its exposome).