By: Dana Sackett, PhD

The harm caused by toxic pollutants starts by changing the internal chemistry of an organism (chemicals in living organisms are called biochemicals).  These initial changes can cascade over time, causing damage at the cellular, tissue, organ, individual, population, and ultimately ecosystem levels.  Because pollutant-driven biochemical alterations precede larger population and ecosystem harm, monitoring biochemical changes can serve as an effective early warning that pollution has begun to impact an environment.

When biochemical changes are measurable and indicate a substance is adversely affecting an organism, they are called biomarkers. You may have heard this term in a medical setting because medical doctors will often take a blood or tissue sample to test for certain biomarkers (chemical and cellular changes) that indicate a particular disease or substance may be causing your body harm. Using biomarkers collected from aquatic ecosystems can help identify and address a pollution problem at an early stage, preventing extensive and potentially irreversible ecosystem damage, and the cost associated with cleaning it up.  A cost that can be expensive economically (through lost revenue and clean-up expenses), socially (such as impacts on public health and housing markets), and environmentally (through the loss of essential ecosystem services like nutrient recycling, food resources, clean water, and others).  Fish and mussel biomarkers in particular are important tools to assess contaminant risk, water quality, ecosystem health, and identity pollutant-caused harm before it becomes widespread.

One biomarker, metallothionein, is a natural protein common in nearly all living things.  Whether in a tiny ocean fish or a person, this protein has the same purpose: to sequester excessive metals and prevent them from harming the cell. While normally at low levels, metallothionein will increase in the tissues of an organism when exposed to excessive metals. Thus, seeing an increase in metallothionein in the tissues of a fish would indicate that fish was being exposed to metals pollution.

Biomarkers can also indicate if a toxic chemical present in the environment is causing injury. For instance, while a scientist may find high concentrations of metals in water, those metals may be bound to particles and unavailable to be taken up by organisms. If that were the case, metallothionein would remain low in the tissues of those organisms, indicating that their bodies were not responding to or being harmed by the metals.  

Biomarkers can also provide different types of information depending on their function in an organism.  Metallothionein detoxifies metals and can determine if organisms are being exposed to that specific class of pollutants.  Other biomarkers can be specific to different types of cellular stress (described here to mean the chemical changes that cells undergo in response to environmental stressors like extreme heat or toxicants, but not emotional stress). Two specific stress proteins called stress70 and cpn60 are stress biomarkers. Their job often includes recognizing other damaged proteins and refolding them into their proper shape, essentially fixing them.  While these stress proteins are typically kept at relatively low concentrations, they will increase when an organism is exposed to a toxic chemical that causes protein damage.

Several detoxifying enzymes that help to break down harmful pollutant-induced free radicals can also serve as biomarkers. For instance, cells exposed to a pollutant that causes excessive free radicals will mount a defense against those free radicals by increasing the amount of detoxifying enzymes.  Thus, measuring this increase can be used to identify that an organism is being exposed to and affected by a harmful pollutant. Other biomarkers can signify DNA, cell, or reproductive damage directly. Scientists have even noted that the pattern and concentration of different biomarkers in comparison to each other can be used to identify the specific contaminant responsible for the biochemical changes.  This is known as stress protein fingerprinting. Another method is the integrated biomarker response index (known as IBR); a method that can combine and summarize multiple biomarker results into a single ‘stress’ index.  

While biomarkers can be extremely useful, there are limitations. One major limitation is that some toxic exposures can cause biochemical changes that resolve naturally, never leading to any further harm.  Another is that some natural processes can cause certain biomarkers to change, even in the absence of a pollutant.  For this reason, if a biomarker is to be useful in preventing ecological harm, it must be strongly linked to the pollutants responsible for those changes, and seen to lead to higher-level ecologically-relevant damage.  

Valuable biomarkers signify that a pollutant causes specific measurable biochemical changes that can lead to individual, population, and ultimately ecosystem harm.  As identifiable and measurable metrics, biomarkers can provide an invaluable early warning system that society can use to prevent widespread damage.    

References and other reading material:

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