Assessing Pollution Effects In Aquatic Environments

Over the last 100 years the number and diversity of man-made chemicals which find their way into aquatic environments has increased enormously.

There are now estimated to be over 100,000 distinct chemical compounds marketed in the European Union alone, and knowledge of the environmental fates and concentrations is restricted to only a small proportion known to be particularly harmful and persistent.

Against this background there has been concern that long-term exposure to complex cocktails of chemical contaminants may be affecting stocks of fish and other animals which live in estuarine and coastal areas. In fact surveys of flounder have shown higher incidences of tumours and fin rot in some polluted estuaries in the UK, other parts of the North Sea and in North America.

Although this sounds alarming, it has been difficult to provide the evidence that pollution is the cause of such ill-effects, or that it is the cause of any declines in population numbers. This is because we know very little about the biological effects of complex mixtures of chemicals, and estuaries, particularly, are highly dynamic and highly exploited environments in which population numbers fluctuate for many reasons not related to pollution.

However, over recent years it has become apparent that particular types of chemicals can have dramatic and measurable effects on individuals. For example, the effect of chemicals which mimic the natural hormone, estradiol, can induce a protein, vitellogenin, many hundred-fold levels above normal in wild, exposed fish. Similarly PCBs can induce a protein, CYP1A, to very high levels.

These proteins and their genes have been proposed as ‘biomarkers’ of pollution, meaning that their measurement in wild flounder, or another animal, can provide evidence that those individuals have been are derived from a point source, such as a sewage outfall, or in a spill situation, such an oil spill. In situations where fish are exposed long-term to complex mixtures of chemicals, biomarkers have proved less effective.

This may not be so surprising because most biomarkers are discovered by acute, short-term, laboratory exposures to single chemicals, a situation quite unlike most environmental exposure scenarios. Very recently techniques for measuring the levels of many thousands of genes simultaneously in practically any organism have been developed.

We have applied this technology in an experimental situation to discover what effects longterm exposure to real polluted estuarine sediments have in flounder. Funded by NERC and in collaboration with Marine Scotland,Aberdeen, we established mesocosms, large tanks, containing sediments collected from polluted UK estuaries and from an unpolluted estuary.

We also collected live flounder from the unpolluted estuary and then put them into the mesocosms for seven months. Thus, the only difference between the mesocosm systems was due to the pollutant status of the sediments. All other natural variables found in estuaries, such as genetic background, salinity, temperature, food source etc were eliminated.

Following this exposure we measured the expression of thousands of genes in these flounders. When polluted flounder were compared to clean it was clear that none of the commonly used biomarker genes were affected, but that the expression of groups of genes with functions in the immune system and in apoptosis was increased.

Apoptosis is a cellular process which brings about the self-destruction of a cell following either an extrinsic signal, or the intrinsic sensing of unsustainable cellular damage. The genes we have found point to the involvement of intracellular chemical damage, which in turn induces the intrinsic apoptotic pathway. Thus, this study points to a mechanism by which long-term exposure to complex chemical mixtures can adversely affect fish, and also provides new candidate biomarkers for these situations.