Parasites used as heavy metal bioindicators

Did you know that more than half of the world’s animal species are parasitic? A parasite is an organism that lives in or on another organism (commonly referred to as the host) and benefits by deriving nutrients from the host species. Parasites can be found almost everywhere in the world, where they latch on to a wide variety of hosts such as ants, fish and humans. One example of a truly gruesome parasite is the tongue-eating louse. This parasite enters through the gills of fish, digging its claws into the fish’s tongue, causing the tongue to fall off! It then replaces the fish’s tongue, feeding on the host’s blood or mucus.

tongue parasite

The tongue eating louse parasite in its fish host (larger louse is the female, smaller louse is the male)


Other examples of parasites include tapeworm, leeches and head lice.

leechesrainbow trout

Examples of parasites: to the left: leeches parasitizing a fish ; to the right: adult tapeworm found in rainbow trout

It is hard to imagine that parasites could be of use to any animal species, especially to humans. However, certain parasites are able to accumulate heavy metals at much higher concentrations than their fish host’s tissues or the environment. Because of this ability, they can be sensitive bioindicators of environmental pollutants as compared to other more commonly used bioindicators (such as fish muscle and various mussel species).


Rainbow trout: Example of a commonly used bioindicator used for freshwater systems.

Note: Bioindicators are used to assess the concentrations of various pollutants in the environment

For example, Brazova and his colleagues (2012, Sensors, 12: 1424-8220) compared heavy metal concentrations in perch, a freshwater fish, with two of its main parasite species, in order to determine which species is a better bioindicator of heavy metal pollution. The study was undertaken at a water reservoir in a region of Slovakia, known for its intensive mining and ore processing activities.

They found that both parasites had significantly higher metal concentrations than the host’s tissues and that the two parasites differed in their propensity to accumulate different pollutants (i.e. there is a species-specific preference from some metals, where one parasite species accumulated certain metals more intensively than the other parasite species and visa-versa). Both parasite species also showed to have a relatively high accumulation of the heavy metal cadmium, whereas there was shown to be a low concentration of cadmium concentrated in the water and bottom sediments of the reservoir that was under study. This implies that even very low concentrations of heavy metals in the environment can be detected using parasites.


An example of perch and one of the main parasites found within the intestines of perch ( used in the study done by Brazova et al. (2012)

Why should we care?

Heavy metals occur naturally in the environment, but anthropogenic activities (such as mining, burning fossil fuels, dumping industrial waste and other industrial activities) have increased concentrations to toxic levels in some habitats. This is frightening as heavy metals cannot be broken down, once released, they accumulate in the environment, especially in lake, marine and estuarine sediments.


mercury sign

The most dangerous heavy metals are cadmium, lead and mercury, which are highly toxic metals that cause major harm to human heath at even very low concentrations

Heavy metals are taken up by organisms that feed in these habitats, becoming increasingly concentrated as one moves up the food chain due to a process termed biomagnification. Humans, as consumers of fish, sit atop the food chain and can therefore be exposed to potentially lethal doses of heavy metals. For example, fish in more than 50 percent of freshwater systems in Sweden are polluted with mercury above World Health Organisation standards; therefore the people inhabiting these areas are advised to refrain from eating these toxic fish. The long-term consumption of fish meat can cause a variety of disorders such as alterations to the nervous system, brain damage, reproductive diseases. Eating contaminated fish meat can also inhibit growth, affect your respiratory system, shorten your lifespan and cause various cancers.

Heavy metal pollution is not just a localised problem, only affecting industrial areas or areas situated near coal-powered stations. There have been reports that high mercury levels exist in relatively remote areas of the world such as in Greenland and the Arctic, showing us that tiny particles of mercury are able to drift over relatively long distances.   There are hence serious environmental, economic and social impacts associated with heavy metal pollution.



Diagram showing how heavy metal concentrations increase in organisms as you move up the food chain

Because parasites are sensitive indicators of environmental pollution and are able to detect even very low levels of pollution, they can be used as early warning indicators of a variety of pollutants as they are more sensitive than other bioindicators and can therefore warn us before pollution levels (especially heavy metal concentrations) reach frighteningly high concentrations in the environment. Parasites can also be used as a monitoring tool in areas where industrial activities are operating nearby or in the case of industries that have stopped operating; parasites can be used to determine whether the area that was affected by the industry is safe to utilize.

Parasites are incredible organisms that play a major role in the functioning of all major ecosystems and we are only starting to understand the many uses that parasites may offer.


Brázová, T., Torres, J., Eira, C., Hanzelova, V., Miklisova, D., Salamun, P. 2012. Perch and its parasites as heavy metal biomonitors in a freshwater environment: the case study of the Ružín Water Reservoir, Slovakia. Sensors 12: 1424-8220.


Sunflowers and Radiation


If you were to ask someone why they think plants are important, you may expect answers such as; “They provide the air we need to breathe”, “they serve as a food source for animals” and “they are great for making tea as well as pretty to have around”.


No matter the answer, I am sure everybody would be able to agree that plants are important as well as incredible! Even today we are still discovering new, exciting ways in which we could use plants so to help make our planet a better place to live in.


I am sure many people are aware that certain plants are able to grow in extremely unfavourable environments, such as in deserts, snowy mountaintops and salt marsh lands. Although what most people probably don’t know, is that certain plants are even able to survive in areas exposed to high radiation and chemical pollution.

A radioactive sign hangs on barbed wire outside a café in Pripyat.

Scientists have taken advantage of this incredible ability displayed by certain plants to live in highly polluted areas, by studying the effectiveness of these plants in removing a wide range of contaminants. They then pushed this further by using these plants to extract pollutants from areas laden with toxic, heavy metals.


Through this, a relatively new technology called phytoremediation was started. Phytoremediation involves the removal or extraction of certain metals through the plants’ root system. These metals then accumulate in the plants’ tissues, without damaging the plant. Pollutants are either removed from the soil and groundwater or they are transformed into less harmful forms.

Heavy metals pose many health risks to the environment as well as on human health. They are non-degradable and therefore just accumulate in the environment. Examples of two toxic, heavy metals are arsenic and mercury. Exposure to arsenic is known to cause skin and lung cancer as well as nausea, diarrhoea and vomiting. Mercury poisoning can lead to mental retardation, blindness and brain damage. One of the main ways that mercury poisoning occurs is through biomagnification (the build-up of metal concentration as the element passes from the lower to the higher trophic levels). Arsenic is found in the soil from smelters and pesticides.


The immobilisation and extraction of these toxic, heavy metals is therefore paramount for if we want to live in a safer, healthier and cleaner environment. Areas where smelting, mining and sludging occur as well as where pesticides and fertilizers are used are therefore high-risk areas.


Although phytoremediation has many benefits such as being low cost, environmentally friendly, as well as limiting the movement of these toxic metals within the soil and preventing soil erosion. There are also a number of problems associated with this technique. Firstly, the ideal plant characteristics for phytoremediation are for these plants to be tall, high yielding, fast growing and easy to harvest. But most of the 400 species identified as a bioaccumulator (an organism that is able to accumulate metals or other contaminants in it’s’ tissues) seem to exhibit a slow growth rate, therefore limiting the amount of metal taken up. Areas where the heavy metal concentration is too elevated also poses a threat as plants begin to become too stressed. So what is the solution?


In the paper written by Paz-Ferreiro and his colleagues, they suggest that the use of biochar combined with phytoremediation could vastly improve remediation efforts.

Biochar is simply charcoal, which comes in many different forms-depending on the waste material it is made from. Biochar has shown many benefits on soil properties, increasing the plant yield, as well as increasing the percentage of germinating seeds and root length.


The soil and atmospheric benefits from using biochar.

In this paper, the authors provide a very comprehensive summary of these two techniques as well as a summary of the various studies performed using biochars and phytoremediation in differently polluted environments. They also stress the importance of further research into the combining of these two techniques for future soil remediation.


One example of phytoremediation in action is with using sunflowers to remove radiation. Since the tragedy of the Chernobyl nuclear reactor leak in 1986, sunflowers have been planted all around the region where the disaster took place. Effectively, sunflowers can removed up to 95% of the radiation in the soil as well as in the contaminated water. These sunflowers are then harvested, to which the radioactive material within the plant is properly disposed of.



Sunflowers planted for phytoremediation at the Chernobyl nuclear reactor.

Although, not all phytoremediation efforts have been as successful such as with the Chernobyl disaster. The survival of these plants depends greatly on the level of toxicity of the contaminated land as well as the general condition of the soil. This is where biochars can become of great value. Future phytoremediation efforts using biochars may prove to be a lot more effective in terms of heavy-metal accumulation as well as the long term survival of the plant. Phytoremediation combined with biochar is the way forward, especially in today’s’ world of massive corporations, industries and development.


Where have all our penguins gone?


             Image Source: Flickr. By: Paul Mannix 

The African penguin (more commonly known as the Jackass penguin) is found on over 20 small islands and mainland areas along the south-west and south coast of Africa. They are currently listed as ‘Endangered’ on the IUCN Red List, which is attributed to a combination of factors. Some of the more well- known factors include oil spills, egg harvesting by humans and disturbance to nesting sites. Recently, the effects of global warming are also indirectly creating detrimental affects on the remaining number of African penguins found along the western coast of South Africa. Indirect effects of global warming include a decline in the amount of food sources as well as a change in the availability of food sources for the penguins.

Large-scale commercial fisheries (such as purse-seine trawling as well as climate change impacts on water temperatures) have caused shifts in the type and amount of prey available to the penguins in their feeding range. Roy et al. (2007) noticed that both anchovies and sardines, the two main prey items for African penguins, have shifted more eastward of South Africa due to the changes in water temperature. A study done by Crawford et al. (2011) investigated how the distribution shift of anchovies and sardines will impact on the African penguin population numbers in South Africa.



African penguin distribution map: The number of breeding penguins in South Africa has declined from about 56 000 pairs in 2001 to 21000 pairs in 2009, while the global population is recorded as 26 000 pairs in 2009.

During breeding, African penguins need to search for food within 40 km of their breeding or nesting site. The further that a penguin travels searching for food, the more energy it expends, which ultimately means less food for the chicks. In addition, adults also face a food shortage which leads to a deterioration in their health. Therefore, the feeding conditions before breeding influence breeding success as well as the decision to participate in breeding.


African penguins nesting site at St Croix Island

Crawford et al. (2011) found that due to the eastward shift in sardines and anchovies as well as the overall decline in these prey species; that the number of penguin nests and breeding pairs has declined exponentially. They found that the biomass of these prey species and number of penguin nests were highly correlated.

They also found that the local availability of these prey species to the penguin breeding sites is also a highly important factor influencing the African penguin numbers. They inferred this by the observation that the abundance of anchovy was still very high, although the anchovy distribution shifted further away from the penguin breeding sites.



African penguin feeding

Besides the obvious importance of this study in bringing to our attention the massive decline in African penguin numbers over the years, it also emphasizes to us the need for better fisheries management. We need to consider not only the overall abundance of prey but also the local availability of the prey species.

An important follow up study to this one, as suggested by Crawford et al. (2011) would be to investigate the influence of fishing on the availability of food to the African penguins and then further determine how this can possibly be managed. Another very important factor to consider is that the distribution shift of these prey species not only affects African penguins, but also other animals such as the Cape gannet and Cape cormorants. Since the change in shift of these fish prey species, Cape gannet breeding success has declined rapidly as well, inadvertently impacting on the gannet population.



African penguin with chick

This article demonstrates to us just how fragile our planet really is. Everything operates in an almost balanced scale whereby the global change drivers created by humans, can easily and noticeably tip this balanced scale quite easily without us even noticing. Only until a species such as the iconic and adorable Jackass penguin starts to disappear do people begin to take notice and action to avoid their possible extinction.

This paper is worth a read and provides a good synthesis of information to the changes that the African penguins have undergone since the late 1990s.



Crawford, R. J. M.,  Altwegg, R., Barham, B. J., Barham, P.J., Durant, J.M., Dyer, B.M., Geldenhuys, D., Makhado, A. B. Pichegru, L., Ryan, P.G., Underhill, L.G., Upfold, L., Visagie, J., Waller, L.J., Whittington, P.A. 2011. Collapse of South Africa’s penguins in the early 21st century. African Journal of Marine Science 33(1): 139-156.