Plastics & methylmercury

An overview of methylmercury in Newfoundland and Labrador

In the province of Newfoundland and Labrador (NL), methylmercury is a contaminant of growing concern (Durkalec et al. 2016). While mercury is naturally occurring in many aquatic ecosystems and sediments in both its elemental and toxic (methylated) forms, industrial projects, including hydroelectric dams such as the widely protested Muskrat Falls project, are forecast to make otherwise non-toxic forms of elemental mercury bioavailable, attaching to organic materials that may be consumed by animals including humans (Schartup et al. 2015, Durkalec et al. 2016). Organic or methylated mercury is a highly toxic contaminant, and can cause developmental impairment in children (Rice et al. 2003), memory and visual problems, increased risk of cardiovascular disease (Salonen et al. 1995, Guallar et al. 2002, Choi et al. 2009), and autoimmune disorders (Hultman 1999). It also affects fish, marine mammals, and fish-eating birds by decreasing reproductive success (Weiner 2003, Scheuhamme et al 2007).

While  methylmercury exists primarily in water, the most acute route of exposure for humans is through the consumption of fish and marine mammals that live in water (Wiener et al 2003). A key problem of methylmercury is that it biomagnifies, meaning that, once ingested, the toxicant stays in the bodies of animals, eventually accumulating in those at the top of the food web (Figure 1).

Example of biomagnification. Image and caption from Rochman 2015.

InNewfoundland and Labrador, those most susceptible to harm by mercury poisoning tend to be rural, low-wage, and Indigenous peoples that regularly consume country foods (local plants and animals) that are not only culturally important, but less expensive and more accessible than food purchased from stores (Durkalec et al. 2016). As such, methylmercury contamination, is an environmental justice and social equity issue, with exposure unevenly distributed by geography, income, and cultural practices.


Methylmercury and Plastics

Marine plastics absorb industrial chemicals and heavy metals, including methylmercury. Because of their ubiquity in the marine environment and ability to absorb contaminants, it has been suggested that microplastics (plastics < 1mm) might be used to assess the presence of toxicants (such as carcinogenic compounds PCBs, PAHs) and trace metals in aquatic environments (Rochman 2015; Hong et al. 2017). While recent research has focused on the relationship and mechanisms through which organic contaminants are absorbed or released by plastics, relatively little research has examined metals, including mercury.

Through a literature search, we found 13 studies to have examined the relationship between metals and microplastics. Of these thirteen studies, only 5 examined concentrations of mercury, and zero studies specifically looked at methylmercury. Experiments have shown that all plastic types absorb metals (Rochman et al. 2014), and many do so at concentrations that are harmful to wildlife (Lavers et al. 2016; Graca et al. 2014). Plastics absorb metals in both marine and freshwater environments (Ashton et al. 2010; Wang et al. 2015; Turner and Holmes 2015). Mercury, which is highly attracted to charged materials, does so independently of factors such as pH (Turner and Holmes 2015).

Importantly, metals are found in their highest concentrations on microplastics that have been in the ocean long enough to break down by sun and waves (a process called photodegredation) or become associated with organisms or organic matter (referred to as biofouling) (Ashton et al. 2010; Rochman et al. 2014). Whereas newer plastics are considered to be less dangerous because fewer chemicals and metals have been attracted to them (Rochman 2015), plastics that have been in the ocean long enough to break down become charged and thus metals—coming from water, soil, and atmosphere— attach themselves to these plastics, making them increasingly toxic (Nakashima et al. 2012; Turner and Holmes 2012; Graca et al. 2014). This is particularly important for understanding the relationships of plastics and metals to food webs because even though plastics can pass through the bodies of animals, the organic matter that grows on plastics and attracts mercury is likely to be digested and absorbed once the plastic is ingested, potentially becoming a source of contamination. Animal studies have demonstrated that metals may leach out of plastics and into the bodies of animals once digested (Lavers and Bond; Lavers et al. Hodsen et al. 2016).

Example of fouled plastics. Image and caption from Holmes (2013).

Citizen Science of methylmercury?

Despite the fact that plastics are known to absorb metals found in the environment, microplastics aren’t yet a good citizen science tool for monitoring methyl mercury. Current methods used to analyze metals in plastics are prohibitively expensive, and many involve cleaning organic matter off of plastics (Hong et al. 2017)—what might remove the methylmercury we would want to analyze. Moreover, more studies need to be done that examine methylmercury specifically in order to better understand its interactions with plastics.

Currently, the only citizen science method for assessing methylmercury involves the use of dragonfly larvae as biological indicators that are then send to a lab for analysis. Biological indicators refer to a particular type of organisms whose contaminant loads consistently correlate with those of their surrounding environment. Dragonfly larvae have been found to be particularly useful as bioindicators because they are (1) aquatic dwelling; (2) located at the bottom of the food chain, and (3) contain a life stage that’s sufficiently long (upwards of 5 years) to accumulate methylmercury contamination (Jeremiason et al. 2016). The Dragonfly Larvae Project is a citizen science project sponsored by the US National Parks Services and several American universities. The project uses citizen scientists (people without science degrees) to collect dragonfly larvae in order to assess and compare ecosystem health in parks across the United States.  People are provided instructions to identify, collect, store, and send dragonfly larvae to labs for methylmercury analysis (Nelson et al. 2017). While analysis still requires the use of expensive lab equipment, the method helps community members have access to information regarding methylmercury content in their local food webs.

CLEAR would consider coordinating such a project if community members and partners that could analyze contaminants were interested.


Works Cited