Impacts: Microplastics, the silent killer

Hello all!

In a follow up from my previous post, I will elaborate more on microplastic debris pollution in the marine environment, which accumulate in organisms over time without directly causing death.

Microplastics: the silent killer

"Of the 5.25 trillion particles of plastic we estimated were in the oceans [...] 95% were smaller than a grain of rice," said Stiv Wilson.

Microplastics and their effects are critical to understanding the magnitude of impact on marine life.  As plastics do not decompose readily, they accumulate in the environment over a long period of time and concentrate toxic pollutants. Pollutants include additives used in the plastic compounding process during the manufacturing process, toxicity from intermediates in the degradation of plastic (eg. styrene) and the persistent organic pollutants (POPs) in seawater that may be absorbed into the microplastic fragments. These are ingested by aquatic organisms, with the impacts briefly listed below:

1. Reduced health and ecological function of the individual: exposure of microplastic fragments to zooplanktons such as copepods and bivalve larvae has shown to cause reduced algal ingestion rates in the example of the copepod Centropages typicus.

2. Increased mortality rates: prolonged exposure to microplastics has also been observed to lead to increased mortality rates in the copepod Tigriopus japonicus over successive generations.

3. Catastrophic ecological transfers: POPs within microplastics and their impacts may be transferred across trophic levels within the food chain, observed within the planktonic food web between copepods and mysid shrimps.

4. Human health dangers from toxic accumulation: in the case of 'nanoplastics' (0.001μm to 0.1μm) with their current undetectable dimensions, they may de-aggregate in the gastrointestinal tract to be absorbed by the body, or that they may translocate across the gut epithelium (lining of the intestines) resulting in toxicity exposure to humans.

Nonetheless, there are still challenges as the impacts of microplastics are still fairly inconclusive at this stage. Adverse effects of plastic transfer and potential bioaccumulation in organisms are still fiercely debated at present as they have only been observed under extreme laboratory conditions in the abovementioned studies; current technology does not allow for the quantification and measurement of nanoplastics in organisms, and therefore there are uncertain effects on human health. In the best estimate on impacts of human health, it was also studied to be negligible, as the majority of microplastics are found in digestive tracts of marine animals, which are often disposed of before human consumption. Even in consumption of bivalves in whole (eg. mussels), a worst case estimate of exposure to microplastics would be 7 µg, considered a negligible amount relative to the total dietary intake of persistant, bioaccumulative and toxic substances (PBTs) by humans.

Summary

Macroplastics and their immediate impacts on marine organisms (recall: "serial killer") are clear disruptions to the natural ecological balance, and an obvious telltale sign that our plastic consumption inflicts direct harm on the environment. Microplastics and their impacts are much less conspicuous, as they have only been evident under extreme laboratory conditions which do not represent the fatigue of the oceans. Unfortunately, the present lack of direct health risk from ingesting minute quantities of microplastics or nanoplastics as written above also serves little to discourage individuals from irresponsible use of plastics.

Nevertheless, it is important to consider the projected increase of plastic waste in the future (recall: introduction post), which may concomitantly possibly magnify the presently undetected impacts of plastic consumption in humans through the 7-step causal chain I constructed in Figure 10.  The micro-nanoplastic problem is especially important given that the particle distribution will increasingly shift towards microplastics and nanoplastics in aquatic environments with fragmentation over time.

Figure 10. Flowchart representing the causal chain of increased plastic waste in the environment, with accompanying ecological effects on aquatic organisms and impacts on human health highlighted in red (Source: Author).

The causal chain drafted above also serves as an effective summary for our discussion in this week's entry. In my following post, I will talk more about the impacts of microplastics from microfibre and microbeads, something I came across in social media lately.

See you!