Bacterial Bioremediation For Poisonous Plastic Pollution

Problem

Plastic has been a steadily increasing problem since it’s development. For its versatility, moisture resistance, durability, and cheapness it has become an essential in our daily lives, with close to 300 million tons produced globally each year [1]. Yet an alarming amount of the plastic we use is taken straight to a rubbish dump, where it may sit for up to 1000 years, waiting for UV radiation to slowly break it apart [1]. This is not the only place it ends up, with massive amounts entering the ocean, leaking chemicals, being ingested by marine life, and congregating in plastic ‘islands’ [1]. Plastic has been shown to impact at least 267 different animal species, killing over 100,000 turtles and birds every year [1,2]. Marine organisms ingest plastic and associated chemicals, so that these toxins enter food chains and accumulate in resources for humans [2]. One chemical leached by polyethylene terephthalate (PET) plastic has been linked to allergies, asthma, cancer, and issues with multiple organs [1]. Other chemicals have also been linked to birth defects, development issues and infertility [2]. We are exposed to these toxins when eating fish contaminated with plastic - we are quite literally poisoning ourselves. Plastic waste, particularly in the marine environment, has economic impacts as well. Coastal tourism is reduced in value by plastic waste, and the fishing industry suffers both from having to increase maintenance of vessels and from fish getting caught in plastic debris, rather than nets [3]. These issues are worsened in small island developing states where there is a heavier reliance on exploiting marine resources [3]. The threats of plastic are compounded by the fact that we continue to produce massive amounts, despite knowing that it won’t go away. With an unwillingness to give it up, and an environment accumulating plastic waste, something needs to be done.

Solution

Bioremediation is the use of living organisms to clean up pollutants and has been thus far unexploited in the breakdown of plastic. This is because plastic is not something that has previously been in the environment, and organisms are not adapted to use it as a food source [1]. However, recently a bacterium called I. sakaiensis was discovered that can breakdown PET plastic [4]. It uses the enzyme PETase to break PET down to mono(2-hydroxylethyl) terephthalic acid (MHET), which can then be broken down further with the enzyme MHETase to terephthalic acid (TPA) [4]. Work has already been done to modify PETase and increase its efficiency [4]. I propose the use of a modified strain of I. sakaiensis (or other appropriate bacteria with genes from I. sakaiensis inserted) for PET bioremediation. The first modification to make would be to further the metabolic breakdown of PET, as the current product TPA may pose a threat to human health [5]. This could be done by isolating genes for the degradation of TPA in other bacterial species and then introducing the pathway to I. sakaiensis via a plasmid or transposons [6]. Transposons randomly insert themselves randomly into the genome so may affect the function of other genes in I. sakaiensis, making the preferred method of insert a plasmid [6]. Modified bacteria will need to be contained and monitored. To monitor bacteria a reporter gene, such as green fluorescent protein, can be inserted with a promoter that activates in the presence of PET [6]. When fluorescence is expressed bacteria will be breaking down PET [6]. This allows online monitoring of PET presence and availability, in a cheaper way than other monitoring methods [7]. Bacteria can also be contained by genetic modifications, using suicide genes. The expression of a suicide gene can be under the control of a cascade that is ultimately controlled by the presence/absence of PET. If PET is absent, the suicide gene will be activated, and the bacteria will die [6]. This will prevent bacteria from entering the environment - bacteria will be confined to the site of PET. While this process has been focused on degradation of PET alone, if it is successful these techniques can be applied to other plastic compounds.

However, employing these modified bacteria will not be as simple as releasing them at a dump. Plastic is not only single-use, it is used for things such as electrical insulation, drainage systems and vehicle interiors. If all plastic around the world disappeared there would be serious consequences. Plastic degradation must occur within a containment facility. Containing the bacteria will not only avoid New Zealand’s strict regulations around the release of genetically modified organisms (GMOs), it will remove risks posed by release of bacteria into environment [8]. PET plastic can be collected and transported to the facility, bacteria can be introduced, and degradation progress can be monitored online. The suicide gene above will mean that once degradation is complete, bacteria will die. Although dead, there is still a risk that modified gene fragments will be taken up by bacteria in the environment through the process of transformation (horizontal gene transfer) [9]. If genes for antibiotic resistance are used during the modification of bacteria this could pose a serious health risk to humans [10]. To avoid this, DNA must be destroyed before any waste is removed from the facility. One way to do this is to degrade DNA by heating it [11]. These processes of modifying bacteria and degrading PET can be used, in combination with reduced plastic consumption, to solve our plastic waste problem.

Risk/Benefit Analysis

Environmental Implications

To reveal the benefits of plastic bioremediation, we need only to look at the problems caused by plastic. Plastic accumulates and suffocates both animals and economies, as well as leaking toxins into the ocean that poison everything from zooplankton to us [1]. Removing plastic from the environment to break down into safer compounds will stop these problems, but it will not be able to clean-up microplastics and chemicals already in the ocean and food-chains. Environmental benefits are not limited to the ocean. New Zealand in 1997 had 327 (legal) landfills, some close to full, and this figure will have only grown since [12]. It is estimated that 20% of the waste in landfills is plastic [13]. As plastic will not break down there we will eventually run out of landfill space, and it may overflow into the environment and urban areas [1]. Removing the problem from sight has not removed the problem, as so many of us have been tricked into thinking.

The bioremediation of plastic waste will help to prevent the filling or expansion of landfills and may free up space for more positive things. Current breakdown of PET results in TPA, a toxic chemical, but by engineering the metabolic pathway of PET breakdown, we are hopeful that we will be able to end up with a compound suitable for use as a biofuel, such as ethanol [5]. Biofuels produce significantly less CO2 than fossil fuels and do so without making us reliant on a non-renewable resource [14]. It is yet to be determined if any products of this breakdown will cause harm to the environment. If CO2 is also produced in the breakdown of PET, the overall carbon footprint may be greater than fossil fuels. Possibly the biggest environmental concern when using GMOs is what will happen if they get out. For example, a loss-of-function mutation in the suicide gene may enable the bacteria to exist outside of the containment facility. Although unlikely, there is a slim chance of modified bacteria entering the environment. If this occurs, it is unclear what will happen.

Literature is conflicted about whether bacteria designed for contained conditions are capable of surviving in the environment [7]. It may be that bacteria that gets out will die naturally, but it may also be that they survive. The major concern is that genes will be passed from modified bacteria to bacteria in the environment [10]. This is especially concerning if antibiotic resistance genes are used to select bacteria modified by plasmids [10]. If genes for antibiotic resistance were passed to human pathogens, for example, the risk to humans could be much greater than the risks ever posed by plastic. We cannot know how disastrous or benign our bacteria may be in the environment.

Social Implications

There seems to be agreement that rather than utilising bioremediation, we eliminate the need for it - stop producing plastic waste [1,7]. This does not consider the plastic waste that we have already accumulated. Do we have the right to try and remedy our mistakes, or now that we have made them do we simply have to bear the consequences? If we begin the bioremediation of plastic, this may be taken as a free pass on producing plastic waste, exacerbating the current issue. If we solve the problem, we will not learn from it, but if we do not attempt a solution the problem will continue to harm us, and other beings, in the future. To employ bioremediation in the clean-up of plastic everyone would need to make an effort to reduce waste and dispose of it properly. The use of volunteers to collect pre-existing plastic from the environment will be needed too. A social concern with this is that some people will not want to reduce their plastic consumption or will not dispose of plastic properly. It is easy to have the mindset that one person’s output will not destroy the world, and I will admit that I once thought this myself.

However, if everyone permits this thought, then it is not one person’s output, it is everyone's. This could potentially be controlled by taxing single-use plastics, and imposing fines on people who do not dispose of plastic waste properly. Another social barrier to employing plastic bioremediation is public mistrust of GMOs [10]. This is especially noticeable in New Zealand, where strict regulations make it virtually impossible to release GMOs [8]. There is also the idea that bacteria are bad, as many people are only presented with bacteria in the context of disease. While this idea is not entirely unfounded, as some bacteria suitable for bioremediation do cause disease, we would not be using any pathogenic bacteria for plastic degradation [10]. Public distrust of GMOs could be minimised by improving science comprehension and must be weighed against the threat of plastic. The social benefits of doing this can also be considered. Biodegrading plastic will stop the accumulation of toxins in marine life, and the ingestion of such toxins by us [2].

Although it will not address the toxins already released, it will prevent the problem from worsening. As these toxins already border on violating multiple human rights (the right to life, the right to adequate standard of living), it is vital that we do all that we can to remedy it [15]. Removing plastic will help people who rely on fishing or coastal tourism [3]. As people in small island developing states are especially affected by plastic waste, they will be especially benefited by its removal [3]. New Zealand is closely associated with many such states (the Cook Islands, Samoa, Fiji, etc. ), so involvement in plastic clean-up would help to strengthen our ties to them. Economic ImplicationsThe economic implications would need to be fully explored before implementing this process. The current costs of processing plastic waste must be contrasted with the potential costs of engineering and growing bacteria, building and maintaining a containment facility, processing waste and removing products of degradation.

These costs may be offset by combining the facility with other things, such as solar panels for green energy or production of a biofuel, as discussed earlier. Another consideration is how the use of GMOs may affect New Zealand's 'Green' image, and thus affect our tourism industry. This would be influenced by global perceptions of genetic engineering, as well as the absence of plastic in our environment. Finally, we would consider how, once we begin removing plastic from the ocean, marine ecosystems will have the opportunity to thrive, allowing a subsequent boost in fishing, aquaculture industries, and coastal tourism [3]. It is worth noting here that plastic is not the only thing harming marine life, and the extent of this 'boost' may be limited. Bioremediation is usually confined to degradation that cannot be done in a cheaper way [7]. As there are no other viable options for plastic degradation the costs should be considered secondary.

Advice

We cannot afford to let hundreds of millions of tons of plastics leach toxins into our environment [1]. Even if we stopped producing plastic today, the problems caused by it would persist for centuries. We know that our plastic usage is not environmentally, socially, or economically responsible. It poisons our oceans, our animals and our people [1,2]. It hurts fisheries and coastal tourism [1]. New Zealand’s current mantra of reduce-reuse-recycle only prevents more mess being made [13]. It does not clean-up what's already there. I believe bioremediation is the only viable solution. It will allow us to quickly and safely get rid of plastic waste. We can increase the efficiency and safety of this by genetically modifying our bacteria [4,5,6].

We cannot ignore the problem on the off-chance plastic eating organisms evolve naturally in New Zealand. We must act, and we must act as soon as we can. Most of the environmental risks are a worst-case scenario, and do not compare to the current harm done by plastics. Public mistrust of GMO's and any expenses to biodegrade plastic must be secondary to the harm being done to human life. Risks should be managed appropriately. Bacteria should be contained and controlled in specialised facilities to prevent environmental harm and reduce risks to human health. People may initially resist changes to plastic consumption and disposal - you can't always trust people to do the right thing. Fines can be imposed on people who refuse to dispose of plastic waste properly, and volunteers can collect plastics from our environment and urban areas. While currently this solution is limited to the degradation of PET, with the potential of discovering or engineering enzymes to break down other plastics, if this is employed successfully we will be able to apply it to all plastic waste. It is my belief that we have an obligation to remedy the harm we have done to the planet. We can begin to do this through the bioremediation of PET plastic.