Friday, 22 December 2017

PET projects: can nature destroy plastic pollution?

One of key markers of the Anthropocene - the as yet unofficial term for a human-impacted global environment - is the deposition of manmade pollutants on land and in the oceans. A prominent component of these pollutants is plastic-based consumer waste; as I mentioned in March 2010, the UK was then using 17.5 billion plastic bags each year. Happily, the introduction of a charge on lightweight plastic shopping bags in the UK has reduced usage by a fantastic 85%. Various nations have introduced similar or even better legislation, but unfortunately in that key polluter the USA only California has had any success in overcoming corporate lobbying. The situation in Trump's America looks unlikely to change any time soon, since several states have even prohibited such bans at a county level!

Therefore, despite the best endeavours of some nations, most recently Kenya, around 80 million tonnes of polyethylene-based packaging and bags are still being produced worldwide each year. The amount that is recycled varies considerably from nation to nation, with the US Environmental Protection Agency recording only 12% of America's plastic as being recycled. As a result, it is estimated that about 12 million tonnes of plastic is annually deposited in the oceans, with even deep-sea species found to have been contaminated.

We've all seen images of beaches on the most remote, uninhabited islands smothered by tiny, multicoloured pieces of plastic, but apart from being unsightly, what are the potential dangers to the global ecosystem and humans in particular? By ingesting plastics, animals risk either choking or starving to death, or being poisoned by chemicals leaching from the material. Even if the latter doesn't quickly kill the critter (which could be anything from sea birds to turtles to baleen whales), substances such as Bisphenol A can build up in their system. In addition to the damage caused to the animals themselves, toxins can upset a species' reproductive cycle. For instance, some of the leached chemicals mimic estrogen, potentially inhibiting development of male offspring.

Of course, with such a range of species being affected, isn't it feasible that there will be knock-on effects to the human food chain? Even if there aren't obvious reductions in commercially-caught species, there is a high likelihood that wider food webs could be severely altered - and not for the good - or even that we on the verge of ingesting copious amounts of microscopic plastic particles. Even people who never eat seafood won't be able to avoid it, since animal feed may contain contaminated fishmeal.

It isn't just the obvious items that are the key pollutants, either: plastic microbeads (i.e. less than 1mm along their longest side) are prominent in rinse-off personal care products. Whoever invented them clearly has zero environmental credentials, bearing in mind there's no ability to recycle or reuse them; in fact, about 8 quadrillion microbeads get washed down the plug hole every day.  The World Trade Organisation is making some inroads into their removal - here in New Zealand their manufacture and sale will be banned by the middle of next year - but research has found they are already pretty much ubiquitous in the environment wherever these products are in use.

Therefore it makes sense to tackle the problem as soon as possible. Since some countries are reticent to implement legislation, or like China and India are having difficulties enforcing it, there is much to be said for seeking ways to degrade plastic waste in the most efficient way possible. Research over the past decade has revealed an astonishing conclusion: only about 1% of the expected amount of waste material has been found in the oceans. Either it is rapidly being buried in the sea bed, or more likely, something is breaking it down. Is this possible? Last year, a team of Japanese researchers found a microbe called Ideonella sakaiensis that is able to digest polyethylene terephthalate (PET), which is used in such mass-produced items as drink bottles. This suggests that there may be marine microorganisms with a taste for human waste, diligently destroying our plastic rubbish and preventing even worse effects on ocean life.

The Japanese research hints that it may be possible to use vats of these microbes to break down at least waste PET and then recycle it, with a much greater efficiency than is currently possible. Without interference, PET is thought to take between four hundred and one thousand years to completely degrade, presumably depending on the shape and thickness of the item. In contrast, Ideonella is able to digest the material in only six weeks. About 56 million tonnes of PET, mostly for bottles, is produced each year. Here in New Zealand, less than half of this material is recycled, the first (conventional) PET recycling plant having started work in August. So there's plenty of scope for a natural solution, should it become usable on an industrial scale.

This begs the question: are there any other critters with similar capabilities?  Last month a team at Texas Tech University reported that caterpillars of the pantry moth Plodia interpunctella have been able to thrive on polyethylene. Research showed that their digestive system contains various species of bacteria - different from the gut microbes in caterpillars that eat natural foods - which are capable of breaking down the plastic. However, what worries me is that if these microbes become selected for in the wild, will this change have the same sort of disastrous result that the inadvertent artificial selection of MRSA has had?

Some worm species are known to eat natural polymers similar to man-made plastics, such as the beeswax in hives, and so have been tested for their ability to break down plastic as well. Further research is required to determine whether the work is being done by microbes in the worms' digestive systems, but one issue with worm-digested plastic is that by-products include the toxic ethylene glycol. Apart from bacteria, Chinese researchers using plastic waste from Pakistan have found that the fungus Aspergillus tubingensis can degrade polyester polyurethane. After some years of disappointing results in mycoremediation (the use of fungi to break down man-made materials) this may prove to be a breakthrough.

The big question then is has nature done it again? After all, it does have about three and a half billion years' head start on the human race. Plastic waste is clearly a big issue and for the majority of humanity who live away from the sea (or rubbish dumps, for that matter) it's fairly easy to think "out of sight, out of mind". However, it pays to highlight the potential danger of changing ecosystems on a global scale, including the extinction of unseen and unknown species, including microbes that are vital to maintaining stability. I've previously mentioned the problems with concentrating on a few key 'poster' organisms at the expense of those that may play a pivotal role - now, or in the future - to our nutritional, pharmaceutical or technological needs. Therefore we need to be certain that the solution won't be as bad as the problem, when it comes to using nature itself to destroy the waste we unthinkingly generate. Surely a good compromise would be to minimise the amounts of plastic rubbish we generate in the first place?