It is hardly a hundred years since chemists learnt to make long molecules called polymers and plastics in the lab. Polymers have since been hailed on one hand as the greatest boon to mankind for their manifold uses, and the greatest bane - thanks to the way that they have cluttered the environment. The most common synthetic polymer or plastic used in everyday life is polyethylene terephthalate or PET (also known as Terylene or Dacron). An estimated 311 million tonnes of plastics are produced every year (and 50 million tons of PET alone). Unfortunately many of them, such as PET, are not degraded, digested or broken down like naturally occurring polymers (such as proteins, carbohydrates and fats). We use plastic in everyday life, use and discard them, hardly recycling them (to the extent we actually can), and as a result plastics have cluttered the earth and its oceans. J.R. Jambeck and others have estimated that as much as 5 trillion pieces of plastic have reached and are found in ocean beds across the globe ( Science, 13 February 2015). That is anywhere between 5 to 13 million tonnes of them, lying and affecting the health of ocean life (and an area of about 1.4 million square kilometers, or the area of Northern India). There is no clear estimate of how much plastic is fouling the land mass of the earth, surely it has to be an equal amount.
If only we can find ways to degrade such huge amounts of accumulated plastics! And the best agents to do so would be biological life forms such as bacteria which multiply by the millions in days and are themselves completely biodegradable! (Indeed, recall how, way back in 1980, Dr Anand Chakraborty of GE R&D Center at Schenectady, NY, isolated one such microbe that would eat off oil spills). It is towards this challenge that research has been going on, and the latest effort which shows some success has been published in the March 11, 2016 issue of Science by a Japanese group, led by Dr Kanji Miyamoto of Keio University, Kanagawa. The group concentrated on looking for and identifying bacteria from the PET bottle recycling sites, and found one such microbe that they have named Ideonella sakaiensis (the first name identifies the family and the second honours the geographic location where they found the bacterium).
I. sakaiensis sticks to the surface of the PET bottle, secretes one molecule named which they named PET-ase (the suffix “– ase” denotes an enzyme molecule), which breaks down PET into a smaller building block abbreviated as MHET. MHET is now taken up and broken down by another enzyme in the microbe’s cell (called MHET hydrolase) and hydrolysed to produce ethylene glycol and terephthalic acid - the two small molecules (called monomers) using which the polymer PET is made in the first place! We should admire I sakaiensis for its efficiency as a safe biodegradable agent. Biologists will now wonder about how this microbe, which all these centuries and millennia had never known PET (until 70 years ago) has suddenly found (or generated) enzymes to degrade this new man-made polymer. Myriad and wonderful are the ways of mutations and natural selection!
Two interesting points emerge from the Japanese work. One is: can we now isolate the ethylene glycol and terephthalic acid, the two monomers, and reuse them to make PET? This offers a nice self-contained set up where the PET bottles and plastics discarded after use are biodegraded back to the starting materials in a bio-reactor, and then taken to the polymer synthesising unit which remakes the PET.
The other point is more challenging and surely there are molecular biologists already working on it. That is: why not clone the genes that express the enzyme PET-ase and MHET hydrolase into some other properly chosen microbe (other than I.sakaiensis ), using genetic engineering methods and thus attempt to biodegrade the vast mountains of PET fouling the earth? If one can do this for PET, surely it can be done for other polymers and plastics. To write these sentences is easy, but to work on it and succeed takes effort and single minded devotion, but worthy of a Nobel.
Even more changeling is the issue of how to clear the millions of tons of plastics fouling the oceans beds. Even if bugs are founds that can biodegrade them, will they be safe for the oceans and their life forms? But this needs to be done and as they say “nothing ventured nothing gained”. It can perhaps first be done in silico using the methods of computational and system biology, to look for optimal ways to do so and then try on a lab scale. Clearly the Kanagawa group has kindled an exciting chapter in environment sciences with their work.
D. BALASUBRAMANIAN
dbala@lvpei.org
