Tuesday, 6 February 2018

Bacteria Evolved to Eat Plastic


ILLUSTRATION: P. HUEY/SCIENCE
Feeding on plastic | Science

One of the more subtle aspects of evolution, and one that creationists are either unable or unwilling to understand, is the difference between genetic information and the meaning of that information. A paper published in March 2016 by a Japanese team, illustrates that principle very well. It is about the discovery of a strain of bacteria that has evolved the ability to digest the man-made plastic, poly(ethylene teraphthalate) (PET).

Currently, the world-wide production of PET is over 50 million tons and none of it was biodegradable - until now!

Abstract
Poly(ethylene terephthalate) (PET) is used extensively worldwide in plastic products, and its accumulation in the environment has become a global concern. Because the ability to enzymatically degrade PET has been thought to be limited to a few fungal species, biodegradation is not yet a viable remediation or recycling strategy. By screening natural microbial communities exposed to PET in the environment, we isolated a novel bacterium, Ideonella sakaiensis 201-F6, that is able to use PET as its major energy and carbon source. When grown on PET, this strain produces two enzymes capable of hydrolyzing PET and the reaction intermediate, mono(2-hydroxyethyl) terephthalic acid. Both enzymes are required to enzymatically convert PET efficiently into its two environmentally benign monomers, terephthalic acid and ethylene glycol.


Analysis has shown that the bacteria use two different enzymes to achieve this. One enzyme, PETase, breaks PET into an intermediate, mono(2-hydroxyethyl) terephthalic acid (MHET), when the bacteria is in contact with the PET surface. This intermediate then passes into the bacterial cell where the second enzyme, MHET hydrolase, completes the process, producing the two starting monomers, terephthalic acid and ethylene glycol. The bacteria is then able to use these simple molecules in it's own growth processes.

As well as helping dispose of the 50 million tons of PET produced annually, this also raises the possibility of recovering the monomers to be recycled into new plastics, so reducing our dependence on fossil fuels.

Plastics are not a naturally occurring substances, even though they are organic in the sense that they are polymers of sub-units, each of which is based on a carbon skeleton with various combinations of oxygen, hydrogen, nitrogen and maybe sulphur and/or phosphorus, etc. As such, the chemical bonds between the sub-units should, in theory be able to be broken down with the right enzymes, just as they are with the 'natural' polymers such as proteins, fats and carbohydrates.

However, living systems have evolved over billions of years in the presence of these natural polymers so there has been an advantage in being able to digest them. Indeed, all the basic physiological processes have evolved to break down and build up various organic molecules. They have not evolved in the presence of man-made plastics like PET, so what would be the advantage in evolving or adapting an existing enzyme to break them down before there were plastics? In fact, without plastics a mutation in the gene to make an existing enzyme so that it no longer does it's job would normally be regarded as harmful and so a degradation of the information.

But what if the information now means 'digest plastic' in the presence of plastic? The same change of information but this time not only no degradation but an enhancement. What has changed is the meaning of that information. Now, the meaning is not just 'stop doing whatever it was you were doing' but 'stop whatever it was you were doing and do something more useful instead'.

And the meaning is determined by the environment. Without plastic the meaning was almost exactly the opposite of the meaning with plastic.

Creationist frauds tell their dupes that, for unexplained reason traditionally blamed on the Second Law of Thermodynamics (2LOT) so it sound like science, no new information can arise. This is nonsensical of course since there is nothing in the 2LOT that prevents gene duplication or the substitution of one nucleotide in DNA for another, but even if it were true, what creationist frauds never tell their dupes is that the meaning of information can be changed by the environment, as we see in this paper. Man-made plastics have added a new potential source of energy and building blocks in the environment, so their presence has the potential to change the meaning of mutations in bacteria and any other organisms capable of benefiting from them.

Evolution is, of course, adapting to environmental change.

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