Could microbes munch our waste plastic?

We all know about problematic plastic filling the oceans and destroying marine life. But what is plastic, how is it made and why does it last for so long? Plus, could we get bacteria to biodegrade it for us? Georgia Mills got the low-down from Cambridge University chemist Steven Lee...

Steven - A plastic, when chemists think of plastics, what we’re thinking about is repeat monomer units, so the word polymer you may have heard is that’s how chemist’s don’t really think about plastics. That’s essentially, a bit long chain molecule that consists of much smaller molecules linked together typically via a carbon link.

Georgia - So a monomer is like a little unit, a little pod, and then a polymer is a load of these strung together?

Steven - Exactly. Poly meaning many, and mer meaning unit, comes from the Greek polymer.

Georgia - Ah, it all makes sense. So how do you make a plastic?

Steven - There’s lots of ways to make a plastic and they have very large meanings. But I think in the general terms when people think about a plastic spade or some wrapper, the way you do that is you take a small molecule that has a carbon/carbon double bonding and you apply by various heats or chemical reactions you can encourage that to form long long chains. That typically can be done by small molecules that come from the petrochemical industry for instance.

Georgia - Why does it stick around for so long?

Steven - The reason is there’s not the traditional method for things that degrade. When we think about something’s rusting or trees or organic waste being digested, there are biological organisms that have spent billions of years evolving lots of complex pathways to be able to break those bonds. In the case of cellulose that’s a polysaccharide, that’s a polymer as well but it’s made of lots of sugar molecules. But in the case of a polymer like a plastic bag at a shopping centre, that’s typically made of polythene or polyethylene and so that’s a very different kind of chemistry - one that’s got to break a carbon/carbon bond. That’s much harder for the natural world to break down and so, unfortunately, we haven’t had the bacteria and other environments and haven’t had to time to deal with that.

Georgia - Right. Something like cellulose which is actually kind of similar to a plastic can be broken down just because it’s been around for millions of years within plants?

Steven - That’s true, it’s called a biopolymer. There’s three kinds of biopolymers, but what we’re talking about here is this unusual class which we see every day but, on the timescale of evolution is a relative newcomer and we’ve only had the first completely synthetic plastic since 1907.

Georgia - Why can’t we just recycle it? Why can’t we put it like in the opposite way through the plastic machine?

Steven - Yeah. That’s a thermodynamic answer to that. It to do with all the technology to understand why plastics don’t mix is the same chemistry and the same physics that we worked out in the industrial revolution. In fact, what it’s to do with is that if you take two compounds and try and mix them together, sometimes they mix, sometimes the dont, but polymers don’t like to do that because they don’t gain as much entropy - this term about how disordered something is. So, what that really means is because polymers don’t mix, if I just take a mixture of my compounds and just try and heat them up to kind of regenerate a big slush of plastic, it means that they all phase separate. They’re much like oil and water don’t mix, you get the same thing with plastics, so what that really means is that it’s quite difficult to separate them out in order to recycle them in a way that, for instance, aluminium or metal isn’t quite as difficult.

Georgia - I guess, quite often within the same product you have a few different types of plastic all in one place?

Steven - Absolutely. If you look very carefully on the bottom of your plastic baby or on the bottom of your bottle of diet coke, you’ll see that there’s a little mark and that terms the type of plastic. What that means is that if you have polyethylene or polythene, that’s possibly recyclable with other clumps of polythene, but it’s not if you use polymethyl methacrylate or polystyrene or any other synthetic polymer we use traditionally today.

Georgia - Could we use nature to help us then. Is there anything out there that might be able to munch away at these carbon bonds?

Steven - There’s president in the literature for various bacteria and also fungus to degrade very specific types of plastic. In biology there’s lots of ways in which we can digest plastic, but the chemistry to be able to digest these specific compounds is unusual and, therefore, doesn’t exist at lot. There absolutely could be, but we haven’t found anything yet. There have been some studies where the way people actually look for them is they look in and around plastic plants because they think if the bacteria could start eating plastic it would be a limitless source of food. It’s an all you can eat buffet of energy to the bacterium, so there is a clear push for them to be able to use it. Unfortunately, normally what happens is we have to find one, because of all the chemistry that I was talking about is very specific, it means that bacteria have to evolve different ways to be able to digest specific plastics. So you don’t get one bacteria that will eat all plastics, but there are samples of bacteria that eat very specific kinds but just not very very well at the moment.

Georgia - Very briefly, do you think we could modify something in a lab - a super bacteria - to just go into the oceans and destroy all the plastic and solve all of our problems?

Steven - Yeah. “Catch a spider to kill a fly.” It’s possible and I think loads of people are working on really interesting ways to be able to modify bacteria. We have lots and lots of genetic tools in the laboratory to be able to change bacteria to do our bidding, as it were. The trouble is is that the specificicity to be able to break this carbon/carbon bond in my plastic bag and not break the carbon bond in me is very different, and that’s the challenge going forward.