Additives and contaminants of plastics

Plastics have been known for a long time to be able to concentrate toxic chemicals, and sometimes these can be up to a million times more concentrated than in the surrounding seawater! I personally think that it's these chemicals that are actually more likely to cause problems to the organisms that eat the plastics, than the plastics themselves will. The chemicals tend to stick to plastics because they are hydrophobic - repelled by water, like oil - but then they are known to be able to become unstuck when they're in the stomachs of the animals that may eat them. A recent study identified just over 900 chemicals that are likely associated with plastics, and 63 of these are highest for human health hazards, and 68 for environmental hazards.

The chemicals that I'm talking about include plastic additives like plasticizers (one of the ones that you're more likely to have heard of is BPA, or seeing that things are "BPA-free") - things that are added to plastics during manufacturing to give them desirable properties - as well as others that stick to the plastics when they are in the environment, like polychlorinated biphenyls (used in electrical equipment) and pesticides (e.g. DDT). While many of these chemicals can cause death in high concentrations, they also have harmful effects at lower, sub-lethal concentrations. Many of these chemicals have been shown to cause cancers and to have endocrine-disrupting effects - they behave like the hormones that all organisms naturally have - and this can lead to intersex organisms. This means that the animal would have characteristics of both females and males, but it can lead to them changing sex entirely, too. Other chemicals can change the behaviour of organisms, which might also lead to problems reproducing normally, or in finding food or avoiding predators. One of the problems with them being stuck to plastics is that they might be more likely to travel longer distances on the plastics than they would have done on their own.

While these compounds are problematic, luckily, they do seem to be more biodegradable than the plastics themselves. One of the parts of my PhD has been looking at the degradation of plasticizers (I chose six commonly used ones) by marine microbes. I was able to isolate lots of different bacteria (about 40) that are able to grow using the plasticizers as their only source of energy, but some of them are much better at growing on them than others. I then narrowed this down to two bacteria that could grow with all six plasticizers that I'm looking at, and I'm currently trying to see which enzymes they use for this. I want to try to find out if the enzymes they use are the same for all of the plasticizers (so if a bacteria can degrade one plasticizer, then it could degrade all of them), and whether this is the same for different bacteria, too. To do this, I'm using proteomics. I wrote before about how we can use sequencing to look at all of the genes within an organism, well proteomics looks at all of the proteins that are actually produced by an organism. By comparing the enzymes they produce under different conditions, we can then try to work out which enzymes might be responsible for each function, or step in a process.

(Photo: Some of my bacterial isolates growing on agar plates with a plasticizer. If you look closely, you can even see that the agar gets clearer when the bacteria use up all of the plasticizer.)

I'm still trying to look through all of the data that I've got for this, and work out what's going on, but what does seem apparent is that - if microbes that colonise plastics are indeed using them as a source of food - it's a lot more likely that they are growing on the plasticizers, and other additives and contaminants, than on the plastics themselves.