Marine plastics: food or a free ride?
Plastics are known to be colonised by microorganisms within hours of reaching the oceans. The communities of colonising microorganisms, such as bacteria and micro-algae, are different from those in the surrounding waters, and these communities experience shifts when they are disrupted or disturbed, just as larger ones do on land. We also know that plastics tend to accumulate pollutants, such as plasticisers (things that are added during manufacturing to give desirable properties e.g. BPA) and other toxic chemicals such as fertilisers and heavy metals.
What we still don't know, however, is whether these microorganisms are colonising because they are actually degrading the plastics (or the contaminants), or whether the plastics just make for a nice place to live. Because plastics tend to float, being attached to them keeps the microorganisms in the light, in the relatively warmer waters of the surface, and around other species that may be beneficial to them. They don’t, however, provide an easy food source. Plastics are notoriously difficult to degrade - we are always hearing about how many hundreds of years they are expected to exist in the oceans - and, we have relatively few examples of microbes being able to degrade plastics (even fewer in the oceans).
(Photo: Somewhere in Sabah, Borneo. Taken by me.)
In an example of plastic degradation on land, scientists in Japan found a bacterium, named Ideonella sakaiensis, that is able to break down PET (what plastic bottles are usually made out of) and use this as a food source. Having a characterised pathway like this one tells us which genes are being used to degrade the plastics, and once we know this, we can look for these genes in other organisms. When other scientists have looked for these genes in the oceans, they've found that such genes are very rare. If they are indeed rare, this suggests that perhaps the microorganisms aren't degrading the plastics, or at least, this job doesn't fall to a single bacterium alone. Maybe the degradation is being carried out by several organisms, a community, or maybe it isn't being carried out at all.
All surfaces that enter the marine environment go through typical stages of community succession. The first things that colonise tend to be species that are just good at colonising; good at attaching to a surface, but probably not a lot else. We then see an increase in (or a selection for) organisms that may be able to degrade the material that they are attaching to, or other things in the surrounding water. Finally, these are then succeeded by other community members; the "cheaters". These are the ones that can't really degrade anything by themselves, but can benefit from those that do degrade, either by eating the bacteria themselves, or using the by-products of that degradation. By looking at the organisms that are colonising plastics in these different phases, we can identify bacteria that might be able to degrade plastics, and then use them to further test these ideas.
Research into the colonisation of plastics is still relatively new. At the moment, we don’t really know whether plastics are being used as a food source, or if they’re just a convenient place to live life. This is one of the things that I’m hoping to shed light some light on through my PhD research; by looking at whether marine bacteria are even capable of degrading plastics, we can see which scenario is more likely.