I came across the above image last week, depicting the vortex of plastic caught up in the Northern Pacific Gyre. Why do all those plastic bags just hang around in the ocean? We all know that plastic doesn’t biodegrade but–why doesn’t plastic biodegrade? Will that plastic be there forever?
It turns out that yes, the plastic will essentially be there forever.
Merriam-Webster defines a biodegradable object as “capable of being broken down especially into innocuous products by the action of living things.” In general, the “living things” that do the vast majority of the biodegrading are microorganisms. Another way of thinking about biodegredation is that its just another word for eating–i.e. humans have been biodegrading sugars, starches, and proteins for some time now.
For example, we–as a species–are excellent biodegraders of pizza. Imagine: you consume a slice of ‘Za and, with the help of our trillions of bacterial symbionts, tear it apart at a molecular level by stripping electrons off organic molecules and shuffling them through our mitochondria. The electron shuffling transfers energy to our cells which we use for a variety of purposes before dumping the electrons onto an oxygen molecule. After all that, we end up with:
- No More Pizza
- Useful Energy (despite any incidental food comas)
- CO2 and Water
This is a highly evolved process and bacteria do more or less the same thing. But the group “bacteria” contains a huge array of species and metabolic diversity. Essentially, for any molecule that contains free energy, there exists some microorganism that has evolved a metabolic infrastructure which is used to extract from the molecule a thermodynamic dividend–in the same way humans do with pizza.
So why don’t plastic grocery bags biodegrade? From nature’s point of view, polyethylene is a fairly new development. In principle, it can produce exploitable energy for microoganisms. But that energy is tough to get at and virtually no bacteria have yet evolved a metabolism that can process PE.
The single known exception comes from the genera Sphingomonas as was demonstrated in 2008 by a 16 year old from Canada. He found that with 3 months of contact he could convert more than 30% of the PE mass to CO2 using Sphingomonas. To put that in perspective, respiration tests using common bacterial communities produce biodegredation estimates of 500 to 1000 years which (as Juliet Lapidos notes) is a more professional sounding way of saying, “we have no idea but it takes an effing long time!”