Wednesday, June 15, 2016

Throwing the baby (fish) out with the (ocean) water

Concerns about oceans plastics first focused on just the existence of a trillion pieces of plastic floating around. Now concerns are switching to the impact of the plastic on the ocean environment and the entities living there. A recent report in Science which got a lot of free publicity (since it was in Science after all) found that ocean perch hatchlings prefer microparticles to their regular food, gain less weight and don't respond as well to danger signals, meaning they are more likely to be eaten by predators. And this is all on top of have a reduced hatching rate in the first place.

Rather than studying fish in the natural environment, this was all done in a lab. Ocean water was filtered and put into 1 L bottles that had an aeration system. Plastic particles were added at a low concentration (10,000 particle/m3) and high concentration (80,000 particle/m3). Water without any particles was the control.

I'm amazed that all the results can be attributed to a physical entity - a microplastic particle. After all, a plastic particle is too large to induce much in the way of a (bio)chemical reaction, and even plastic molecules themselves are too large to induce a reaction either. So how does this happen strictly via physics?

It doesn't. There is some chemistry involved, but it is not chemistry with plastic as a starting reagent. Something else such as unreacted monomer, residual catalyst, etc. is the real culprit and that rogue actor was never identified. Since we are now talking chemistry, the universality of the results is called into question.

Surprisingly, the researchers used polystyrene microspheres that they got off the shelf from Polysciences. Why?

Why polystyrene, with a specific gravity of 1.04? In the Supplemental Information (SI), the researchers noted that about 60 - 70% of the particles settled to the bottom. First off, how did they come up with that number? No information is provided. But secondly, is that value the same for both the high concentration tanks and the low concentration tanks? Really? Because as an engineer, I seriously question how a simply designed tank could achieve that same degree of flotation/separation across a multitude of concentrations.

In the main article, the authors infer that the low hatch rates with higher concentration of plastic is due to an unnamed chemical:"
"This suggests that polystyrene particles may be chemically affecting larvae in both average and high concentrations, as exposure potentially reduces hatching rates of fertilized P. fluviatilis eggs."
But then they remark in the SI:
"Thus, fertilized eggs may have been in direct contact with some of the polystyrene microparticles and that is why the authors are unable to say that the reduced hatching success of larvae exposed to microplastic particles are solely due to a chemical effect."
In other words, the reduced hatching results may be due to nothing more than the eggs being buried by the settling particles?

Would the results be different with polyethylene and polypropylene, both of which are far more common in oceans (having been produced in much larger quantities and more often as single-use articles) and both of which have an specific gravity less than one. Flotation would eliminate the burying-the-eggs-alive problem and any leachates from the olefin plastics would be quite different chemically than that of the polystyrene.

And why not run a set of experiments with unfiltered ocean water - a real control (adding more plastic as needed). Then there would be some idea of how well these results mimic reality.

Sorry, but I'm just not impressed much with this report. The authors took a rather complicated system and threw out so much of it that there is likely little chance that the results will ever translate back into the real world. Reducing a complicated system is often a good idea, but you have to take care to not overly reduce it. Unfortunately, that is what happened here.

Previous Years

June 15 - 2015 - Is a Vegan Tesla even Possible?

June 15, 2011 - Sustainability

June 15, 2007 - PVA - Err, is that alcohol or acetate?

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