Can anyone with a better understanding of the chemistry help me understand the level of plastic “degradation” this represents? My (low) understanding is that we know of several processes that partially break plastic down, but nothing that actually metabolizes it or turns it into useful / non-toxic material.

Is this a breakthrough, or just another example of partial breakdown without truly consuming the plastic?

Plastic is a polymer, like cellulose. It's well known that microbes have evolved some enzymes to breakdown biopolymers - cellulase, which is present in rumen fluid. It is also known that pseudomonas has enzymes that are capable of breaking down hydrocarbon chains in oil - since that is the precursor for plastic polymers it should be no surprise that there are pseudomonas strains capable of breaking down plastic polymers. This paper claims that there is such a strain that is dominant in this rumen fluid that does so through a hydrolysis reaction (likely using an enzyme produced in that strain as a catalyst). Hydrolysis cleaves terephthalic acid in this process - a monomer that contains an aromatic ring. So essentially the final size of the polymers depends on the placement of that unit in that particular polymer. You need several enzymes working in sequence to degrade a complex polymer fully, but it is possible.

https://en.wikipedia.org/wiki/Terephthalic_acid

Op question made me think of a few others.

Why is a molecules with high energy density so difficult to break down?

Why are there so few natural decomposers of plastics?

Is this because bacteria haven't evolved the correct enzymes, and many other non plastics globally have bacteria consumers that we can just assume that they decompose naturally?

On the other hand (having no idea about the subject) I can imagine that the fact that plastics can't be broken down so easily is part of the reason why we find it so useful, for example to protect foods.

"bacteria haven't evolved the correct enzymes"

Yet. The results might not be that great for humanity either. Especially if the bacteria get, shall we say, overly eager and efficient?... and plentiful.

That said, there's plenty of obscure bacteria having a decent stab at the problem and having a good time. Humans are helping them along as well.

Like all decently powerful tools there's a possibility for harm and hope.

I am inclined to prefer the scenario where plastic is more readily decomposed, rather the situation where we are in now, where plastic is entering the food chain and organisms, including ourselves.

Other than climate change, I think plastic pollution is the issue of our time.

This is a fiction book about the aftermath of an oil spill, developing enzymes to digest the hydrocarbons and how it all then gets a bit out of control..

https://www.goodreads.com/book/show/86452.Ill_Wind

Sounds like on of Neal Stephenson's books: https://www.nealstephenson.com/zodiac.html

Reminds me of Doomwatch:The Plastic Eaters, from 1970.

See https://en.wikipedia.org/wiki/Doomwatch

OR more dramatically, good ole ICE-9 from Cat's Cradle

> Why are there so few natural decomposers of plastics?

Because plastics weren't naturally occurring. Give it time.

Maybe a lot of time. The main reason we have cheap coal is from nature failing to find a solution to break down lignin during the Carboniferous period (and really particularly during the Carboniferous rainforest collapse) hundreds of millions of years ago.

Sometimes nature is very slow. Like geological timescales.

Lignin can be broken down in aerobic conditions by peroxidase and a series of other fungal enzymes, it only forms those energy dense forms under anaerobic conditions and probably requires high pressures. The reason it is so difficult and only breaks down through oxidation is because it is quite complex, unlike the homogenous polymer chains of plastic that have many repeating monomers.

https://www.researchgate.net/figure/Chemical-structure-of-li...

>Why is a molecules with high energy density so difficult to break down?

Generally, even if a given reaction is energetically favourable, if the activation energy or enabling conditions for the reaction to occur are high/rare respectively, then the rate of the energetically favourable reaction will be slow.

It's like water in a cup; sure it would prefer to be on the ground, but absent something tipping the cup over, it'll stay inside the current local energy minima without any complaints.

There are many forms of plastic made from many different chemicals. Fortunately some of the most common ones like polyethylene are chemically harmless to life. Some plastics like PVC and some synthetic rubbers may contain hazardous additives but most plastics are quite harmless.

It seems to me that with humans making so much of these 'harmless' (or at least relatively-low-toxicity) plastics, we have essentially created a giant niche waiting there for the first micro (or even perhaps macro) organism(s) that can live off this stuff, they are srsly going to have a massive field day once their day roles around!

It could of course be fairly troubling when pieces of our various machines/buildings/etc start to decompose earlier than expected! This assumes quite some luck in terms of mutation to be fair, and maybe humans might also help with speeding that up too? could be good!?

Why don't people worry about sand? Nothing eats it. It can mechanically irritate animals (see pearls). How is polyethylene different? The reddit driven hysteria about plastics in general is vastly out of purportion to the actual harm done by plastics.

I remember news stories in India of cows dying because of ingesting too much plastic.

https://wewantrefill.com/cows-india-die-consuming-plastic-wa...

I can see it now, Big Meat will start feeding cows plastic because it's cheaper

Not just cheaper, but also better for the environment.

/s

malk, now with vitamin BPA!

Putting the xenoestrogens in estrogen.

Would it not be more likely to find microbes that can break down plastic on plastic itself?

(Why are we looking in cow's stomachs for "eco-sustainable plastic degradation and recycling processes" I guess is my question?)

Cow stomachs are a good place to find digestive microbes. Nice temperature, humidity, ph and what not.

We did! The other well known bacteria that can break down plastic were found in a plastic waste facility in Japan.

I think it's pretty clearly just a case of more is better

This reminds me of that tale/urban legend/reality when, once upon a time, some cows were eating the old Trabant cars (that had an outer body made od some type of plastic).

Pretty sure that was pigs.

I remember seeing that scene in some Serbian movie.

Edit: It's a scene from Black cat, white cat by Emir Kusturica.

The Trabant bodywork was made of Duroplast[0], a thermosetting resin plastic reinforced with cotton waste from the Soviet Union.

[0] https://en.wikipedia.org/wiki/Duroplast

Given you can (sort of?) make plastic from milk I'm not surprised.

Cows are amazing!