>Usually these bioengineered yeasts need pretty specific growth conditions in order to eat

Until, say, they mutate independently. Now, I have no idea how likely that is, but I think the fear is not baseless. There's a fun SF book, "Directive 51", which explores this a bit. Not a bad book to cuddle up with.

Good thing microorganisms are famous for their slow rate of change in dynamic environments…

Of course, the lysine contingency.

> bioengineered yeasts need pretty specific growth conditions

In a natural environment this will not hold. Organisms are unstable; they change all the time, simpler ones even more so. Mutants with a faster growth rate will, up to a certain maximum, take over from slower growing or growth-deficient mutants. There isn't really a compelling growth advantage for plastic-degrading organisms, which is one reason why plastic stays so long in the environment.

All pre-processing steps further add to the cost, so we will year after year hear about it - and then wonder why nothing happens and plastics keep compounding in the ocean. And this is not new either: https://en.wikipedia.org/wiki/Diamond_v._Chakrabarty

Yeah, that was for plot reasons...

for all the reasons in the book/movie/television show 'The Andromeda Strain'.

Mutant 59: The Plastic Eaters, a 1971 British sci-fi environmental disaster novel written by medical scientist Kit Pedler and television screenwriter Gerry Davis.

No wikipedia page, but goodreads has it here:

https://www.goodreads.com/book/show/99852816-mutant-59

Not plastic, but 'Cats Cradle' by Kurt Vonnegut has something kind of like that.

I've heard that the prevailing theory now is that it wasn't the lack of fungus that made coal. Instead, the earth was covered in tropical wetlands, and trees couldn't decay in the anoxic water.