Microorganisms are probably far less affected by vacuum than larger ones, as long as they can get some nutrients from time to time. And since most engines used in space produce plain water (and engines are fired frequently around the ISS), I think the only really difficult thing about the environment is radiation... and microorganisms might also be more resistant to this.

Tardigrades can endure some of the most extreme conditions, including the vacuum of space and solar radiation, and can go without food or water for more than 10 years: http://en.wikipedia.org/wiki/Tardigrade

Not the same as plankton, but still quite interesting, I think!

It's almost vacuum up there, so the water from the engines is diluted and diluted and the microorganism can't get enough of it. Probably they are only not-dying there, like inside a big freezer without air, only waiting frozen. It's not clear than they are viable, and that if we put them in a good environment they will thrive. Perhaps they are only microcorpses.