Very similar to the Xbox 360 indie game "Solar" except on the microscopic scale. You play as an organism trying to eat other smaller organisms to gain in size. Approach something larger than yourself and you get absorbed instead. Eventually you encounter smarter organisms with their own unique defenses.

Steep difficulty curve after the intro missions, and a lot has to do with luck of the randomly generated maps (which can be re-randomized with a button press). Eventually you will probably rage quit like I almost did and swear the game off forever... That is until you get the itch to try again until you want to throw your computer out the window.

If you can get it cheap, it comes recommended.