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What's in that gunk?

Remember the gunk in the fish tanks? Some of that's caused by diatoms, a range of largish phytoplankton species with elaborate silica walls, and some by Microcystis aeruginosa, a tiny species of cyanobacteria.

Diatoms are generally benign, helping to regulate nutrient levels and providing a food source (or even toothpaste, if you collected enough of them).

M. aeruginosa, on the other hand, is anything but benign: it tends to reproduce explosively, producing "blooms" (like the one at the right) that not only use up dissolved oxygen but also produce toxins, so that fish die from both asphixiation and poisoning.

These are the organisms that poor Nemo needs to count. Look at the pictures below to get an idea of how different they are: