Today I found Cochlodinium polykrikoides - a marine dinoflagellate. It travels generally as a chain (2, 4 or 8 individuals). Every alga is about 30-40 µm long.
It is beautiful because it has a very vivid jellow color.
It is harmful because it causes sometimes fish kills and it is very poisonous.
To my surprise it is also very useful, because it develops an antiviral activity wich is used in the treatment of HIV (see Google).
Images made through a microscope. All subject types.