I have been intrigued with metallic bacteria since the first time I mistook it for an oil slick. The way to tell is to poke the surface with a stick. If the surface breaks up into distinct pieces, it is not oil, but a thin deposit of metal particles secreted by bacteria. The bacteria are chemoautotrophs that live by oxidizing iron and manganese ions in water. The colors are caused by a process called thin film interference and are determined by the thickness of the film.
