Slimes, oil-like films, and rock coatings are often made by bacteria that are reacting to the natural presence of minerals, such as iron, manganese, copper, and sulfur, in the water. Bacteria live in wet areas, including on the water surface, in the water column, and in the lake sediment.
Some bacteria obtain energy and perform other life functions by transforming those naturally occurring minerals to different chemical forms. These bacteria are of no threat to human health and have been involved in the iron and manganese cycles for billions of years. Some bacteria are very useful because they remove harmful materials from water.
Bacteria produce different color films, coatings, and slimes. Bacteria that precipitate (settle out of water as a solid) copper minerals may make turquoise blue films. Green and purple bacterial slimes may appear when sulfur is present, while white slimes occur in the presence of aluminum, sulfur, or calcium minerals. Iron bacteria produce brown or reddish-brown deposits.