Some oil-like films, coatings, and slimes, although they may look bad, are natural phenomena. These phenomena can be caused by bacteria such as Thiobacillus ferrooxidans (Iron Bacteria).
Slimes, oil-like films, and rock coatings are often made by bacteria that are reacting to the presence of minerals in the water. Bacteria live in wet areas, including: on the water surface, in the water column, and in the sediment. These bacteria are of no threat to human health and have been involved in the iron and manganese cycles for millions of years. Some bacteria are very useful because they remove harmful materials from water.
Bacteria create oil-like films when they attach themselves to the water surface. Sunlight bounces off the films, giving them an oily appearance. To test the difference between a bacterial film and oil floating on the water, break the film. If the film stays broken, it is a natural bacterial film. If it flows back into place, it is an immiscible hydrocarbon based substance, which indicates pollution.
Bacteria that precipitate copper minerals may make turquoise blue films. Green and purple bacterial slimes may appear when sulphur is present, while white slimes occur in the presence of aluminium, sulphur, or calcium minerals.
