What is Black Algae?

Black algae are aquatic photosynthetic bacteria called cyanobacteria. They are single-celled but grow in large colonies. They contain chlorophyll but they also contain compounds called phycobilins, which mask the green color of chlorophyll. There are two types of phycobilins: (1) phycocyanin, a blue pigment, which gives the cyanobacteria their name and (2) phycoerythrin, a red pigment, which exists in red or pink algae (often found around sinks or drains). Cyanobacteria are very important organisms that assist in the growth of many types of plants. They are one of very few organisms that can convert inert atmospheric nitrogen into a form that plants can use such as ammonia or nitrate.

These bacteria can grow somewhat protected from the surrounding environment. This is due to the formation of a “sticky layer” on the outside of the cell. The process of photosynthesis carried out by the bacteria depletes carbon dioxide in the surrounding water. A decrease in carbon dioxide concentration causes the precipitation of calcium carbonate in that area. So the calcium carbonate, along with any other sediment that may precipitate, become trapped within the sticky layer. The bacteria will then grow through and over the sediment continuing to photosynthesize and grow. This process will occur over and over again forming more layers and making the black algae more difficult to treat. Brushing is extremely important when treating black algae because that protective layer has to be broken in order for the sanitizer or algicide to come in contact with the cyanobacteria.