What is didymo?

Didymo is a member of the group of single-celled aquatic plants (freshwater algae) known as diatoms. Although it is microscopic, didymo can form dense colonies called algal blooms which can be seen with the naked eye. Diatoms are unique in that their cell walls contain silica (a major element in sand), which is why didymo feels gritty when touched. Didymo also contains chlorophyll, a pigment which enables it to make its own food by using energy from the sun. Under optimum growing conditions, didymo cells ooze large amounts of a mucus-like substance (mucilage) which attaches didymo firmly to underwater surfaces. Young colonies look like raised pimples on the surfaces of river rocks, but as the mucilage elongates to form stalks, the colonies form impenetrable mats which form thick strands and can cover all surfaces, including other plants, logs and debris.