(mer-is-mo-pea-dee-a) is a blue-green alga or cyanobacterium. The genus is distinguished by the square-packed pattern of the coccoid cells. It is common in sediments, but the number of cells which occur in a colony may vary from only a few to thousands. As a cyanobacterium, the photosynthetic pigments are located throughout the cytoplasm. Differential interference contrast.