Also present are gliding "Beggiatoa" filaments (1–4 mm wide) that are non-pigmented but contain highly refractive intracellular granules of elemental sulfur. Numerous gram-negative bacteria (small rods) are also present but not identifiable using light microscopy. The bacterial population has been characterized using molecular techniques and was found to contain over 500 species of bacteria that are different from bacterial communities found in the water column, healthy coral tissue, or dead coral skeleton. The functional role of this diverse population of bacteria is not known. Black band disease affects 42 species of coral in a worldwide distribution. The only known reservoir is within cyanobacterial biofilms that are present on sediments in depressions of healthy black band disease susceptible corals. Black band disease significantly affects boulder star coral in the reef ecosystem, allowing for more staghorn coral to grow.