The cover depicts Gloeotrichia echinulata, a large colonial diazotrophic cyanobacterium, under an epifluorescent microscope which emits ultraviolet light. Using ultraviolet light, the filaments glow red from chlorophyll, while other pigments give various hues of purple, which may be a hint about the health of the cells (image created by the US Geological Survey and retrieved via Flickr; the image was modified by MIC). The cover is published under the Creative Commons Attribution (CC BY) license.