Microbial mats are multi-layered sheets of microorganisms, composed mainly of bacteria and archaea. The photograph shows patches of encrusting red and green algae lying beneath filamentous bacterial mats on rock surfaces (at East Diamante volcano, Mariana Islands, USA, 190 m/623 ft depth). The small white corals on the rocks are about 5 cm/2 in long. Thus, here the chemosynthetic and photosynthetic communities overlap (image by Submarine Ring of Fire 2004 Exploration, NOAA Vents Program); image modified by MIC. The cover is published under the Creative Commons Attribution (CC BY) license.