Cover photograph (Copyright © 2006, American Society for Microbiology. All Rights Reserved.): Light micrograph of photosynthetic organisms from the photic zone of the hypersaline microbial mat of Guerrero Negro, Baja California Sur, Mexico. Molecular studies of this mat show it to harbor 42 phyla of bacteria, including 15 novel candidate divisions, organized spatially according to gradients of oxygen and sulfide. (Photo courtesy of Lee Bebout.) (See related article on page 3685.)