Cover photograph (Copyright © 2006, American Society for Microbiology. All Rights Reserved.): Autofluorescence micrograph of the red-pigmented filamentous cyanobacterium Planktothrix rubescens (strain 110), which occurs in deep and physically stratified freshwater lakes of the temperate region of the Northern Hemisphere. The efficient light conversion in the green part of the light spectrum is caused by phycobiliproteins (phycocyanin and phycoerythrin), resulting in the ability of Planktothrix to live under low light intensities in deep water layers. Naturally occurring Planktothrix strains show numerous genotypes containing transposons that lead to mutations and inactivate the biosynthesis of the nonribosomal peptide microcystin, which has been shown to be toxic to various aquatic biota. Magnification, 200×. Image created by Rainer Kurmayer. (See related article on page 117.)