Nutritional interdependencies Bacteria and archaea show a wide range of nutritional specialism. Not every organism can synthesize essential components and may need to trade for them. Taking as an example a diverse and interesting family of enzyme cofactors—the cobalt-containing cobamides, which include vitamin B 12 —Sokolovskaya et al. reviewed the interdependencies among microorganisms for this suite of nutrients. Cobamides are required for many processes, from catabolism of carbon sources to nucleotide biosynthesis, and are needed by a majority of microbes, from those in the gut to those in the oceans. Availability of cobamides is patchy and habitat specific, and nonspecific scavenging may not be adequate to obtain a specific cobamide structure required by an organism. Therefore, a variety of mutualisms have evolved to deliver and import specific structural variants of cobamides between organisms or among consortia of eukaryotes and prokaryotes by an equal variety of subtle and distinct mechanisms. Science , this issue p. 48