Multidisciplinary

Secondary metabolites are ubiquitous in bacteria, but by definition, they are thought to be nonessential. Highly toxic secondary metabolites such as patellazoles have been isolated from marine tunicates, where their exceptional potency and abundance implies a role in chemical defense, but their biological source is unknown. Here, we describe the association of the tunicate Lissoclinum patella with a symbiotic α-proteobacterium, Candidatus Endolissoclinum faulkneri, and present chemical and biological evidence that the bacterium synthesizes patellazoles. We sequenced and assembled the complete Ca . E. faulkneri genome, directly from metagenomic DNA obtained from the tunicate, where it accounted for 0.6% of sequence data. We show that the large patellazoles biosynthetic pathway is maintained, whereas the remainder of the genome is undergoing extensive streamlining to eliminate unneeded genes. The preservation of this pathway in streamlined bacteria demonstrates that secondary metabolism is an essential component of the symbiotic interaction.

Fixing on a Marine Partnership Nitrogen fixation by microorganisms determines the productivity of the biosphere. Although plants photosynthesize by virtue of the ancient incorporation of cyanobacteria to form chloroplasts, no equivalent endosymbiotic event has occurred for nitrogen fixation. Nevertheless, in terrestrial environments, nitrogen-fixing symbioses between bacteria and plants, for example, are common. Thompson et al. (p. 1546 ) noticed that the ubiquitous marine cyanobacterium UCYN-A has an unusually streamlined genome lacking components of the photosynthetic machinery and central carbon metabolism—all suggestive of being an obligate symbiont. By using gentle filtration methods for raw seawater, a tiny eukaryote partner for UCYN-A of less than 3-µm in diameter was discovered. The bacterium sits on the cell wall of this calcifying picoeukaryote, donating fixed nitrogen and receiving fixed carbon in return.