Versatile bacterial symbionts of shipworms contribute to wood digestion, fix nitrogen and produce secondary metabolites

Shipworms, bivalve mollusks that burrow into and consume wood, host intracellular gamma proteobacterial symbionts in bacteriocytes located in the host gill. The bacteria produce cellulolytic enzymes and fix nitrogen, and were proposed to support the host's ability to survive on a food source that is resistant to digestion and poor in nitrogen. Symbiont genomes reveal a surprising number of putative secondary metabolite biosynthesis pathways, leading us to propose that these symbionts may also contribute chemically to the symbiosis. We observed that the wood digestion organ portion of the gut is nearly devoid of microbes, allowing the host to consume the sugars resulting from wood digestion without competition. We demonstrated that cellulolytic enzymes produced by the symbionts in the gill are transported to the wood digestion organ, and we hypothesize that secondary metabolites also follow this route and suppress competing environmental microbes in the wood digestion organ. The candidate compounds found include a new family of catecholate siderophores, the turnerbactins, and tartrolon E, both of which are not only produced by the symbionts in culture, but can be detected by mass spectrometry in extracts of the intact association, suggesting that they may play a vital role in the symbiosis.