Symbiosis, Chemical Biology of

Abstract The term “symbiosis” was defined by Anton de Bary in his monograph “Die Erscheinung der Symbios” as “the living together of unlike organisms” (1). His studies were based on the formation of lichens, which are the result of an association between a fungus and an alga or cyanobacterium. The definition was coined in the end of the nineteenth century but is regarded by most symbiosis researchers as largely valid today. Accordingly, any specific association between two or more species can be classified as symbiosis. It should be noted that many scientists use symbiosis in a more restricted way to denote a mutually beneficial relationship. This article will give an overview of various biologic manifestations of symbiosis and discuss selected examples, where primary or secondary metabolites play a crucial role in the association. If the partners in a symbiosis differ in size, the larger member is termed “host” and the smaller member is termed “symbiont” or “symbiote.” The more common term “symbiont” will be used here. One general way to distinguish between various forms of symbiosis is to identify the location of the attachment of the symbiont to the host. Symbionts that live on the host surface, including internal surfaces like the digestive tube, participate in ectosymbiosis (Greek: ɛκτoς = outside). If a symbiont is localized within the tissues of the host, the association is termed “endosymbiosis” (Greek: ɛνδoν = within). Endosymbionts can be found either in the extracellular space or intracellularly.