Symbiosis in the Ocean Microbiome

Abstract

Symbiotic interactions are widespread in Earth’s ecosystems including the marine environment. “Living together” describes a spectrum of interactions ranging from predation and parasitism to the positive interactions of commensalism and mutualism most commonly associated with the term symbiosis. Many well-known symbioses in the marine environment involve associations between microbes and multicellular organisms such as corals but there are diverse microbe-microbe symbiotic interactions that have been described for decades if not centuries from microscopic observations. Microbe-microbe symbioses have been challenging to study in part because of their small size, our inability to establish and culture them in the laboratory, and the ineffectiveness or inappropriateness of the methods that have been used to study macroscopic species. However, technical advances in nucleic acid sequencing, bioinformatics, isotopic approaches, and imaging have begun to provide new insights into these diverse and abundant interactions. The application of culture-independent approaches has revealed that microbial interactions in the marine microbiome range from metabolite exchanges between free-living planktonic cells to epibiotic and intracellular endosymbiotic interactions that bridge the symbiosis – organelle transition. Here we provide a brief overview of symbiosis and then focus on two specific vignettes in the oceanic plankton—N₂-fixing and planktonic rhizarian symbioses—that illustrate how cutting-edge approaches and methodologies are providing new insights into the establishment and functioning of these associations.