| Summary: | Most eukaryotic organisms live in intimate association with diverse communities of Bacteria, and sometimes Archaea, which can have a profound influence on the ecology and evolution of their eukaryotic hosts. Microbial symbionts can expand the metabolic capabilities and functional traits associated with their host, with cascading impacts to nutrient cycles in the surrounding ecosystem. Kelp, brown macroalgae in the order Laminariales, play an important role in the global carbon cycle by creating highly productive underwater forests in temperate coastal marine ecosystems. Photosynthetic kelp blades harbor abundant microbial communities, providing a large surface area that is rich in organic carbon resources, yet we know little about the diversity, community structure, and functional role of the kelp microbiome. In four chapters, this dissertation research 1) characterized the microbiome of the bull kelp, Nereocystis luetkeana, and the giant kelp, Macrocystis pyrifera, through field surveys across spatial and temporal scales, 2) examined microbial community assembly on N. luetkeana blade surfaces through in situ experiments, 3) quantified carbon fixation and dissolved organic carbon (DOC) release by the canopy-forming kelps N. luetkeana and M. pyrifera, and 4) determined the functional role of Bacteria associated with N. luetkeana by reconstructing bacterial genomes from metagenomes. The kelp microbiome was specific to each host kelp species, distinct from microbial communities in the surrounding seawater, and displayed significantly geographic and temporal variation. Quantifying carbon cycling by canopy-forming kelps demonstrated that kelps release ~16% of their total fixed carbon into the surrounding seawater, providing a consistent metabolic resource to heterotrophic bacteria. The most abundant bacterial genus on kelp blades, Granulosicoccus, was ubiquitous across spatial and temporal scales and became abundant on new meristematic kelp blade tissues within days. Genomes of Granulosicoccus sp. revealed that these bacteria have the ability to assimilate dissolved organic carbon resources, synthesize cobalamin (vitamin B12), and reduce oxidized nitrogen sources to ammonium, which may provide the host kelp with reduced nitrogen and vitamins. Overall, this dissertation research expanded our knowledge of the factors structuring the kelp microbiome, examined microbial settlement on kelp tissues, quantified carbon cycling in kelp forests, and provided insight into the metabolic functions of key bacterial taxa in the kelp microbiome that persisted across spatial and temporal scales.
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