The microbiome of the canopy-forming kelps, Nereocystis and Macrocystis, from the outer Olympic Coast to the Puget Sound

Canopy-forming kelps create vast underwater forests that are among the most productive marine ecosystems. In addition to providing vital habitat for macroscopic organisms, kelps also host an abundant microbial community in their surface mucus layer. In the Salish Sea, two canopy-forming kelps with contrasting life histories co-occur; Macrocystis pyrifera, a perennial species, and Nereocystis luetkeana, an annual species. Kelp-associated microbial communities were sampled along a spatial gradient, including sites from the outer Olympic Coast, Strait of Juan de Fuca, and Puget Sound. We characterized the microbial communities associated with each kelp species and the surrounding seawater using next-generation Illumina sequencing of 16S rRNA genes. Our preliminary results show that canopy-forming kelps host complex microbial symbiont communities that are host-specific and distinct from free-living bacteria in the surrounding seawater. Based on fluorescence microscopy data, up to 25 million bacteria live on just one square centimeter of N. luetkeana tissue. In addition to examining spatial variation in the kelp microbiome, we examined the impact of N. luetkeana and M. pyrifera kelp forests on the surrounding seawater chemistry. We used an enriched carbon stable isotope tracer experiment (13C-bicarbonate) to demonstrate simultaneous carbon fixation and dissolved organic carbon (DOC) production by N. luetkeana from Tatoosh Island, WA. During the daytime, carbon fixed by N. luetkeana is leaked into the surrounding seawater at a rate of 10 μmol DOC per hour per gram of dry mass. This carbon presents an abundant food resource for heterotrophic microbes in the surrounding seawater. Given the immense surface area of kelp forests, the kelp microbiome has potentially far-reaching impacts on kelp forest productivity and coastal nutrient cycling.