Foundational kelp species reveal links between host traits, the environment and the associated microbial community

Canopy kelp are foundational species in coastal ecosystems and host diverse bacterial communities. Here, we test the association between bull kelp (Nereocystis luetkeana) host traits, blade-associated bacterial taxa and seawater environmental features across nine sites spanning more than 200 km in Washington state. Traits related to kelp fitness, environmental features and microbial community structure differed geographically. Kelp carbon fixation and tissue nitrogen content were greater at outer coast locales, compared with more inland locales in central and south Puget Sound. Geographic differences in carbon fixation rates, tissue nitrogen and bulb diameter were positively correlated with seawater nutrients and negatively correlated with sea surface temperature. Bacterial taxa showed differentiation among sites, and blade-associated bacterial densities were higher at the outer coast site compared with the most inland site. Yet, 11 bacterial genera were present in at least 80% of the samples; these taxa probably serve as core members of the N. luetkeana microbiome and show both positive and negative correlations with host health and environmental features. We show that there are strong interrelationships between kelp traits, seawater features and bacterial community composition with implications for the health of this highly productive foundational species in coastal ecosystems.