An experimental test of the influence of microbial manipulation on sugar kelp (

Kelp are valued for a wide range of commercial products and their role in kelp forest ecosystems, making kelp cultivation a rapidly expanding economic sector. Microbes associated with kelp and other macroalgae play a critical role in processes such as nutrient exchange, chemical signaling, and defense against pathogens. Thus, manipulating the microbiome to enhance macroalgal growth and resilience is a promising yet underexplored approach for sustainable kelp cultivation. The core microbiome hypothesis suggests that the bacteria that are consistently found on a host (the core microbes) are likely to have a disproportionate impact on host biology, making them an attractive target for microbiome manipulation. In this study, we surveyed wild IMPORTANCE: Microorganisms consistently associated with hosts are widely thought to be more likely to impact host biology and health. However, this intuitive concept has not been experimentally evaluated. This study formalizes this concept as the Core Microbiome Influences Host Function hypothesis and experimentally tests this hypothesis in sugar kelp (Saccharina). The distribution of bacteria on wild kelp and core microbes was first identified by compiling a broad dataset of the kelp microbiome sampled across space and time. Bacterial cultures were isolated from the surface of sugar kelp and individually grown in laboratory co-culture with sugar kelp spores to assess the ability of bacterial isolates to influence kelp growth and development. In support of the core influences host function hypothesis, isolates belonging to bacterial genera that are more strongly associated with wild sugar kelp are more likely to influence development in laboratory experiments.