Exploring the Impact of Microbial Manipulation on the Early Development of Kelp (Saccharina latissima) using an Ecological Core Microbiome Framework.

Kelp cultivation is a rapidly expanding economic sector, as many kelp species are valued for a wide range of commercial products. Microbes associated with kelp and other macroalgae are increasingly recognized for their critical role in nutrient exchange, chemical signaling, and defense against pathogens during host settlement, development, and beyond. Microbes that are found at high frequency on a host and specifically enriched on a host are referred to as core microbes. The core microbiome hypothesis suggests that bacteria that are consistently present in a host's microbiome are likely to have a disproportionate impact on host physiology. This makes these core bacteria an attractive target for manipulating microbial communities. In this chapter, we first surveyed wild S. latissima to identify core bacterial taxa that were frequent and enriched on S. latissima compared to the surrounding environment. We hypothesized that core bacteria would be equally associated with both wild and cultivated S. latissima because the consistent association would persist despite cultivation conditions that aim to remove microbes. Here, we showed that core bacteria found in wild kelp are nearly absent in cultivated juvenile sporophytes during their time at nurseries and initial outplant stages. We then measured the effect of bacteria on kelp development in co-culture experiments on S. latissima with and without bacterial inoculation in co-culture experiments over four different trials. These trials used bacterial isolates cultured from wild S. latissima, representing 37 genera, including three genera that part of the core identified from wild S. latissima. Our results revealed that bacterial inoculation can have both positive and negative effects on kelp development. There was a weak positive correlation between the frequency of the bacterial genus in the wild and the effect development in kelp culture.