Core bacterial inoculation in sugar kelp cultivation: Examining bacterial colonization and its effect on microbiome in a co-culture system

The kelp microbiome is variable in space and time, but there is a small set of core bacterial taxa that are consistently associated with kelp. Core bacteria are an attractive target for manipulation; as stated by the core microbiome hypothesis, bacteria that are consistently present are more likely to have a functional impact. Bacteria associated with macroalgae enhance growth, influence development, and protect against disease. In kelp, I showed in Chapter 3 that bacteria influence development rates and the core are more likely to positively influence. It is unclear whether core bacteria are consistently found on kelp because they are vertically inherited or if they repeatedly colonize the kelp at a later time point. This remains an open question due to two key facts: firstly, while the core bacteria are consistently associated with kelp, they are not always present, indicating some variability; and secondly, these core bacteria are not observed during the nursery or juvenile stages of cultivated kelp. Here, I surveyed juvenile stages of sugar kelp and found that core bacterial taxa were almost absent on the juvenile sporophytes in their natural habitat. I experimentally tested whether core bacteria are more likely to colonize kelp than other bacteria at two stages of development. I found that core bacteria effectively colonize sugar kelp in a deterministic manner, regardless of the timing of their inoculation. These results suggest that the core taxa are acquired repeatedly from the environment at a later time point. These findings suggest that targeting naturally prevalent core bacterial taxa for probiotic development could offer advantages, as deterministic colonization will make it easier to manipulate the microbiome.