Ecological and Evolutionary Drivers of Morphology and Alginate Content in Giant Kelp (Macrocystis pyrifera)

The giant kelp, Macrocystis pyrifera, exists as one species with distinct morphological variants—or “ecomorphs”—in different populations, yet the mechanism for this variation is unclear. One ecomorph (“pyrifera”) has a conical, mound-shaped base while the other (“integrifolia”) has a flattened, elongated base that forms denser kelp beds. My research on the drivers of these distinct phenotypes can inform decisions about management and restoration of kelp populations, as well as selective breeding for certain traits in aquaculture. First, I assessed the patterns of giant kelp ecomorph spatial distribution and genetic differentiation across its Western Hemisphere range. I collected samples from 18 kelp populations in Chile and California and used whole-genome sequencing to evaluate the degree of genetic divergence among populations. My results demonstrate that “pyrifera” and “integrifolia” are genetically distinguishable, yet divergence patterns differ between Northern and Southern Hemispheres. Next, I empirically tested the effect of local environment on giant kelp morphology. I collected reproductive tissue from adult individuals of both ecomorphs naturally occurring at the same location, cultured the spores in the laboratory, and outplanted the resulting baby kelp to the seafloor in a common ocean environment. Weekly monitoring revealed distinguishing morphological features at multiple developmental stages, ultimately culminating in morphologically distinct adults which also differed in reproductive timing. By including a mixed treatment with equal numbers of spores from each ecomorph, I observed the morphologies that emerge in a competitive context. This study also revealed for the first time how the characteristic “integrifolia” morphology forms in nature. Finally, I investigated ecomorph variation in production of alginate, one of the most commercially important chemicals from kelp. I compared alginate yield and composition of “pyrifera” and “integrifolia” populations across varied environments using kelp samples from 15 populations (a subset of those used in the genetic differentiation study). Alginate yield and composition were significantly different between the two morphs and were also related to the depth at which the kelp grew. Collectively, this work reveals a genetic basis to giant kelp morphology as well as eco-physiological divergence between the two ecomorphs, giving reason to consider them as distinct species