Maturing giant kelp develop depth-specific microbiomes

Giant kelp (Macrocystis pyrifera) is a photosynthetic macroalga that produces dissolved organic carbon (DOC), essential for marine bacteria and food webs. The bacterial communities residing on giant kelp blades consume and compete for complex carbohydrates, contributing to the microbiome community structure. In this study, we investigate how the microbiome changes in response to the age and depth of giant kelp blades and assess how these changes relate to differences in the hosts photophysiology. We find that the microbial community increases in richness and evenness as kelp blades age. While the microbiomes of juvenile blades are stochastic, communities on mature blades coalesce into less variable, depth-specific community types. Differentially abundant genera in mature microbiomes include members of Bacteroidia and Gammaproteobacteria, known for carbohydrate degradation, and Planctomycetes, which often produce protective secondary metabolites. These shifts in microbiome communities are associated with increased maximum quantum yield of photosystem II of mature blades; therefore, they may be linked to enhanced DOC exudation. By shedding light on these dynamics, our study contributes to a better understanding of the complex interplay between macroalgae, their respective microbiomes, and the surrounding marine environment.