Macrocystis pyrifera bacterial diversity

Bacteria are integral to marine carbon cycling. They transfer organic carbon to higher trophic levels and remineralise it into inorganic forms. Kelp forests are highly productive ecosystems within the coastal ocean, yet the role of bacteria in transforming kelp carbon is poorly understood. We surveyed the taxonomic composition and functional capacity for polysaccharide degradation of cultured and uncultured bacterial communities associated with Macrocystis pyrifera, a foundational species in Pacific kelp forests. The majority of cultured bacteria (66%) could degrade and metabolise alginate, the primary polysaccharide in kelp tissue. This activity was not phylogenetically restricted, but instead found across diverse genera in the Bacteroidetes and Proteobacteria. Further, alginate activity was not phylogenetically conserved, and varied between isolates - even those with identical 16S rRNA sequences. Thus, taxonomy is unreliable for inferring functional ability. Finally, community functional profiles derived from shotgun metagenomic sequencing contained several alginate enzyme families and these were consistently represented within taxonomically diverse communities across Macrocystis and the water column. The wide distribution and poor phylogenetic signal of alginate activity observed from cultured taxa, coupled with the presence of alginate metabolic pathways within the broader bacterial communities, suggests that alginate activity is likely widespread amongst bacteria in kelp forests.