Polyelectrolyte mannan from diatoms reshapes sunlit ocean microbiome

Algae catalyze half of the global solar powered reduction of carbon dioxide to organic molecules. While this organic synthesis shapes genomes of heterotrophic microbiota globally, many corresponding structures remain elusive. We show that diatoms release a mannan polyelectrolyte selecting for bacteria with a specifically adapted enzymatic cascade. NMR spectroscopy, chromatography, automated glycan assembly and enzymatic dissection resolved the structure of a C6 sulfated α-1,3-mannan with exceptional simplicity for a sulfated polysaccharide from algae. Biochemical, physiological and structural analyses demonstrated that some bacteria can hydrolyze this mannan with a cascade of just four enzymes. Circumnavigation metagenome data revealed corresponding genes in Atlantic, Pacific, Southern and Indian Ocean surface waters. We show that this sulfated mannan may exert substantial selection pressure on marine microbiota around the globe.