Sulfated mannan helps diatoms domesticate their microbiome

Background Diatoms are a keystone phylum in Earth’s ecosystems, responsible for substantial oxygen production and carbohydrate fixation that fuel global food webs. They host diverse microbiomes, yet how diatoms preferentially recruit bacteria with complementary metabolic traits remains unknown. Results We discovered that diatoms exude a C6-sulfated α-1,3-mannan that serves as a selective carbon source for adapted bacteria. Its structure was resolved using NMR spectroscopy, chromatography, chemical synthesis, and enzymatic dissection. Biochemical, physiological, and structural analyses demonstrated that specialized Bacteroidota employ a four-enzyme pathway to metabolize this glycan. Metagenomic and transcriptomic data revealed that sulfated mannan utilization loci are globally abundant and actively expressed in surface ocean bacterioplankton. Because this mannan provides only carbon, oxygen, sulfur, and hydrogen, bacteria must obtain other essential elements elsewhere, reinforcing metabolic interdependence. Conclusions We propose that diatoms use sulfated mannans to attract beneficial partners and exclude competitors, thereby engineering a microbiome that enhances their productivity and underpins marine carbon cycling.