Phosphate deprivation restricts bacterial degradation of the marine polysaccharide fucoidan

Brown algae and diatoms convert carbon dioxide into the polysaccharide fucoidan, which sequesters carbon in the ocean despite the prevalence of marine bacterial fucoidanase genes. Bacteria with fucoidanase genes also have high-affinity phosphate transporters, suggesting that phosphate could impact fucoidan degradation and subsequent carbon sequestration. Here, to test this hypothesis, we assembled a system consisting of a microalga that produces and a bacterium that degrades fucoidan. The fixation of carbon dioxide into fucoidan by the microalga Glossomastix sp. PLY432 occurred independent of the phosphate concentration. In contrast, the fucoidan-degrading Verrucomicrobiaceae bacterium 227 was inhibited by a lack of phosphate. Degradation of the structurally simpler polysaccharide laminarin was less affected by the phosphate concentration. Phosphate deprivation enabled the fixation of carbon dioxide in fucoidan and disabled its degradation. These conclusions suggest that phosphate deprivation could be a potential strategy to promote the fixation and sequestration of carbon dioxide as fucoidan.