Abstract Marine algae annually synthesize gigatons of glycans from carbon dioxide, exporting it within sinking particles into the deep-sea and underlying sea floor, unless those glycans are digested before by bacteria. Identifying algal glycans in the ocean remains challenging with the molecular resolution of conventional analytic techniques. Whether algal glycans are digested by heterotrophic bacteria during downward transport, before they can transfer carbon dioxide from the ocean surface into the deep-sea or the sea floor, remains unknown. In the Red Sea Shaban Deep, where at 1500 m water depth a brine basin acts as a natural sediment trap, we found its high salt and low oxygen concentration accumulated and preserved exported algal glycans for the past 2500 years. By using monoclonal antibodies specific for glycan structures, we detected fucose-containing sulfated polysaccharide, β-glucan, β-mannan and arabinogalactan glycans, synthesized by diatoms, coccolithophores, dinoflagellates and other algae living in the sunlit ocean. Their presence in deep-sea sediment demonstrates these algal glycans were not digested by bacteria. Instead they moved carbon dioxide from the surface ocean into the deep-sea, where it will be locked away from the atmosphere at least for the next 1000 years. Considering their global synthesis, quantity and stability against degradation during transport through the water column, algal glycans are agents for carbon sequestration. Significance statement Algae and plants use the greenhouse gas carbon dioxide to synthesize polymeric carbohydrates, or glycans, for energy storage, structural support and as protection against invasion by microbes. Glycans provide protection, are carbon sinks and enable carbon sequestration for as long as they are not digested by bacteria or other organisms, which releases the carbon dioxide back in to the atmosphere. In this study, we show that non-digested algal glycans sink into the deep ocean and into marine sediment. Thus, glycans are more than food for animals and prebiotics for bacteria, they are also molecules that remove carbon dioxide from the atmosphere and transfer it to the deep-sea, where it can be stored for 1000 years and longer.

Not digested: algal glycans move carbon dioxide into the deep-sea