Algae synthesise structurally complex glycans to build a protective barrier, the extracellular matrix. One function of matrix glycans is to slow down microorganisms that try to enzymatically enter living algae and degrade and convert their organic carbon back to carbon dioxide. We propose that matrix glycans lock up carbon in the ocean by controlling degradation of organic carbon by bacteria and other microbes not only while algae are alive, but also after death. Data revised in this review shows accumulation of algal glycans in the ocean underscoring the challenge bacteria and other microbes face to breach the glycan barrier with carbohydrate active enzymes. Briefly we also update on methods required to certify the uncertain magnitude and unknown molecular causes of glycan-controlled carbon sequestration in a changing ocean.

Algal glycans

Marine carbon cycle

Polysaccharides

Carbohydrates

Microalgae

Phytoplankton

Fucoidan

fucose-containing sulphated polysaccharide

high molecular weight dissolved organic carbon

polysaccharide utilisation locus

carbohydrate-active enzyme

© 2022 The Author(s). Published by Elsevier Ltd.