Synergistic degradation of fucoidans in the ocean.

Fucoidans, a class of complex polysaccharides produced by brown algae and diatoms, contribute to long-term carbon sequestration owing to their resistance to microbial degradation1,2. Although individual microorganisms can break down portions of these polysaccharides3-5, it remains unclear whether complete breakdown is possible in nature and, if so, by what mechanisms. Here we show that fucoidans are degraded through synergistic interactions between specialized bacteria with complementary metabolic functions. Using metabolomic analysis of a reconstructed marine consortium, we uncovered metabolic guilds of bacteria that preferentially degrade either the sulfated fucose backbone or the side branches of rare monomers. This functional division of labour leads to an unexpectedly high number of synergistic interactions between different degraders that enhanced degradation efficiency up to 97.1%. Despite varying fucoidan structures across different types of algae6, the metabolic functions of degraders remained conserved, enabling quantitative prediction of degradation outcomes based on community and substrate composition. The frequent co-occurrence of functionally complementary fucoidan degraders in ocean metagenomes suggests that synergistic degradation is a globally relevant strategy. Our findings suggest that the environmental turnover of complex biopolymers depends not only on individual metabolic capabilities of degraders but also on ecological interactions shaped by substrate architecture. This work provides a mechanistic framework for understanding carbon cycling in the ocean and for engineering synthetic microbial consortia to degrade recalcitrant polysaccharides.