Fucoidan is the second most abundant polysaccharide in brown macroalgae, contributing up to 50% of the dissolved organic matter released daily from these algae into the ocean, substantially influencing the marine carbon cycle. Due to its complex chemical structure, fucoidan is highly resistant to microbial degradation and accumulates on the surface of macroalgae, where it contributes to regulating their microbiome. Here, we used a range of microfluidic assays to quantify the chemotactic and physiological responses of marine bacteria to fucoidan. Our results reveal that all the strains tested (14 out of 14) are attracted to fucoidan produced by two different species of brown macroalgae. Attraction occurred only at high fucoidan concentrations (0.1 to 1 mg/mL), such as those expected in the immediate vicinity of algal surfaces, whereas low concentrations elicited no response or modest chemorepulsion. Although fucoidan did not support bacterial growth under our experimental conditions, it did not impair the utilization of other carbon sources. Together, these findings suggest that fucoidan gradients can act as chemical cues guiding marine bacteria toward the nutrient-rich microenvironment of macroalgal surfaces, pointing to a potential ecological role of fucoidan in shaping macroalgal microbiota, bacterial behavior, and ultimately carbon cycling in the ocean. ### Competing Interest Statement The authors have declared no competing interest. Swiss National Science Foundation, https://ror.org/00yjd3n13 Simons Foundation, https://ror.org/01cmst727

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Microbes with complementary functions work together synergistically to degrade fucoidan molecules.

Microbes with complementary functions work together synergistically to degrade fucoidan molecules.

New research reveals how communities of marine bacteria divide the task of degrading fucoidan, a key player in ocean carbon storage.

New research reveals how communities of marine bacteria divide the task of degrading fucoidan, a key player in ocean carbon storage.

Cómo las comunidades de bacterias marinas trabajan en equipo para descomponer el fucoidan, un complejo polisacárido fundamental para el almacenamiento de carbono oceánico.

New research reveals how communities of marine bacteria divide the task of degrading fucoidan, a key player in ocean carbon storage.

Deep in the ocean, brown algae and diatoms produce a complex carbohydrate molecule called fucoidan, which helps form their protective outer layers.

Des bactéries marines s'associent pour décomposer l'un des molécules de stockage de carbone les plus résistantes de l'océanUne étude récente publiée dans Nature révèle que des communautés microbiennes marines sont capables de décomposer le fucoïdan, une molécule complexe produite par les algues brun

New research reveals how communities of marine bacteria divide the task of degrading fucoidan, a key player in ocean carbon storage.

Efficient degradation of fucoidans depends on complementary bacterial guilds that cooperatively break backbone and side-chain sugars, revealing a conserved, globally relevant mechanism that shapes mar...

Chemicals and reagents Fucoidan from F. vesiculosus (Sigma-Aldrich, F8190, lot no. 0000485452) was used as the primary substrate throughout this study. Additional F. vesiculosus fucoidans were obtained from Marinova (FVF2021547) and Biosynth (YF57714). Fucoidans from other species were sourced from Fucus serratus (Biosynth, YF09360), Fucus evanescens (OceanBasis and extracted as described previously60), Cladosiphon okamuranus (Biosynth, YF146834), Durvillaea potatorum (Biosynth, YF157165), Ecklonia maxima (Biosynth, YF157166) and Laminaria hyperborea (TheFucoidanStore, LowEndo Fucoidan).

Fucoidan from F. vesiculosus (Sigma-Aldrich, F8190, lot no. 0000485452) was used as the primary substrate throughout this study. Additional F. vesiculosus fucoidans were obtained from Marinova (FVF2021547) and Biosynth (YF57714). Fucoidans from other species were sourced from Fucus serratus (Biosynth, YF09360), Fucus evanescens (OceanBasis and extracted as described previously60), Cladosiphon okamuranus (Biosynth, YF146834), Durvillaea potatorum (Biosynth, YF157165), Ecklonia maxima (Biosynth, YF157166) and Laminaria hyperborea (TheFucoidanStore, LowEndo Fucoidan). For derivatization, 1-phenyl-3-methyl-5-pyrazolone (PMP) was obtained from Sigma-Aldrich (M70800). Internal standards for LC–MS analysis included d-galactose-13C6 (Sigma-Aldrich, 605379), d-mannose-13C6 (Sigma-Aldrich, 592994) and PMP-[d5] (CAS 1228765-67-0), custom-synthesized by BOC Sciences (Shirley). LC–MS-grade solvents and reagents were acetonitrile (Honeywell), methanol (Honeywell), ethanol (Sigma-Aldrich), formic acid (Sigma-Aldrich) and ammonium formate (Merck). Ultrapure water was produced using a Q-POD system (Merck). Unless otherwise stated, all other chemicals were of analytical grade and sourced from Sigma-Aldrich. Bacterial growth media Throughout this study, three distinct media detailed in Supplementary Table 1 were used for (1) enrichment of bacteria, (2) isolation of bacteria on solid medium and (3) routine cultivation of bacterial isolates in MBL medium52. All media mimicked the ionic composition of coastal seawater and contained 340 mM NaCl, 15 mM MgCl2, 6.75 mM KCl and 1 mM CaCl2...