Marine bacteria are generally considered consumers of algal polysaccharides. Here we demonstrate that algal polysaccharides can also function as bioactive regulators that attenuate bacterial metabolism. Using purified fucoidan as a model molecule, we show that exposure of phylogenetically diverse marine bacteria leads to reduced respiration, intracellular ROS production and biofilm formation without substantially affecting growth. Transcriptomic analyses further reveal coordinated activation of membrane stress responses and suppression of respiration- and nutrient acquisition-related pathways, indicating a transition toward a low-metabolic physiological state. Because these responses occur in bacteria lacking fucoidan-degrading capacity, they likely result from direct interactions between polysaccharides and bacterial cell surfaces rather than substrate metabolism. We propose that algal polysaccharides not only fuel microbial metabolism but also regulate microbial carbon processing by suppressing bacterial carbon remineralization. This study introduces a previously unrecognized mechanism linking algae-derived polysaccharides to microbial physiology and the fate of marine organic carbon, with important implications for understanding biological carbon sequestration in a changing ocean.

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