Glycans shape microbiomes in the ocean and the gut, driving key steps in the global carbon cycle and human health. Yet, our ability to track microbial glycan turnover across microbiomes is limited, as identifying active degraders without prior genomic knowledge remains a key challenge. Here, we introduce an activity-based fluorescence resonance energy transfer (FRET) probe that enables direct visualization and quantification of glycan metabolism in complex microbial communities. As a proof of concept, we investigated α-mannan degradation, a prominent polysaccharide in algal blooms. Using automated glycan assembly, we synthesized a mannan hexasaccharide bearing a fluorescein–rhodamine FRET pair. The probe was validated using a recombinantly expressed endo -α-mannanase (GH76) from Salegentibacter sp. Hel_I_6. It was shown to function in cell lysates, pure cultures, and complex microbiomes (via plate assays and microscopy). This probe enabled spatiotemporal visualization of in situ α-mannan turnover in a marine microbiome. Glycan FRET probes are versatile tools for tracking glycan degradation across biological scales from single enzymes to microbiomes.
