Unique microbes released by retreating glaciers are rarely propagated to coastal ecosystems

Glacial retreat releases a vast reservoir of microbes into downstream ecosystems, yet the functional consequences of this transfer are unknown. Here we used genome-resolved metagenomics across a Svalbard glacier-foreland-fjord continuum and reconstructed 309 metagenome-assembled genomes (MAGs) to track microbial fate. Glacial taxa dominated foreland soils (75% of MAGs) but were rare in fjord sediments (14%), which hosted endemic marine lineages (83%). Although heterotrophy was ubiquitous, carbon fixation pathways were primarily encoded by glacial MAGs. The foreland was a denitrification hotspot, whereas sulfur oxidation dominated in the fjord. Genomic traits for cryoprotection and complex carbon degradation further distinguished glacial from marine taxa. We conclude that glaciers host functionally distinct microbiota, but strong environmental filtering at the land-sea interface limits functional propagation to marine ecosystems, implying glacier loss may disrupt terrestrial microbial biodiversity and biogeochemistry more profoundly than coastal systems. Metabolically unique glacial microbes dominate foreland communities but rarely persist in fjords, showing limited functional transfer across the land-sea interface, based on genome-resolved metagenomics along a glacier-to-fjord gradient