Summary The biology of biofilm in deep‐sea environments is barely being explored. Here, biofilms were developed at the brine pool (characterized by limited carbon sources) and the normal bottom water adjacent to T huwal cold seeps. Comparative metagenomics based on 50 Gb datasets identified polysaccharide degradation, nitrate reduction and proteolysis as enriched functional categories for brine biofilms. The genomes of two dom...