Background: The Mariana Trench is the deepest known site in the Earth's oceans, reaching a depth of similar to 11,000m at the Challenger Deep. Recent studies reveal that hadal waters harbor distinctive microbial planktonic communities. However, the genetic potential of microbial communities within the hadal zone is poorly understood. Results: Here, implementing both culture-dependent and culture-independent methods, we perform extensive analysis of microbial populations and their genetic potential at different depths in the Mariana Trench. Unexpectedly, we observed an abrupt increase in the abundance of hydrocarbon-degrading bacteria at depths > 10,400m in the Challenger Deep. Indeed, the proportion of hydrocarbon-degrading bacteria at > 10,400m is the highest observed in any natural environment on Earth. These bacteria were mainly Oleibacter, Thalassolituus, and Alcanivorax genera, all of which include species known to consume aliphatic hydrocarbons. This community shift towards hydrocarbon degraders was accompanied by increased abundance and transcription of genes involved in alkane degradation. Correspondingly, three Alcanivorax species that were isolated from 10,400m water supplemented with hexadecane were able to efficiently degrade n-alkanes under conditions simulating the deep sea, as did a reference Oleibacter strain cultured at atmospheric pressure. Abundant n-alkanes were observed in sinking particles at 2000, 4000, and 6000m (averaged 23.5 mu g/gdw) and hadal surface sediments at depths of 10,908, 10,909, and 10,911m (averaged 2.3 mu g/gdw). The delta H-2 values of n-C-16/18 alkanes that dominated surface sediments at near 11,000-m depths ranged from -79 to -93%, suggesting that these sedimentary alkanes may have been derived from an unknown heterotrophic source. Conclusions: These results reveal that hydrocarbon-degrading microorganisms are present in great abundance in the deepest seawater on Earth and shed a new light on potential biological processes in this extreme environment.