The hadal zone remains one of the least-explored marine biospheres on Earth. Accumulated evidence suggests the existence of abundant microbial communities and active microbial carbon turnover in the sediment of hadal trenches, making the hadal biosphere a "hotspot" of organic carbon degradation in the deep ocean. However, little is known about the composition and metabolic potentials of hadal microbial communities. In this study, we utilized metagenomic sequencing and in-depth bioinformatic analysis to study the composition and functional capacities of microbial communities in the surface sediments of the "Challenger Deep", the Mariana Trench (water depth 10 853 m). The ultimate goal was to reveal the potential mechanism of hadal microorganisms to drive the biogeochemical cycles in the deep ocean and the consequential ecological effects. The results showed that Proteobacteria (Gammaproteobacteria, Alphaproteobacteria, Betaproteobacteria, Deltaproteobacteria), Thaumarchaeota, Bacteroidetes, Actinobacteria, and Planctomycetes were the dominant taxa in the surface sediments of the Mariana Trench. Functional annotation with particular focus on the metabolism related to carbon, nitrogen, and sulfur cycles showed that the studied microbial communities in the Mariana Trench dominated by heterotrophic processes, with the capability to degrade a wide range of organic carbon (OC) compounds, including some recalcitrant OC, such as phthalate or other aromatic compounds. The findings suggest a strong capability of the analyzed microbial communities for degradation of organic carbon compound, supporting their active roles in processes of organic matter degradation. In addition to the heterotrophic metabolism, the results also showed that autotrophic metabolic processes such as ammonia oxidation may occupy an important position in the analyzed microbial community, suggesting that the microbially fixed carbon may be an important source of organic carbon in the hadal sediment of the Mariana Trench.