Hadal trench sediments represent a remote and poorly explored ecosystem characterized by extreme hydrostatic pressure and high organic carbon deposition. We hypothesized microbes in trench sediments could utilize ancient organic matter deposited long ago, thereby influencing the cycling and fate of the well-deposited organic carbon. Here we examined the chemical properties of organic carbon alongside microbial composition and activities in an approximately 3-meter sediment core collected from the Challenger Deep of the Mariana Trench. Our findings revealed a dramatic increase in the production of dissolved organic matter (DOM) with recalcitrant properties at greater depths, correlating with higher organic carbon deposition. The recalcitrance of produced DOM was reflected by an elevated presence of aromatic molecular structures, a high nominated oxidative state of carbon, and a large contribution of recalcitrant carboxyl-rich-alicyclic-molecules. These chemical characteristics were closely associated with specific microbial groups at different depths of sediments. In the deep layers, laminated diatom mats deposited during the Last Glacial Maximum provided a substantial source of particulate organic matter that could be transformed into labile DOM. This process further enhanced the production and accumulation of recalcitrant DOM within the Mariana Trench sediment, suggesting occurrence of microbial carbon pump in the production and storage of recalcitrant DOM in sediments. This study underlies an active and successive endogenous microbial transformation of well-deposited particulate organic carbon under extreme hydrostatic pressure, highlighting the need for a nuanced understanding of the ultimate fate of organic carbon in marine sediments.
 

Mariana Trench; Sediment; Dissolved Organic Matter; FT-ICR MS; Diatom Blooms; Carbon Sequestration