Hadal sediments are recognized as organic carbon depocenters with intensified microbial activity compared
to adjacent abyssal sites due to focusing of relatively labile organic materials. However, the sources and turnover
of hadopelagic organic carbon and its linkages to microbial activities have not been studied. We present the first
synergic research on particulate organic carbon, dark carbon fixation, and size-fractionated microbial community
respiration proxy over the Atacama Trench. The results demonstrate that all parameters attenuate rapidly
from surface to mesopelagic water (
1000 m). Progressing deeper, values remain relatively stable throughout
bathypelagic (
4000 m) and abyssopelagic (
6000 m) waters. However, in the hadopelagic zone (> 6000 m),
highly variable values indicate dynamic organic carbon sources and microbial activities in the deepest trench.
On average, 71% of the microbial community respiration proxy is attributable to particle-associated communities,
indicating importance of particles for microbial metabolism. No apparent relationship was observed
between the microbial community respiration proxy and microbial 16S rRNA gene abundance below the epipelagic
depth, indicating variable supply and quality of organic carbon likely constrained heterotrophic activities
rather than microbial abundances in the deep ocean. The depth-integrated dark carbon fixation (> 1000 m)
accounts for 11.5%  7.6% of the surface net primary production, of which 2.9%  0.4% is from hadopelagic
depth. Dark carbon fixation is thus an important in situ organic carbon source for hadal life. This study suggests
that high variability in organic carbon sources and microbial activities in the hadopelagic trench cannot be simply
extrapolated from findings in the shallower dark ocean (e.g., 1000–6000 m).

Atacama Trench,Particulate organic carbon,dark carbon fixation,microbial community respiration