Characteristics of the archaeal and bacterial communities in core sediments from Southern Yap Trench via in situ sampling by the manned submersible Jiaolong.

The hadal environment is the deepest part of the ocean and harbors a significant number of unique microbial communities. Here, we collected core sediment samples of Southern Yap Trench with the deep-sea manned submersible Jiaolong and analyzed the microbial community structure and abundance in the samples through high-throughput sequencing and real-time fluorescence quantitative PCR (qPCR), taking physicochemical parameters into account to explore potential environmental drivers and metabolic pathways therein. Considering the typical "V-shape" topography and frequent sediment collapses on trench walls, the core sediments of Southern Yap Trench harbored distinct microbial populations with fluctuating distributions and metabolic processes dominated by Proteobacteria and Thaumarchaeota. To discover the main potential metabolic processes of microbes, functional genes were detected by qPCR. The abundance of bacteria was greater than that of archaea in Southern Yap Trench sediments. The abundance of ammonia-oxidizing archaea (AOA), ammonia-oxidizing bacteria (AOB), sulfate-reducing bacteria (SRB) and denitrifying bacteria (denitrifier) decreased with increasing depth and decreasing total organic carbon (TOC%) and total nitrogen (TN%) and showed a positive and significant correlation with TOC% (P < 0.01), TN% (P < 0.01), TOC/TN molar ratio (C/N ratio) (P < 0.01) and median grain size (P < 0.01). From the perspective of function based on the 16S rRNA gene, aerobic ammonium oxidization, carbon assimilation, and chemoheterotrophic function may be the dominant processes in Southern Yap Trench sediments. Moreover, considering the isolated geomorphological and hydrological characteristics of Southern Yap Trench, we hypothesized that the distinct hadal microbial ecosystem was driven by the endogenous recycling of organic matter in the hadal sediments associated with the trench geomorphology.