The hadal trenches are the most remote and inaccessible habitats on earth and were once believed to be pristine. A recent study has reported the detection of high levels of persistent organic pollutants (POPs), including polybrominated diphenyl ethers (PBDEs) and polychlorinated biphenyls (PCBs), in endemic amphipods from two hadal trenches (Mariana and Kermadec) in the Western Pacific, implicating that the trenches are indeed polluted. However, a fundamental question remains unanswered, if and to what extent such the physical environment of the trenches is polluted by POPs. In this study, we sampled Mariana, Mussau, and New Britain trenches and analyzed samples of amphipods, sediment, and suspended particulate matter (SPM). Our results show that the amphipods contained elevated levels of PCBs and PBDEs, comparable to those reported in the earlier study. We also detected significantly high concentrations (up to 1343 ng g–1 lw) of chlorinated pesticides, such as dichlorodiphenyltrichloroethanes and chlordanes. Furthermore, four brominated natural products (BNPs), which structurally resembled methoxylated brominated diphenyl ethers or polybrominated biphenyls, were identified in the endemic amphipods. However, neither POPs nor BNPs were detected in sediments or SPM. Taken together, we propose that the POPs detected in endemic amphipods likely resulted from bioaccumulation by feeding on polluted large detritus (e.g., carrion) falling to the trench bottoms from the surface ocean.

The characteristics of the rhizosphere microbial community across different cultivation years (from 1, 3, 5, 15, 20, and 50 years) in saline-alkaline paddy soils in Songnen Plain of China were investigated based on sequence variation of 16S rDNA using Illumina MiSeq sequencing. The results showed that the microbial community diversity varied across cultivation years, showing higher diversity in cultivation years >15 than in cultivation years <15. The dominant microbial community of the rhizosphere was mainly composed of Proteobacteria, Acidobacteria, Firmicutes, and Bacteroidetes. Furthermore, soil microbial diversity appeared to be affected directly by changes in soil properties corresponding to cultivated years. Diversity of Proteobacteria decreased as cultivated years increased; however, that of Acidobacteria showed the opposite direction. In addition, the soil microbial communities were clustered into two main groups: one from the sites cultivated for fewer than 15 years, and the other from the sites cultivated for more than 15 years. The abundance of nitrogen-fixing microorganisms in the soil sample was significantly higher in soils cultivated for under 15 years than in those cultivated for over 15 years (P < 0.05). Moreover, there was an obvious negative correlation between the cultivated years and Methanosarcina. Our findings on the dynamics of microbial community and its specific function in response to variable soil conditions are important for understanding and improving physical and chemical characteristics of saline-alkaline soil in Songnen Plain of China.

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